MySQL Reference Manual for version 3.23.48.

1 General Information About MySQL

This is the MySQL reference manual; it documents MySQL Version 3.23.48. As MySQL is work in progress, the manual gets updated frequently. There is a very good chance that this version is out of date, unless you are looking at it online. The most recent version of this manual is available at http://www.mysql.com/documentation/ in many different formats. If you have a hard time finding information in the manual, you can try the searchable PHP version at http://www.mysql.com/documentation/manual.php.

MySQL is a very fast, multi-threaded, multi-user, and robust SQL (Structured Query Language) database server.

MySQL is free software. It is licensed with the GNU GENERAL PUBLIC LICENSE http://www.gnu.org/. See section 1.3 MySQL Licensing and Support.

The MySQL home page provides the latest information about MySQL.

The following list describes some useful sections of the manual:

IMPORTANT:

Reports of errors (often called bugs), as well as questions and comments, should be sent to the mailing list at mysql@lists.mysql.com. See section 1.2.22.3 How to Report Bugs or Problems. The mysqlbug script should be used to generate bug reports. For source distributions, the mysqlbug script can be found in the `scripts' directory. For binary distributions, mysqlbug can be found in the `bin' directory. If you have found a sensitive security bug in MySQL, you should send an email to security@mysql.com.

If you have any suggestions concerning additions or corrections to this manual, please send them to the manual team at docs@mysql.com.

This is a reference manual; it does not provide general instruction on SQL or relational database concepts. If you want general information about SQL, see section 1.2.2 General SQL Information and Tutorials. For books that focus more specifically on MySQL, see section 1.2.1 Books About MySQL.

1.1 MySQL, MySQL AB, and Open Source

1.1.1 What Is MySQL

MySQL, the most popular Open Source SQL database, is provided by MySQL AB. MySQL AB is a commercial company that builds its business providing services around the MySQL database. See section 1.1.2 What Is MySQL AB.

MySQL is a database management system.
A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as MySQL. Since computers are very good at handling large amounts of data, database management plays a central role in computing, as stand-alone utilities, or as parts of other applications.
MySQL is a relational database management system.
A relational database stores data in separate tables rather than putting all the data in one big storeroom. This adds speed and flexibility. The tables are linked by defined relations making it possible to combine data from several tables on request. The SQL part of MySQL stands for "Structured Query Language" - the most common standardized language used to access databases.
MySQL is Open Source Software.
Open Source means that it is possible for anyone to use and modify. Anybody can download MySQL from the Internet and use it without paying anything. Anybody so inclined can study the source code and change it to fit their needs. MySQL uses the GPL (GNU General Public License) http://www.gnu.org, to define what you may and may not do with the software in different situations. If you feel uncomfortable with the GPL or need to embed MySQL into a commercial application you can buy a commercially licensed version from us.
Why use MySQL?
MySQL is very fast, reliable, and easy to use. If that is what you are looking for, you should give it a try. MySQL also has a very practical set of features developed in very close cooperation with our users. You can find a performance comparison of MySQL to some other database managers on our benchmark page. See section 5.1.4 The MySQL Benchmark Suite. MySQL was originally developed to handle very large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Though under constant development, MySQL today offers a rich and very useful set of functions. The connectivity, speed, and security make MySQL highly suited for accessing databases on the Internet.
The technical features of MySQL
For advanced technical information, see section 6 MySQL Language Reference. MySQL is a client/server system that consists of a multi-threaded SQL server that supports different backends, several different client programs and libraries, administrative tools, and several programming interfaces. We also provide MySQL as a multi-threaded library which you can link into your application to get a smaller, faster, easier to manage product.
MySQL has a lot of contributed software available.
It is very likely that you will find that your favorite application or language already supports MySQL.

The official way to pronounce MySQL is ``My Ess Que Ell'' (not MY-SEQUEL). But we try to avoid correcting people who say MY-SEQUEL.

1.1.2 What Is MySQL AB

MySQL AB is the Swedish company owned and run by the MySQL founders and main developers. We are dedicated to developing MySQL and spreading our database to new users. MySQL AB owns the copyright to the MySQL server source code and the MySQL trademark. A significant amount of revenues from our services goes to developing MySQL. See section 1.1.1 What Is MySQL.

MySQL AB has been profitable providing MySQL from the start. We don't get any outside funding, but have earned all our money ourselves.

We are searching after partners that would like to support our development of MySQL so that we could accelerate the development pace. If you are interested in doing this, you can email partner@mysql.com about this!

MySQL AB has currently 20+ people (http://www.mysql.com/development/team.html) on its payroll and is growing rapidly.

Our main sources of income are:

The MySQL core values show our dedication to MySQL and Open Source.

We want MySQL to be:

MySQL AB and the people of MySQL AB:

1.1.3 About This Manual

This manual is currently available in Texinfo, plain text, Info, HTML, PostScript, and PDF versions. The primary document is the Texinfo file. The HTML version is produced automatically using a modified version of texi2html. The plain text and Info versions are produced with makeinfo. The Postscript version is produced using texi2dvi and dvips. The PDF version is produced with pdftex.

This manual is written and maintained by David Axmark, Michael (Monty) Widenius, Jeremy Cole, and Paul DuBois. For other contributors, see section E Credits.

1.1.4 Conventions Used in This Manual

This manual uses certain typographical conventions:

constant
Constant-width font is used for command names and options; SQL statements; database, table and column names; C and Perl code; and environment variables. Example: ``To see how mysqladmin works, invoke it with the --help option.''
`filename'
Constant-width font with surrounding quotes is used for filenames and pathnames. Example: ``The distribution is installed under the `/usr/local/' directory.''
`c'
Constant-width font with surrounding quotes is also used to indicate character sequences. Example: ``To specify a wild card, use the `%' character.''
italic
Italic font is used for emphasis, like this.
boldface
Boldface font is used for access privilege names (for example, ``do not grant the process privilege lightly'') and occasionally to convey especially strong emphasis.

When commands are shown that are meant to be executed by a particular program, the program is indicated by a prompt shown before the command. For example, shell> indicates a command that you execute from your login shell, and mysql> indicates a command that you execute from the mysql client program: 

shell> type a shell command here
mysql> type a mysql command here

Shell commands are shown using Bourne shell syntax. If you are using a csh-style shell, you may need to issue commands slightly differently. For example, the sequence to set an environment variable and run a command looks like this in Bourne shell syntax: 

shell> VARNAME=value some_command

For csh, you would execute the sequence like this: 

shell> setenv VARNAME value
shell> some_command

Often, database, table, and column names must be substituted into commands. To indicate that such substitution is necessary, this manual uses db_name, tbl_name and col_name. For example, you might see a statement like this: 

mysql> SELECT col_name FROM db_name.tbl_name;

This means that if you were to enter a similar statement, you would supply your own database, table, and column names, perhaps like this: 

mysql> SELECT author_name FROM biblio_db.author_list;

SQL statements may be written in uppercase or lowercase. When this manual shows a SQL statement, uppercase is used for particular keywords if those keywords are under discussion (to emphasize them) and lowercase is used for the rest of the statement. For example, you might see the following in a discussion of the SELECT statement: 

mysql> SELECT count(*) FROM tbl_name;

On the other hand, in a discussion of the COUNT() function, the same statement would be written like this: 

mysql> select COUNT(*) from tbl_name;

If no particular emphasis is intended, all keywords are written uniformly in uppercase.

In syntax descriptions, square brackets (`[' and `]') are used to indicate optional words or clauses: 

DROP TABLE [IF EXISTS] tbl_name

When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (`|'). When one member from a set of choices may be chosen, the alternatives are listed within square brackets (`[' and `]'): 

TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)

When one member from a set of choices must be chosen, the alternatives are listed within braces (`{' and `}'): 

{DESCRIBE | DESC} tbl_name {col_name | wild}

1.1.5 History of MySQL

We once started out with the intention of using mSQL to connect to our tables using our own fast low-level (ISAM) routines. However, after some testing we came to the conclusion that mSQL was not fast enough nor flexible enough for our needs. This resulted in a new SQL interface to our database but with almost the same API interface as mSQL. This API was chosen to ease porting of third-party code.

The derivation of the name MySQL is not perfectly clear. Our base directory and a large number of our libraries and tools have had the prefix ``my'' for well over 10 years. However, Monty's daughter (some years younger) is also named My. Which of the two gave its name to MySQL is still a mystery, even for us.

1.1.6 The Main Features of MySQL

The following list describes some of the important characteristics of MySQL:

1.1.7 How Stable Is MySQL?

This section addresses the questions ``How stable is MySQL?'' and ``Can I depend on MySQL in this project?'' We will try to clarify some issues and to answer some of the more important questions that seem to concern many people. This section has been put together from information gathered from the mailing list (which is very active in reporting bugs).

At TcX, MySQL has worked without any problems in our projects since mid-1996. When MySQL was released to a wider public, we noticed that there were some pieces of ``untested code'' that were quickly found by the new users who made queries in a manner different than our own. Each new release has had fewer portability problems than the previous one (even though each has had many new features).

Each release of MySQL has been usable, and there have been problems only when users start to use code from the ``gray zones.'' Naturally, outside users don't know what the gray zones are; this section attempts to indicate those that are currently known. The descriptions deal with Version 3.23 of MySQL. All known and reported bugs are fixed in the latest version, with the exception of the bugs listed in the bugs section, which are things that are design-related. See section 1.4.7 Known errors and design deficiencies in MySQL.

MySQL is written in multiple layers and different independent modules. These modules are listed below with an indication of how well-tested each of them is:

The ISAM table handler -- Stable
This manages storage and retrieval of all data in MySQL Version 3.22 and earlier. In all MySQL releases there hasn't been a single (reported) bug in this code. The only known way to get a corrupted table is to kill the server in the middle of an update. Even that is unlikely to destroy any data beyond rescue, because all data are flushed to disk between each query. There hasn't been a single bug report about lost data because of bugs in MySQL.
The MyISAM table handler -- Stable
This is new in MySQL Version 3.23. It's largely based on the ISAM table code but has a lot of new and very useful features.
The parser and lexical analyser -- Stable
There hasn't been a single reported bug in this system for a long time.
The C client code -- Stable
No known problems. In early Version 3.20 releases, there were some limitations in the send/receive buffer size. As of Version 3.21, the buffer size is now dynamic up to a default of 16M.
Standard client programs -- Stable
These include mysql, mysqladmin, mysqlshow, mysqldump, and mysqlimport.
Basic SQL -- Stable
The basic SQL function system and string classes and dynamic memory handling. Not a single reported bug in this system.
Query optimizer -- Stable
Range optimizer -- Stable
Join optimizer -- Stable
Locking -- Gamma
This is very system-dependent. On some systems there are big problems using standard OS locking (fcntl()). In these cases, you should run the MySQL daemon with the --skip-locking flag. Problems are known to occur on some Linux systems, and on SunOS when using NFS-mounted file systems.
Linux threads -- Stable
The major problem found has been with the fcntl() call, which is fixed by using the --skip-locking option to mysqld. Some people have reported lockup problems with Version 0.5. LinuxThreads will need to be recompiled if you plan to use 1000+ concurrent connections. Although it is possible to run that many connections with the default LinuxThreads (however, you will never go above 1021), the default stack spacing of 2 MB makes the application unstable, and we have been able to reproduce a coredump after creating 1021 idle connections. See section 2.6.1 Linux Notes (All Linux Versions).
Solaris 2.5+ pthreads -- Stable
We use this for all our production work.
MIT-pthreads (Other systems) -- Stable
There have been no reported bugs since Version 3.20.15 and no known bugs since Version 3.20.16. On some systems, there is a ``misfeature'' where some operations are quite slow (a 1/20 second sleep is done between each query). Of course, MIT-pthreads may slow down everything a bit, but index-based SELECT statements are usually done in one time frame so there shouldn't be a mutex locking/thread juggling.
Other thread implementions -- Beta - Gamma
The ports to other systems are still very new and may have bugs, possibly in MySQL, but most often in the thread implementation itself.
LOAD DATA ..., INSERT ... SELECT -- Stable
Some people thought they had found bugs here, but these usually have turned out to be misunderstandings. Please check the manual before reporting problems!
ALTER TABLE -- Stable
Small changes in Version 3.22.12.
DBD -- Stable
Now maintained by Jochen Wiedmann (wiedmann@neckar-alb.de). Thanks!
mysqlaccess -- Stable
Written and maintained by Yves Carlier (Yves.Carlier@rug.ac.be). Thanks!
GRANT -- Stable
Big changes made in MySQL Version 3.22.12.
MyODBC (uses ODBC SDK 2.5) -- Gamma
It seems to work well with some programs.
Replication -- Beta / Gamma
We are still working on replication, so don't expect this to be rock solid yet. On the other hand, some MySQL users are already using this with good results.
BDB Tables -- Beta
The Berkeley DB code is very stable, but we are still improving the interface between MySQL and BDB tables, so it will take some time before this is as tested as the other table types.
InnoDB Tables -- Beta
This is a recent addition to MySQL. They appear to work good and can be used after some initial testing.
Automatic recovery of MyISAM tables - Beta
This only affects the new code that checks if the table was closed properly on open and executes an automatic check/repair of the table if it wasn't.
MERGE tables -- Beta / Gamma
The usage of keys on MERGE tables is still not that tested. The other part of the MERGE code is quite well tested.
FULLTEXT -- Beta
Text search seems to work, but is still not widely used.

MySQL AB provides e-mail support for paying customers, but the MySQL mailing list usually provides answers to common questions. Bugs are usually fixed right away with a patch; for serious bugs, there is almost always a new release.

1.1.8 How Big Can MySQL Tables Be?

MySQL Version 3.22 has a 4G limit on table size. With the new MyISAM in MySQL Version 3.23 the maximum table size is pushed up to 8 million terabytes (2 ^ 63 bytes).

Note, however, that operating systems have their own file size limits. Here are some examples:

Operating System File Size Limit
Linux-Intel 32 bit 2G, 4G or more, depends on Linux version
Linux-Alpha 8T (?)
Solaris 2.5.1 2G (possible 4G with patch)
Solaris 2.6 4G
Solaris 2.7 Intel 4G
Solaris 2.7 ULTRA-SPARC 8T (?)

On Linux 2.2 you can get bigger tables than 2G by using the LFS patch for the ext2 file system. On Linux 2.4 there exists also patches for ReiserFS to get support for big files.

This means that the table size for MySQL is normally limited by the operating system.

By default, MySQL tables have a maximum size of about 4G. You can check the maximum table size for a table with the SHOW TABLE STATUS command or with the myisamchk -dv table_name. See section 4.5.5 SHOW Syntax.

If you need bigger tables than 4G (and your operating system supports this), you should set the AVG_ROW_LENGTH and MAX_ROWS parameter when you create your table. See section 6.5.3 CREATE TABLE Syntax. You can also set these later with ALTER TABLE. See section 6.5.4 ALTER TABLE Syntax.

If your big table is going to be read-only, you could use myisampack to merge and compress many tables to one. myisampack usually compresses a table by at least 50%, so you can have, in effect, much bigger tables. See section 4.7.4 myisampack, The MySQL Compressed Read-only Table Generator.

You can go around the operating system file limit for MyISAM data files by using the RAID option. See section 6.5.3 CREATE TABLE Syntax.

Another solution can be the included MERGE library, which allows you to handle a collection of identical tables as one. See section 7.2 MERGE Tables.

1.1.9 Year 2000 Compliance

MySQL itself has no problems with Year 2000 (Y2K) compliance:

You may run into problems with applications that use MySQL in a way that is not Y2K-safe. For example, many old applications store or manipulate years using 2-digit values (which are ambiguous) rather than 4-digit values. This problem may be compounded by applications that use values such as 00 or 99 as ``missing'' value indicators.

Unfortunately, these problems may be difficult to fix, because different applications may be written by different programmers, each of whom may use a different set of conventions and date-handling functions.

Here is a simple demonstration illustrating that MySQL doesn't have any problems with dates until the year 2030: 

mysql> DROP TABLE IF EXISTS y2k;
Query OK, 0 rows affected (0.01 sec)

mysql> CREATE TABLE y2k (date date, date_time datetime, time_stamp timestamp);
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO y2k VALUES 
    -> ("1998-12-31","1998-12-31 23:59:59",19981231235959),
    -> ("1999-01-01","1999-01-01 00:00:00",19990101000000),
    -> ("1999-09-09","1999-09-09 23:59:59",19990909235959),
    -> ("2000-01-01","2000-01-01 00:00:00",20000101000000),
    -> ("2000-02-28","2000-02-28 00:00:00",20000228000000),
    -> ("2000-02-29","2000-02-29 00:00:00",20000229000000),
    -> ("2000-03-01","2000-03-01 00:00:00",20000301000000),
    -> ("2000-12-31","2000-12-31 23:59:59",20001231235959),
    -> ("2001-01-01","2001-01-01 00:00:00",20010101000000),
    -> ("2004-12-31","2004-12-31 23:59:59",20041231235959),
    -> ("2005-01-01","2005-01-01 00:00:00",20050101000000),
    -> ("2030-01-01","2030-01-01 00:00:00",20300101000000),
    -> ("2050-01-01","2050-01-01 00:00:00",20500101000000);
Query OK, 13 rows affected (0.01 sec)
Records: 13  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM y2k;
+------------+---------------------+----------------+
| date       | date_time           | time_stamp     |
+------------+---------------------+----------------+
| 1998-12-31 | 1998-12-31 23:59:59 | 19981231235959 |
| 1999-01-01 | 1999-01-01 00:00:00 | 19990101000000 |
| 1999-09-09 | 1999-09-09 23:59:59 | 19990909235959 |
| 2000-01-01 | 2000-01-01 00:00:00 | 20000101000000 |
| 2000-02-28 | 2000-02-28 00:00:00 | 20000228000000 |
| 2000-02-29 | 2000-02-29 00:00:00 | 20000229000000 |
| 2000-03-01 | 2000-03-01 00:00:00 | 20000301000000 |
| 2000-12-31 | 2000-12-31 23:59:59 | 20001231235959 |
| 2001-01-01 | 2001-01-01 00:00:00 | 20010101000000 |
| 2004-12-31 | 2004-12-31 23:59:59 | 20041231235959 |
| 2005-01-01 | 2005-01-01 00:00:00 | 20050101000000 |
| 2030-01-01 | 2030-01-01 00:00:00 | 20300101000000 |
| 2050-01-01 | 2050-01-01 00:00:00 | 00000000000000 |
+------------+---------------------+----------------+
13 rows in set (0.00 sec)

This shows that the DATE and DATETIME types will not give any problems with future dates (they handle dates until the year 9999).

The TIMESTAMP type, which is used to store the current time, has a range up to only 2030-01-01. TIMESTAMP has a range of 1970 to 2030 on 32-bit machines (signed value). On 64-bit machines it handles times up to 2106 (unsigned value).

Even though MySQL is Y2K-compliant, it is your responsibility to provide unambiguous input. See section 6.2.2.1 Y2K Issues and Date Types for MySQL's rules for dealing with ambiguous date input data (data containing 2-digit year values).

1.2 MySQL Information Sources

1.2.1 Books About MySQL

For the latest book information, with user comments, please visit http://www.mysql.com/portal/books/html/index.html.

While this manual is still the right place for up to date technical information, its primary goal is to contain everything there is to know about MySQL. It is sometimes nice to have a bound book to read in bed or while you travel. Here is a list of books about MySQL and related subjects (in English).

By purchasing a book through these hyperlinks provided herein, you are contributing to the development of MySQL.

MySQL
Available Barnes and Noble
Publisher New Riders
Author Paul DuBois
Pub Date 1st Edition December 1999
ISBN 0735709211
Pages 800
Price $49.99 US
Downloadable examples samp_db distribution
Errata are available here

Foreword by Michael ``Monty'' Widenius, MySQL Moderator.

In MySQL, Paul DuBois provides you with a comprehensive guide to one of the most popular relational database systems. Paul has contributed to the online documentation for MySQL and is an active member of the MySQL community. The principal MySQL developer, Monty Widenius, and a network of his fellow developers reviewed the manuscript, and provided Paul with the kind of insight no one else could supply.

Instead of merely giving you a general overview of MySQL, Paul teaches you how to make the most of its capabilities. Through two sample database applications that run throughout the book, he gives you solutions to problems you're sure to face. He helps you integrate MySQL efficiently with third-party tools, such as PHP and Perl, enabling you to generate dynamic Web pages through database queries. He teaches you to write programs that access MySQL databases, and also provides a comprehensive set of references to column types, operators, functions, SQL syntax, MySQL programming, C API, Perl DBI, and PHP API. MySQL simply gives you the kind of information you won't find anywhere else.

If you use MySQL, this book provides you with:


MySQL & mSQL
Available Barnes and Noble
Publisher O'Reilly
Authors Randy Jay Yarger, George Reese & Tim King
Pub Date 1st Edition July 1999
ISBN 1-56592-434-7, Order Number: 4347
Pages 506
Price $34.95

This book teaches you how to use MySQL and mSQL, two popular and robust database products that support key subsets of SQL on both Linux and Unix systems. Anyone who knows basic C, Java, Perl, or Python can write a program to interact with a database, either as a stand-alone application or through a Web page. This book takes you through the whole process, from installation and configuration to programming interfaces and basic administration. Includes plenty of tutorial material.

Sams' Teach Yourself MySQL in 21 Days
Available Barnes and Noble
Publisher Sams
Authors Mark Maslakowski and Tony Butcher
Pub Date June 2000
ISBN 0672319144
Pages 650
Price $39.99

Sams' Teach Yourself MySQL in 21 Days is for intermediate Linux users who want to move into databases. A large share of the audience is Web developers who need a database to store large amounts of information that can be retrieved via the Web.

Sams' Teach Yourself MySQL in 21 Days is a practical, step-by-step tutorial. The reader will learn to design and employ this open source database technology into his or her Web site using practical, hands-on examples to follow.

E-Commerce Solutions with MySQL
Available Barnes and Noble
Publisher Prima Communications, Inc.
Authors N/A
Pub Date January 2000
ISBN 0761524452
Pages 500
Price $39.99

No description available.

MySQL and PHP from Scratch
Available Barnes and Noble
Publisher Que
Authors N/A
Pub Date September 2000
ISBN 0789724405
Pages 550
Price $34.99

This book puts together information on installing, setting up, and troubleshooting Apache, MySQL, PHP3, and IMP into one complete volume. You also learn how each piece is part of a whole by learning, step-by-step, how to create a web-based e-mail system. Learn to run the equivalent of Active Server Pages (ASP) using PHP3, set up an e-commerce site using a database and the Apache web server, and create a data entry system (such as sales, product quality tracking, customer preferences, etc) that no installation in the PC.

Professional MySQL Programming
Available Barnes and Noble
Publisher Wrox Press, Inc.
Authors N/A
Pub Date Late 2001
ISBN 1861005164
Pages 1000
Price $49.99

No description available.

Professional Linux Programming
Available Barnes and Noble
Publisher Wrox Press, Inc.
Authors N/A
Pub Date September 2000
ISBN 1861003013
Pages 1155
Price $47.99

In this follow-up to the best-selling Beginning Linux Programming, you will learn from the authors' real-world knowledge and experience of developing software for Linux; you'll be taken through the development of a sample 'DVD Store' application, with 'theme' chapters addressing different aspects of its implementation. Meanwhile, individual ``take-a-break'' chapters cover important topics that go beyond the bounds of the central theme. All focus on the practical aspects of programming, showing how crucial it is to choose the right tools for the job, use them as they should be used, and get things right first time.

PHP and MySQL Web Development
Available Barnes and Noble
Publisher Sams
Authors Luke Welling, Laura Thomson
Pub Date March 2001
ISBN 0672317842
Pages 700
Price $49.99

PHP and MySQL Web Development introduces you to the advantages of implementing both MySQL and PHP. These advantages are detailed through the provision of both statistics and several case studies. A practical web application is developed throughout the book, providing you with the tools necessary to implement a functional online database. Each function is developed separately, allowing you the choice to incorporate only those parts that you would like to implement. Programming concepts of the PHP language are highlighted, including functions which tie MySQL support into a PHP script and advanced topics regarding table manipulation.

Books recommended by the MySQL Developers

SQL-99 Complete, Really
Available Barnes and Noble
Publisher CMP Books
Authors Peter Gulutzan, Trudy Pelzer
Pub Date April 1999
ISBN 0879305681
Pages 1104
Price $55.96

This book contains complete descriptions of the new standards for syntax, data structures, and retrieval processes of SQL databases. As an example-based reference manual, it includes all of the CLI functions, information, schema tables, and status codes, as well as a working SQL database provided on the companion disk.

C, A reference manual
Available Barnes and Noble
Publisher Prentice Hall
Authors Samuel P. Harbison, Guy L. Steele
Pub Date September 1994
ISBN 0133262243
Pages 480
Price $35.99

A new and improved revision of the bestselling C language reference. This manual introduces the notion of "Clean C", writing C code that can be compiled as a C++ program, C programming style that emphasizes correctness, portability, maintainability, and incorporates the ISO C Amendment 1 (1994) which specifies new facilities for writing portable, international programs in C.

C++ for Real Programmers
Available Barnes and Noble
Publisher Academic Press, Incorporated
Authors Jeff Alger, Jim Keogh
Pub Date February 1998
ISBN 0120499428
Pages 388
Price $39.95

C++ For Real Programmers bridges the gap between C++ as described in beginner and intermediate-level books and C++ as it is practiced by experts. Numerous valuable techniques are described, organized into three simple themes: indirection, class hierarchies, and memory management. It also provides in-depth coverage of template creation, exception handling, pointers and optimization techniques. The focus of the book is on ANSI C++ and, as such, is compiler independent.

C++ For Real Programmers is a revision of Secrets of the C++ Masters and includes a new appendix comparing C++ with Java. The book comes with a 3.5" disk for Windows with source code.

Algorithms in C
Available Barnes and Noble
Publisher Addison Wesley Longman, Inc.
Authors Robert Sedgewick
Pub Date April 1990
ISBN 0201514257
Pages 648
Price $45.75

Algorithms in C describes a variety of algorithms in a number of areas of interest, including: sorting, searching, string-processing, and geometric, graph and mathematical algorithms. The book emphasizes fundamental techniques, providing readers with the tools to confidently implement, run, and debug useful algorithms.

Multithreaded Programming with Pthreads
Available Barnes and Noble
Publisher Prentice Hall
Authors Bil Lewis, Daniel J. Berg
Pub Date October 1997
ISBN 0136807291
Pages 432
Price $34.95

Based on the best-selling Threads Primer, Multithreaded Programming with Pthreads gives you a solid understanding of Posix threads: what they are, how they work, when to use them, and how to optimize them. It retains the clarity and humor of Threads Primer, but includes expanded comparisons to Win32 and OS/2 implementations. Code examples tested on all of the major UNIX platforms are featured along with detailed explanations of how and why they use threads.

Programming the PERL DBI: Database Programming with PERL
Available Barnes and Noble
Publisher O'Reilly & Associates, Incorporated
Authors Alligator Descartes, Tim Bunce
Pub Date February 2000
ISBN 1565926994
Pages 400
Price $27.96

Programming the Perl DBI is coauthored by Alligator Descartes, one of the most active members of the DBI community, and by Tim Bunce, the inventor of DBI. For the uninitiated, the book explains the architecture of DBI and shows you how to write DBI-based programs. For the experienced DBI dabbler, this book explains DBI's nuances and the peculiarities of each individual DBD.

The book includes:


1.2.2 General SQL Information and Tutorials

The following book has been recommended by several people on the MySQL mailing list: 

Judith S. Bowman, Sandra L. Emerson and Marcy Darnovsky
The Practical SQL Handbook: Using Structured Query Language
Second Edition
Addison-Wesley
ISBN 0-201-62623-3
http://www.awl.com

The following book has also received some recommendations by MySQL users: 

Martin Gruber
Understanding SQL
ISBN 0-89588-644-8
Publisher Sybex 510 523 8233
Alameda, CA USA

A SQL tutorial is available on the net at http://w3.one.net/~jhoffman/sqltut.htm

1.2.3 Useful MySQL-related Links

Apart from the following links, you can find and download a lot of MySQL programs, tools and APIs from the Contrib directory.

MySQL

1.2.4 Tutorials and Manuals

MySQL Myths Debunked
MySQL used in the real world.
http://www.4t2.com/mysql
Information about the German MySQL mailing list.
http://www2.rent-a-database.de/mysql/
MySQL handbook in German.
http://www.bitmover.com:8888//home/bk/mysql
Web access to the MySQL BitKeeper repository.
http://www.analysisandsolutions.com/code/mybasic.htm
Beginners MySQL Tutorial on how to install and set up MySQL on a Windows machine.
http://www.devshed.com/Server_Side/MySQL/
A lot of MySQL tutorials.
http://mysql.hitstar.com/
MySQL manual in Chinese.
http://www.linuxplanet.com/linuxplanet/tutorials/1046/1/
Setting Up a MySQL-based Web site.
http://www.hotwired.com/webmonkey/backend/tutorials/tutorial1.html
MySQL-Perl tutorial.
http://www.iserver.com/support/contrib/perl5/modules.html
Installing new Perl modules that require locally installed modules.
http://www.hotwired.com/webmonkey/databases/tutorials/tutorial4.html
PHP/MySQL Tutorial.
http://www.useractive.com/
Hands on tutorial for MySQL.

1.2.5 Porting MySQL/Using MySQL on Different Systems

http://www.entropy.ch/software/macosx/mysql/
Binary of MySQL for Mac OS X Client. Includes information of how to build and use MySQL on Mac OS X.
http://xclave.macnn.com/MySQL/
The Mac OS Xclave. Running MySQL on Mac OS X.
http://www.prnet.de/RegEx/mysql.html
MySQL for Mac OS X Server.
http://www.latencyzero.com/macosx/mysql.html
Building MySQL for Mac OS X.
http://www.essencesw.com/Software/mysqllib.html
New Client libraries for the Mac OS Classic (Macintosh).
http://www.lilback.com/macsql/
Client libraries for Mac OS Classic (Macintosh).
http://sixk.maniasys.com/index_en.html
MySQL for Amiga

1.2.6 Perl-related Links

http://dbimysql.photoflux.com/
Perl DBI with MySQL FAQ.

1.2.7 MySQL Discussion Forums

http://www.weberdev.com/
Examples using MySQL; (check Top 20)
http://futurerealm.com/forum/futureforum.htm
FutureForum Web Discussion Software.

1.2.8 Commercial Applications that Support MySQL

http://www.supportwizard.com/
SupportWizard; Interactive helpdesk on the Web (This product includes a licensed copy of MySQL.)
http://www.sonork.com/
Sonork, Instant Messenger that is not only Internet oriented. It's focused on private networks and on small to medium companies. Client is free, server is free for up to 5 seats.
http://www.stweb.org/
StWeb - Stratos Web and Application server - An easy-to-use, cross platform, Internet/Intranet development and deployment system for development of web-enabled applications. The standard version of StWeb has a native interface to MySQL database.
http://www.rightnowtech.com/
Right Now Web; Web automation for customer service.
http://www.icaap.org/Bazaar/
Bazaar; Interactive Discussion Forums with Web interface.
http://www.phonesweep.com/
PhoneSweepT is the world's first commercial Telephone Scanner. Many break-ins in recent years have come not through the Internet, but through unauthorized dial-up modems. PhoneSweep lets you find these modems by repeatedly placing phone calls to every phone number that your organization controls. PhoneSweep has a built-in expert system that can recognize more than 250 different kinds of remote-access programs, including Carbon Copy(TM), pcANYWHERE(TM), and Windows NT RAS. All information is stored in the SQL database. It then generates a comprehensive report detailing which services were discovered on which dial-up numbers in your organization.

1.2.9 SQL Clients and Report Writers

urSQL
SQL Editor and Query Utility. Custom syntax highlighting, editable results grid, exportable result-sets, basic MySQL admin functions, Etc.. For Windows.
MySQL Data Manager
MySQL Data Manager * is platform independent web client (written in perl) for MySQL server over TCP/IP.
http://ksql.sourceforge.net/
KDE MySQL client.
http://www.ecker-software.de
A Windows GUI client by David Ecker.
http://www.icaap.org/software/kiosk/
Kiosk; a MySQL client for database management. Written in Perl. Will be a part of Bazaar.
http://www.casestudio.com/
Db design tool that supports MySQL 3.23.
http://home.skif.net/~voland/zeos/eng/index.html
Zeos - A client that supports MySQL, Interbase and PostgreSQL.
http://www.geocities.com/SiliconValley/Ridge/4280/GenericReportWriter/grwhome.html
A free report writer in Java
http://www.javaframework.de
MySQLExport - Export of MySQL create statements and data in a lot of different formats (SQL, HTML, CVS, text, ZIP, GZIP...)
http://dlabs.4t2.com
M2D, a MySQL Administration client for Windows. M2D supports administration of MySQL databases, creation of new databases and tables, editing, and more.
http://dlabs.4t2.com
Dexter, a small server written in Perl which can be used as a proxy server for MySQL or as a database extender.
http://www.scibit.com/Products/Software/Utils/Mascon.asp
Mascon is a powerful Win32 GUI for administering MySQL databases.
http://www.rtlabs.com/
MacSQL Monitor. GUI for MySQL, ODBC, and JDBC databases for the Mac OS.

1.2.10 Distributions that Include MySQL

http://www.suse.com/
SuSE Linux (6.1 and above)
http://www.redhat.com/
RedHat Linux (7.0 and above)
http://distro.conectiva.com.br
Conectiva Linux (4.0 and above)

1.2.11 Web Development Tools that Support MySQL

http://www.php.net/
PHP: A server-side HTML-embedded scripting language.
http://www.midgard-project.org
The Midgard Application Server; a powerful Web development environment based on MySQL and PHP.
http://www.smartworker.org
SmartWorker is a platform for Web application development.
http://xsp.lentus.se/
XSP: e(X)tendible (S)erver (P)ages and is a HTML embedded tag language written in Java (previously known as XTAGS.)
http://www.dbServ.de/
dbServ is an extension to a web server to integrate database output into your HTML code. You may use any HTML function in your output. Only the client will stop you. It works as standalone server or as Java servlet.
http://www.chilisoft.com/
Platform independent ASP from Chili!Soft
http://www.voicenet.com/~zellert/tjFM
A JDBC driver for MySQL.
http://www.wernhart.priv.at/php/
MySQL + PHP demos.
http://www.dbwww.com/
ForwardSQL: HTML interface to manipulate MySQL databases.
http://www.daa.com.au/~james/www-sql/
WWW-SQL: Display database information.
http://www.minivend.com/minivend/
Minivend: A Web shopping cart.
http://www.heitml.com/
HeiTML: A server-side extension of HTML and a 4GL language at the same time.
http://www.metahtml.com/
Metahtml: A Dynamic Programming Language for WWW Applications.
http://www.binevolve.com/
VelocityGen for Perl and Tcl.
http://hawkeye.net/
Hawkeye Internet Server Suite.
http://www.fastflow.com/
Network Database Connection For Linux
http://www.wdbi.net/
WDBI: Web browser as a universal front end to databases which supports MySQL well.
http://www.webgroove.com/
WebGroove Script: HTML compiler and server-side scripting language.
http://www.ihtml.com/
A server-side Web site scripting language.
ftp://ftp.igc.apc.org/pub/myodbc/README
How to use MySQL with ColdFusion on Solaris.
http://calistra.com/MySQL/
Calistra's ODBC MySQL Administrator.
http://www.webmerger.com
Webmerger - This CGI tool interprets files and generates dynamic output based on a set of simple tags. Ready-to-run drivers for MySQL and PostgreSQL through ODBC.
http://phpclub.net/
PHPclub - Tips and tricks for PHP.
http://www.penguinservices.com/scripts
MySQL and Perl Scripts.
http://www.widgetchuck.com
The Widgetchuck; Web Site Tools and Gadgets
http://www.adcycle.com/
AdCycle - advertising management software.
http://sourceforge.net/projects/pwpage/
pwPage - provides an extremely fast and simple approach to the creation of database forms. That is, if a database table exists and an HTML page has been constructed using a few simple guidelines, pwPage can be immediately used for table data selections, insertions, updates, deletions and selectable table content reviewing.
http://www.omnis-software.com/products/studio/studio.html
OMNIS Studio is a rapid application development (RAD) tool.
http://www.webplus.com
talentsoft Web+ 4.6 - a powerful and comprehensive development language for use in creating web-based client/server applications without writing complicated, low-level, and time-consuming CGI programs.

1.2.12 Database Design Tools with MySQL Support

http://www.mysql.com/documentation/dezign/
"DeZign for databases" is a database development tool that uses an entity relationship diagram (ERD).

1.2.13 Web Servers with MySQL Tools

ftp://ftp.kcilink.com/pub/
mod_auth_mysql, An Apache authentication module.
http://www.roxen.com/
The Roxen Challenger Web server.

1.2.14 Extensions for Other Programs

http://www.seawood.org/msql_bind/
MySQL support for BIND (The Internet Domain Name Server).
http://www.inet-interactive.com/sendmail/
MySQL support for Sendmail and Procmail.

1.2.15 Using MySQL with Other Programs

http://www.iserver.com/support/addonhelp/database/mysql/msaccess.html
Using MySQL with Access.
http://www.iserver.com/support/contrib/perl5/modules.html
Installing new Perl modules that require locally installed modules.

1.2.16 ODBC-related Links

http://www.iodbc.org/
Popular iODBC Driver Manager (libiodbc) now available as Open Source.
http://users.ids.net/~bjepson/freeODBC/
The FreeODBC Pages.
http://genix.net/unixODBC/
The unixODBC Project goals are to develop and promote unixODBC to be the definitive standard for ODBC on the Linux platform. This is to include GUI support for KDE.
http://www.sw-soft.com/products/BtrieveODBC/
A MySQL-based ODBC driver for Btrieve.

1.2.17 API-related Links

http://www.jppp.com/
Partially implemented TDataset-compatible components for MySQL.
http://www.riverstyx.net/qpopmysql/
qpopmysql - A patch to allow POP3 authentication from a MySQL database. There's also a link to Paul Khavkine's patch for Procmail to allow any MTA to deliver to users in a MySQL database.
http://www.pbc.ottawa.on.ca
Visual Basic class generator for Active X.
http://www.essencesw.com/Software/mysqllib.html
New Client libraries for the Mac OS Classic (Macintosh).
http://www.lilback.com/macsql/
Client libraries for the Macintosh.
http://www.essencesw.com/Plugins/mysqlplug.html
Plugin for REALbasic (for Macintosh)
http://www.iis.ee.ethz.ch/~neeri/macintosh/gusi-qa.html
A library that emulates BSD sockets and pthreads on Macintosh. This can be used if you want to compile the MySQL client library on Mac. It could probably even be sued to port MySQL to Macintosh, but we don't know of anyone that has tried that.
http://www.dedecker.net/jessie/scmdb/
SCMDB - an add-on for SCM that ports the MySQL C library to scheme (SCM). With this library scheme developers can make connections to a MySQL database and use embedded SQL in their programs.

1.2.18 Other MySQL-related Links

SAT
The Small Application Toolkit (SAT) is a collection of utilities intended to simplify the development of small, multi-user, GUI based applications in a (Microsoft -or- X) Windows Client / Unix Server environment.
http://www.wix.com/mysql-hosting/
Registry of Web providers who support MySQL.
http://www.softagency.co.jp/mysql/index.en.html
Links about using MySQL in Japan/Asia.
http://abattoir.cc.ndsu.nodak.edu/~nem/mysql/udf/
MySQL UDF Registry.
http://www.open.com.au/products.html
Commercial Web defect tracking system.
http://www.stonekeep.com/pts/
PTS: Project Tracking System.
http://tomato.nvgc.vt.edu/~hroberts/mot
Job and software tracking system.
http://www.cynergi.net/exportsql/
ExportSQL: A script to export data from Access95+.
http://SAL.KachinaTech.COM/H/1/MYSQL.html
SAL (Scientific Applications on Linux) MySQL entry.
http://www.infotech-nj.com/itech/index.shtml
A consulting company which mentions MySQL in the right company.
http://www.pmpcs.com/
PMP Computer Solutions. Database developers using MySQL and mSQL.
http://www.aewa.org/
Airborne Early Warning Association.
http://www.dedserius.com/y2kmatrix/
Y2K tester.

1.2.19 SQL and Database Interfaces

http://java.sun.com/products/jdbc/
The JDBC database access API.
http://www.gagme.com/mysql
Patch for mSQL Tcl.
http://www.amsoft.ru/easysql/
EasySQL: An ODBC-like driver manager.
http://www.lightlink.com/hessling/rexxsql.html
A REXX interface to SQL databases.
http://www.mytcl.cx/
Tcl interface based on tcl-sql with many bugfixes.
http://www.binevolve.com/~tdarugar/tcl-sql/
Tcl interface.
http://www.contrib.andrew.cmu.edu/~shadow/sql.html
SQL Reference Page with a lot of interesting links.

1.2.20 Examples of MySQL Use

http://www.little6.com/about/linux/
Little6 Inc., An online contract and job finding site that is powered by MySQL, PHP3, and Linux.
http://www.delec.com/is/products/prep/examples/BookShelf/index.html
DELECis - A tool that makes it very easy to create an automatically generated table documentation. They have used MySQL as an example.
http://www.worldrecords.com
World Records - A search engine for information about music that uses MySQL and PHP.
http://www.webtechniques.com/archives/1998/01/note/
A Contact Database using MySQL and PHP.
http://modems.rosenet.net/mysql/
Web based interface and Community Calendar with PHP.
http://www.odbsoft.com/cook/sources.htm
Perl package to generate html from a SQL table structure and for generating SQL statements from an html form.
http://www.gusnet.cx/proj/telsql/
Basic telephone database using DBI/DBD.
http://tecfa.unige.ch/guides/java/staf2x/ex/jdbc/coffee-break
JDBC examples by Daniel K. Schneider.
http://www.spade.com/linux/howto/PostgreSQL-HOWTO-41.html
SQL BNF
http://www.ooc.com/
Object Oriented Concepts Inc; CORBA applications with examples in source.
http://www.pbc.ottawa.on.ca/
DBWiz; Includes an example of how to manage cursors in VB.
http://keilor.cs.umass.edu/pluribus/
Pluribus is a free search engine that learns to improve the quality of its results over time. Pluribus works by recording which pages a user prefers among those returned for a query. A user votes for a page by selecting it; Pluribus then uses that knowledge to improve the quality of the results when someone else submits the same (or similar) query. Uses PHP and MySQL.
http://www.stopbit.com/
Stopbit - A technology news site using MySQL and PHP.
http://www.linuxsupportline.com/~kalendar/
KDE based calendar manager - The calendar manager has both single user (file based) and multi-user (MySQL database) support.
http://tim.desert.net/~tim/imger/
Example of storing/retrieving images with MySQL and CGI.
http://www.penguinservices.com/scripts
Online shopping cart system.
http://www.city-gallery.com/album/
Old Photo Album - The album is a collaborative popular history of photography project that generates all pages from data stored in a MySQL database. Pages are dynamically generated through a php3 interface to the database content. Users contribute images and descriptions. Contributed images are stored on the web server to avoid storing them in the database as BLOBs. All other information is stored on the shared MySQL server.

1.2.21 General Database Links

http://www.pcslink.com/~ej/dbweb.html
Database Jump Site
http://black.hole-in-the.net/guy/webdb/
Homepage of the webdb-l (Web Databases) mailing list.
http://www.symbolstone.org/technology/perl/DBI/index.html
Perl DBI/DBD modules homepage.
http://www.student.uni-koeln.de/cygwin/
Cygwin tools. Unix on top of Windows.
http://dbasecentral.com/
dbasecentral.com; Development and distribution of powerful and easy-to-use database applications and systems.
http://www.tek-tips.com/
Tek-Tips Forums are 800+ independent peer-to-peer non-commercial support forums for Computer Professionals. Features include automatic e-mail notification of responses, a links library, and member confidentiality guaranteed.
http://www.public.asu.edu/~peterjn/btree/
B-Trees: Balanced Tree Data Structures.
http://www.fit.qut.edu.au/~maire/baobab/lecture/sld001.htm
A lecture about B-Trees.

There are also many Web pages that use MySQL. See section B Some MySQL Users. Send any additions to this list to webmaster@mysql.com. We now require that you show a MySQL logo somewhere if you wish your site to be added. It is okay to have it on a ``used tools'' page or something similar.

1.2.22 MySQL Mailing Lists

This section introduces you to the MySQL mailing lists, and gives some guidelines as to how to use them.

1.2.22.1 The MySQL Mailing Lists

To subscribe to the main MySQL mailing list, send a message to the electronic mail address mysql-subscribe@lists.mysql.com.

To unsubscribe from the main MySQL mailing list, send a message to the electronic mail address mysql-unsubscribe@lists.mysql.com.

Only the address to which you send your messages is significant. The subject line and the body of the message are ignored.

If your reply address is not valid, you can specify your address explicitly. Adding a hyphen to the subscribe or unsubscribe command word, followed by your address with the `@' character in your address replaced by a `='. For example, to subscribe your_name@host.domain, send a message to mysql-subscribe-your_name=host.domain@lists.mysql.com.

Mail to mysql-subscribe@lists.mysql.com or mysql-unsubscribe@lists.mysql.com is handled automatically by the ezmlm mailing list processor. Information about ezmlm is available at The ezmlm Website.

To post a message to the list itself, send your message to mysql@lists.mysql.com. However, please do not send mail about subscribing or unsubscribing to mysql@lists.mysql.com, because any mail sent to that address is distributed automatically to thousands of other users.

Your local site may have many subscribers to mysql@lists.mysql.com. If so, it may have a local mailing list, so that messages sent from lists.mysql.com to your site are propagated to the local list. In such cases, please contact your system administrator to be added to or dropped from the local MySQL list.

If you wish to have traffic for a mailing list go to a separate mailbox in your mail program, set up a filter based on the message headers. You can use either the List-ID: or Delivered-To: headers to identify list messages.

The following MySQL mailing lists exist:

announce-subscribe@lists.mysql.com announce
This is for announcement of new versions of MySQL and related programs. This is a low volume list all MySQL users should subscribe to.
mysql-subscribe@lists.mysql.com mysql
The main list for general MySQL discussion. Please note that some topics are better discussed on the more-specialized lists. If you post to the wrong list, you may not get an answer!
mysql-digest-subscribe@lists.mysql.com mysql-digest
The mysql list in digest form. That means you get all individual messages, sent as one large mail message once a day.
bugs-subscribe@lists.mysql.com bugs
On this list you should only post a full, repeatable bug report using the mysqlbug script (if you are running on Windows, you should include a description of the operating system and the MySQL version). Preferably, you should test the problem using the latest stable or development version of MySQL before posting! Anyone should be able to repeat the bug by just using mysql test < script on the included test case. All bugs posted on this list will be corrected or documented in the next MySQL release! If there are only small code changes involved, we will also post a patch that fixes the problem.
bugs-digest-subscribe@lists.mysql.com bugs-digest
The bugs list in digest form.
internals-subscribe@lists.mysql.com internals
A list for people who work on the MySQL code. On this list one can also discuss MySQL development and post patches.
internals-digest-subscribe@lists.mysql.com internals-digest
A digest version of the internals list.
java-subscribe@lists.mysql.com java
Discussion about MySQL and Java. Mostly about the JDBC drivers.
java-digest-subscribe@lists.mysql.com java-digest
A digest version of the java list.
win32-subscribe@lists.mysql.com win32
All things concerning MySQL on Microsoft operating systems such as Win95, Win98, NT, and Win2000.
win32-digest-subscribe@lists.mysql.com win32-digest
A digest version of the win32 list.
myodbc-subscribe@lists.mysql.com myodbc
All things about connecting to MySQL with ODBC.
myodbc-digest-subscribe@lists.mysql.com myodbc-digest
A digest version of the myodbc list.
plusplus-subscribe@lists.mysql.com plusplus
All things concerning programming with the C++ API to MySQL.
plusplus-digest-subscribe@lists.mysql.com plusplus-digest
A digest version of the plusplus list.
msql-mysql-modules-subscribe@lists.mysql.com msql-mysql-modules
A list about the Perl support in MySQL. msql-mysql-modules
msql-mysql-modules-digest-subscribe@lists.mysql.com msql-mysql-modules-digest
A digest version of the msql-mysql-modules list.

You subscribe or unsubscribe to all lists in the same way as described above. In your subscribe or unsubscribe message, just put the appropriate mailing list name rather than mysql. For example, to subscribe to or unsubscribe from the myodbc list, send a message to myodbc-subscribe@lists.mysql.com or myodbc-unsubscribe@lists.mysql.com.

If you can't get an answer for your questions from the mailing list, one option is to pay for support from MySQL AB, which will put you in direct contact with MySQL developers. See section 1.3.5 Types of Commercial Support.

The following table shows some MySQL mailing in other languages than English. Note that these are not operated by MySQL AB, so we can't guarantee the quality on these.

mysql-france-subscribe@yahoogroups.com A French mailing list
list@tinc.net A Korean mailing list
Email subscribe mysql your@email.address to this list.
mysql-de-request@lists.4t2.com A German mailing list
Email subscribe mysql-de your@email.address to this list. You can find information about this mailing list at http://www.4t2.com/mysql.
mysql-br-request@listas.linkway.com.br A Portugese mailing list
Email subscribe mysql-br your@email.address to this list.
mysql-alta@elistas.net A Spanish mailing list
Email subscribe mysql your@email.address to this list.

1.2.22.2 Asking Questions or Reporting Bugs

Before posting a bug report or question, please do the following:

If you can't find an answer in the manual or the archives, check with your local MySQL expert. If you still can't find an answer to your question, go ahead and read the next section about how to send mail to mysql@lists.mysql.com.

1.2.22.3 How to Report Bugs or Problems

Writing a good bug report takes patience, but doing it right the first time saves time for us and for you. A good bug report containing a full test case for the bug will make it very likely that we will fix it in the next release. This section will help you write your report correctly so that you don't waste your time doing things that may not help us much or at all.

We encourage everyone to use the mysqlbug script to generate a bug report (or a report about any problem), if possible. mysqlbug can be found in the `scripts' directory in the source distribution, or, for a binary distribution, in the `bin' directory under your MySQL installation directory. If you are unable to use mysqlbug, you should still include all the necessary information listed in this section.

The mysqlbug script helps you generate a report by determining much of the following information automatically, but if something important is missing, please include it with your message! Please read this section carefully and make sure that all the information described here is included in your report.

The normal place to report bugs and problems is mysql@lists.mysql.com. If you can make a test case that clearly demonstrates the bug, you should post it to the bugs@lists.mysql.com list. Note that on this list you should only post a full, repeatable bug report using the mysqlbug script. If you are running on Windows, you should include a description of the operating system and the MySQL version. Preferably, you should test the problem using the latest stable or development version of MySQL before posting! Anyone should be able to repeat the bug by just using ``mysql test < script'' on the included test case or run the shell or perl script that is included in the bug report. All bugs posted on the bugs list will be corrected or documented in the next MySQL release! If there are only small code changes involved to correct this problem, we will also post a patch that fixes the problem.

Remember that it is possible to respond to a message containing too much information, but not to one containing too little. Often people omit facts because they think they know the cause of a problem and assume that some details don't matter. A good principle is: if you are in doubt about stating something, state it! It is a thousand times faster and less troublesome to write a couple of lines more in your report than to be forced to ask again and wait for the answer because you didn't include enough information the first time.

The most common errors are that people don't indicate the version number of the MySQL distribution they are using, or don't indicate what platform they have MySQL installed on (including the platform version number). This is highly relevant information, and in 99 cases out of 100 the bug report is useless without it! Very often we get questions like, ``Why doesn't this work for me?'' then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has been fixed already in newer MySQL versions. Sometimes the error is platform dependent; in such cases, it is next to impossible to fix anything without knowing the operating system and the version number of the platform.

Remember also to provide information about your compiler, if it is related to the problem. Often people find bugs in compilers and think the problem is MySQL-related. Most compilers are under development all the time and become better version by version. To determine whether or not your problem depends on your compiler, we need to know what compiler is used. Note that every compiling problem should be regarded as a bug report and reported accordingly.

It is most helpful when a good description of the problem is included in the bug report. That is, a good example of all the things you did that led to the problem and the problem itself exactly described. The best reports are those that include a full example showing how to reproduce the bug or problem. See section G.1.6 Making a test case when you experience table corruption.

If a program produces an error message, it is very important to include the message in your report! If we try to search for something from the archives using programs, it is better that the error message reported exactly matches the one that the program produces. (Even the case should be observed!) You should never try to remember what the error message was; instead, copy and paste the entire message into your report!

If you have a problem with MyODBC, you should try to generate a MyODBC trace file. See section 8.3.7 Reporting Problems with MyODBC.

Please remember that many of the people who will read your report will do so using an 80-column display. When generating reports or examples using the mysql command line tool, you should therefore use the --vertical option (or the \G statement terminator) for output that would exceed the available width for such a display (for example, with the EXPLAIN SELECT statement; see the example below).

Please include the following information in your report:

If you are a support customer, please cross-post the bug report to mysql-support@mysql.com for higher priority treatment, as well as to the appropriate mailing list to see if someone else has experienced (and perhaps solved) the problem.

For information on reporting bugs in MyODBC, see section 8.3.4 How to Report Problems with MyODBC.

For solutions to some common problems, see See section A Problems and Common Errors.

When answers are sent to you individually and not to the mailing list, it is considered good etiquette to summarize the answers and send the summary to the mailing list so that others may have the benefit of responses you received that helped you solve your problem!

1.2.22.4 Guidelines for Answering Question on the Mailing List

If you consider your answer to have broad interest, you may want to post it to the mailing list instead of replying directly to the individual who asked. Try to make your answer general enough that people other than the original poster may benefit from it. When you post to the list, please make sure that your answer is not a duplication of a previous answer.

Try to summarize the essential part of the question in your reply; don't feel obliged to quote the entire original message.

Please don't post mail messages from your browser with HTML mode turned on! Many users don't read mail with a browser!

1.3 MySQL Licensing and Support

This section describes MySQL support and licensing arrangements:

1.3.1 MySQL Licensing Policy

The formal terms of the GPL license can be found at section K GNU GENERAL PUBLIC LICENSE. Basically, our licensing policy and interpretation of the GPL is as follows:

Note that older versions of MySQL are still using a more strict license. See the documentation for that version for more information. If you need a commercial MySQL license, because the GPL license doesn't suit your application, you can buy one at https://order.mysql.com/.

For normal internal use, MySQL costs nothing. You do not have to pay us if you do not want to.

A license is required if:

A license is NOT required if:

For circumstances under which a MySQL license is required, you need a license per machine that runs the mysqld server. However, a multiple-CPU machine counts as a single machine, and there is no restriction on the number of MySQL servers that run on one machine, or on the number of clients concurrently connected to a server running on that machine!

If you have any questions as to whether or not a license is required for your particular use of MySQL, please read this again and then contact us. See section 1.3.4.2 Contact Information.

If you require a MySQL license, the easiest way to pay for it is to use the license form on MySQL's secure server at https://order.mysql.com/. Other forms of payment are discussed in section 1.3.4.1 Payment information.

1.3.2 Copyrights Used by MySQL

There are several different copyrights on the MySQL distribution:

  1. The MySQL-specific source needed to build the mysqlclient library is licensed under the LGPL and programs in the `client' directory is GPL. Each file has a header that shows which copyright is used for that file.
  2. The client library and the (GNU getopt) library are covered by the ``GNU LIBRARY GENERAL PUBLIC LICENSE.'' See section L GNU LESSER GENERAL PUBLIC LICENSE.
  3. Some parts of the source (the regexp library) are covered by a Berkeley-style copyright.
  4. All the source in the server and the (GNU readline) library is covered by the ``GNU GENERAL PUBLIC LICENSE.'' See section K GNU GENERAL PUBLIC LICENSE. This is also available as the file `COPYING' in the distributions.

One goal is that the SQL client library should be free enough that it is possible to add MySQL support into commercial products without a license. For this reason, we chose the LGPL license for the client code.

This means that you can use MySQL for free with any program that uses any of the free software licenses. MySQL is also free for any end user for his own or company usage.

However, if you use MySQL for something important to you, you may want to help secure its development by purchasing licenses or a support contract. See section 1.3.5 Types of Commercial Support.

1.3.2.1 Copyright Changes

Version 3.22 of MySQL is still using a more strict license. See the documentation for that version for more information.

1.3.3 Example Licensing Situations

This section describes some situations illustrating whether or not you must license the MySQL server. Generally these examples involve providing MySQL as an integral part of a product.

Note that a single MySQL license covers any number of CPUs and mysqld servers on a machine! There is no artificial limit on the number of clients that connect to the server in any way.

1.3.3.1 Selling Products that use MySQL

To determine whether or not you need a MySQL license when selling your application, you should ask whether the proper functioning of your application is dependent on the use of MySQL and whether you include the MySQL server with your product. There are several cases to consider:

1.3.3.2 ISP MySQL Services

Internet Service Providers (ISPs) often host MySQL servers for their customers. With the GPL license this does not require a license.

On the other hand, we do encourage people to use ISPs that have MySQL support, as this will give them the confidence that if they have some problem with their MySQL installation, their ISP will be able to solve the problem for them (in some cases with the help from the MySQL development team).

All ISPs that want to keep themselves up-to-date should subscribe to our announce mailing list so that they can be aware of fatal issues that may be relevant for their MySQL installations.

Note that if the ISP doesn't have a license for MySQL, it should give its customers at least read access to the source of the MySQL installation so that its customer can verify that it is patched correctly.

1.3.3.3 Running a Web Server Using MySQL

If you use MySQL in conjunction with a Web server on Unix, you don't have to pay for a license.

This is true even if you run a commercial Web server that uses MySQL, because you are not selling an embedded MySQL version yourself. However, in this case we would like you to purchase MySQL support, because MySQL is helping your enterprise.

1.3.4 MySQL Licensing and Support Costs

Our current license prices are shown below. To make a purchase, please visit https://order.mysql.com/.

If you pay by credit card, the currency is EURO (European Union Euro) so the prices will differ slightly.

Number of licenses Per copy
1-9 230 EURO
10-24 138 EURO
25-49 117 EURO
50-99 102 EURO
100-249 91 EURO
250-499 76 EURO
500-999 66 EURO

For high volume (OEM) purchases, please contact sales@mysql.com.

For OEM purchases, you must act as the middle-man for eventual problems or extension requests from your users. We also require that OEM customers have at least an extended e-mail support contract. Note that OEM licenses only apply for products where the user doesn't have direct access to the MySQL server (embedded system). In other words, the MySQL server should only be used with the application that was supplied you.

If you have a low-margin, high-volume product, you can always talk to us about other terms (for example, a percent of the sale price). If you do, please be informative about your product, pricing, market, and any other information that may be relevant.

A full-price license is not a support agreement and includes very minimal support. This means that we try to answer any relevant questions. If the answer is in the documentation, we will direct you to the appropriate section. If you have not purchased a license or support, we probably will not answer at all.

If you discover what we consider a real bug, we are likely to fix it in any case. But if you pay for support we will notify you about the fix status instead of just fixing it in a later release.

More comprehensive support is sold separately. Descriptions of what each level of support includes are given in section 1.3.5 Types of Commercial Support. Costs for the various types of commercial support are shown below. Support level prices are in EURO (European Union Euro). One EURO is about 1.06 USD.

Type of support Cost per year
Basic e-mail support. See section 1.3.5.1 Basic E-mail Support. EURO 200
Extended e-mail support See section 1.3.5.2 Extended E-mail Support. EURO 1000
Login support See section 1.3.5.3 Login Support. EURO 2000
Extended login support See section 1.3.5.4 Extended Login Support. EURO 5000
Telephone support See section 1.3.5.5 Telephone Support. EURO 12000

You may upgrade from any lower level of support to a higher level of support for the difference in price between the two support levels.

We do also provide telephone support (mostly emergency support but also 24/7 support). This support option doesn't however have a fixed price but is negotiated for case to case. If you are interested in this option you can email sales@mysql.com and tell us about your needs.

Note that as our sales staff is very busy, it may take some time until your request is handled. Our support staff does however always answer promptly to support questions!

1.3.4.1 Payment information

Currently we can take SWIFT payments, checks, or credit cards.

Payment should be made to: 

Postgirot Bank AB
105 06 STOCKHOLM, SWEDEN

MySQL AB
BOX 6434
11382 STOCKHOLM, SWEDEN

SWIFT address: PGSI SESS
Account number: 96 77 06 - 3

Specify: license and/or support and your name and e-mail address.

In Europe and Japan you can use EuroGiro (that should be less expensive) to the same account.

If you want to pay by check, make it payable to ``MySQL Finland AB'' and mail it to the address below: 

MySQL AB
BOX 6434, Torsgatan 21
11382 STOCKHOLM, SWEDEN

If you want to pay by credit card over the Internet, you can use MySQL AB's secure license form.

You can also print a copy of the license form, fill it in, and send it by fax to:

+46-8-729 69 05

If you want us to bill you, you can use the license form and write ``bill us'' in the comment field. You can also mail a message to sales@mysql.com (not mysql@lists.mysql.com!) with your company information and ask us to bill you.

1.3.4.2 Contact Information

For commercial licensing, please contact the MySQL licensing team. The much preferred method is by e-mail to licensing@mysql.com. Fax is also possible but handling of these may take much longer (Fax +46-8-729 69 05).

If you represent a business that is interested in partnering with MySQL, please send e-mail to partner@mysql.com.

For timely, precise answers to technical questions about MySQL you should order one of our support contracts. MySQL support is provided by the MySQL developers so the standard is extremely high.

If you are interested in placing a banner advertisement on our Web site, please send e-mail to advertising@mysql.com.

If you are interested in any of the jobs listed in our jobs section, please send e-mail to jobs@mysql.com.

For general discussion amongst our many users, please direct your attention to the appropriate mailing list.

For general information inquires, please send e-mail to info@mysql.com.

For questions or comments about the workings or content of the Web site, please send e-mail to webmaster@mysql.com.

1.3.5 Types of Commercial Support

The following is true of all support options:

1.3.5.1 Basic E-mail Support

Basic e-mail support is a very inexpensive support option and should be thought of more as a way to support our development of MySQL than as a real support option. We at MySQL do give a lot of free support in all the different MySQL lists, and the money we get from basic e-mail support is largely used to make this possible.

At this support level, the MySQL mailing lists are the preferred means of communication. Questions normally should be mailed to the primary mailing list (mysql@lists.mysql.com) or one of the other regular lists (for example, win32@lists.mysql.com for Windows-related MySQL questions), as someone else already may have experienced and solved the problem you have. See section 1.2.22.2 Asking Questions or Reporting Bugs.

However, by purchasing basic e-mail support, you also have access to the support address mysql-support@mysql.com, which is not available as part of the minimal support that you get by purchasing a MySQL license. This means that for especially critical questions, you can cross-post your message to mysql-support@mysql.com. (If the message contains sensitive data, you should post only to mysql-support@mysql.com.)

REMEMBER! to ALWAYS include your registration number and expiration date when you send a message to mysql-support@mysql.com.

Note that if you have encountered a critical, repeatable bug, and follow the rules outlined in the manual section of how to report bugs and send it to bugs@lists.mysql.com, we promise to try to fix this as soon as possible, regardless of your support level! See section 1.2.22.3 How to Report Bugs or Problems.

Basic e-mail support includes the following types of service:

1.3.5.2 Extended E-mail Support

Extended e-mail support includes everything in basic e-mail support with these additions:

1.3.5.3 Login Support

Login support includes everything in extended e-mail support with these additions:

1.3.5.4 Extended Login Support

Extended login support includes everything in login support with these additions:

1.3.5.5 Telephone Support

Telephone support includes everything in extended login support with these additions:

1.3.5.6 Support for other table handlers

To get support for BDB tables, InnoDB tables you have to pay an additional 30% on the standard support price for each of the table handlers you would like to have support for.

We at MySQL AB will help you create a proper bug report for the table handler and submit it to the developers for the specific table handler. We will also do our best to ensure that you will get a timely answer or solution from the developers of the table handler.

Even if we are quite confident that we can solve most problems within a timely manner, we can't guarantee a quick solution for any problems you can get with the different table handlers. We will however do our best to help you get the problem solved.

1.4 How Standards-compatible Is MySQL?

This section describes how MySQL relates to the ANSI SQL standards. MySQL has many extensions to the ANSI SQL standards, and here you will find out what they are, and how to use them. You will also find information about functionality missing from MySQL, and how to work around some differences.

1.4.1 MySQL Extensions to ANSI SQL92

MySQL includes some extensions that you probably will not find in other SQL databases. Be warned that if you use them, your code will not be portable to other SQL servers. In some cases, you can write code that includes MySQL extensions, but is still portable, by using comments of the form /*! ... */. In this case, MySQL will parse and execute the code within the comment as it would any other MySQL statement, but other SQL servers will ignore the extensions. For example: 

SELECT /*! STRAIGHT_JOIN */ col_name FROM table1,table2 WHERE ...

If you add a version number after the '!', the syntax will only be executed if the MySQL version is equal to or newer than the used version number: 

CREATE /*!32302 TEMPORARY */ TABLE (a int);

The above means that if you have Version 3.23.02 or newer, then MySQL will use the TEMPORARY keyword.

MySQL extensions are listed below:

1.4.2 MySQL Differences Compared to ANSI SQL92

We try to make MySQL follow the ANSI SQL standard and the ODBC SQL standard, but in some cases MySQL does some things differently:

1.4.3 Running MySQL in ANSI Mode

If you start mysqld with the --ansi option, the following behavior of MySQL changes:

This is the same as using --sql-mode=REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,SERIALIZE,ONLY_FULL_GROUP_BY.

1.4.4 Functionality Missing from MySQL

The following functionality is missing in the current version of MySQL. For a prioritized list indicating when new extensions may be added to MySQL, you should consult the online MySQL TODO list. That is the latest version of the TODO list in this manual. See section 1.6 MySQL and the future (The TODO).

1.4.4.1 Sub-selects

MySQL currently only supports sub selects of the form INSERT ... SELECT ... and REPLACE ... SELECT .... You can however use the function IN() in other contexts.

In many cases you can rewrite the query without a sub-select: 

SELECT * FROM table1 WHERE id IN (SELECT id FROM table2);

This can be re-written as: 

SELECT table1.* FROM table1,table2 WHERE table1.id=table2.id;

The queries: 

SELECT * FROM table1 WHERE id NOT IN (SELECT id FROM table2);
SELECT * FROM table1 WHERE NOT EXISTS (SELECT id FROM table2 where table1.id=table2.id);

Can be rewritten as: 

SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id where table2.id IS NULL

For more complicated subqueries you can often create temporary tables to hold the subquery. In some cases, however this option will not work. The most frequently encountered of these cases arises with DELETE statements, for which standard SQL does not support joins (except in sub-selects). For this situation there are two options available until subqueries are supported by MySQL.

The first option is to use a procedural programming language (such as Perl or PHP) to submit a SELECT query to obtain the primary keys for the records to be deleted, and then use these values to construct the DELETE statement (DELETE FROM ... WHERE ... IN (key1, key2, ...)).

The second option is to use interactive SQL to contruct a set of DELETE statements automatically, using the MySQL extension CONCAT() (in lieu of the standard || operator). For example: 

SELECT CONCAT('DELETE FROM tab1 WHERE pkid = ', tab1.pkid, ';')
  FROM tab1, tab2
 WHERE tab1.col1 = tab2.col2;

You can place this query in a script file and redirect input from it to the mysql command-line interpreter, piping its output back to a second instance of the interpreter: 

prompt> mysql --skip-column-names mydb < myscript.sql | mysql mydb

MySQL 4.0 supports multi-table deletes that can be used to efficiently delete rows based on information from one table or even from many tables at the same time.

1.4.4.2 SELECT INTO TABLE

MySQL doesn't yet support the Oracle SQL extension: SELECT ... INTO TABLE .... MySQL supports instead the ANSI SQL syntax INSERT INTO ... SELECT ..., which is basically the same thing. See section 6.4.2.1 INSERT ... SELECT Syntax. 

INSERT INTO tblTemp2 (fldID) SELECT tblTemp1.fldOrder_ID FROM tblTemp1 WHERE
tblTemp1.fldOrder_ID > 100;

Alternatively, you can use SELECT INTO OUTFILE... or CREATE TABLE ... SELECT to solve your problem.

1.4.4.3 Transactions

As MySQL does nowadays support transactions, the following discussion is only valid if you are only using the non-transaction-safe table types. See section 6.7.1 BEGIN/COMMIT/ROLLBACK Syntax.

The question is often asked, by the curious and the critical, ``Why is MySQL not a transactional database?'' or ``Why does MySQL not support transactions?''

MySQL has made a conscious decision to support another paradigm for data integrity, ``atomic operations.'' It is our thinking and experience that atomic operations offer equal or even better integrity with much better performance. We, nonetheless, appreciate and understand the transactional database paradigm and plan, within the next few releases, to introduce transaction-safe tables on a per table basis. We will be giving our users the possibility to decide if they need the speed of atomic operations or if they need to use transactional features in their applications.

How does one use the features of MySQL to maintain rigorous integrity and how do these features compare with the transactional paradigm?

First, in the transactional paradigm, if your applications are written in a way that is dependent on the calling of ``rollback'' instead of ``commit'' in critical situations, then transactions are more convenient. Moreover, transactions ensure that unfinished updates or corrupting activities are not committed to the database; the server is given the opportunity to do an automatic rollback and your database is saved.

MySQL, in almost all cases, allows you to solve for potential problems by including simple checks before updates and by running simple scripts that check the databases for inconsistencies and automatically repair or warn if such occurs. Note that just by using the MySQL log or even adding one extra log, one can normally fix tables perfectly with no data integrity loss.

Moreover, fatal transactional updates can be rewritten to be atomic. In fact,we will go so far as to say that all integrity problems that transactions solve can be done with LOCK TABLES or atomic updates, ensuring that you never will get an automatic abort from the database, which is a common problem with transactional databases.

Not even transactions can prevent all loss if the server goes down. In such cases even a transactional system can lose data. The difference between different systems lies in just how small the time-lap is where they could lose data. No system is 100% secure, only ``secure enough.'' Even Oracle, reputed to be the safest of transactional databases, is reported to sometimes lose data in such situations.

To be safe with MySQL, you only need to have backups and have the update logging turned on. With this you can recover from any situation that you could with any transactional database. It is, of course, always good to have backups, independent of which database you use.

The transactional paradigm has its benefits and its drawbacks. Many users and application developers depend on the ease with which they can code around problems where an abort appears to be, or is necessary, and they may have to do a little more work with MySQL to either think differently or write more. If you are new to the atomic operations paradigm, or more familiar or more comfortable with transactions, do not jump to the conclusion that MySQL has not addressed these issues. Reliability and integrity are foremost in our minds. Recent estimates indicate that there are more than 1,000,000 mysqld servers currently running, many of which are in production environments. We hear very, very seldom from our users that they have lost any data, and in almost all of those cases user error is involved. This is, in our opinion, the best proof of MySQL's stability and reliability.

Lastly, in situations where integrity is of highest importance, MySQL's current features allow for transaction-level or better reliability and integrity. If you lock tables with LOCK TABLES, all updates will stall until any integrity checks are made. If you only obtain a read lock (as opposed to a write lock), then reads and inserts are still allowed to happen. The new inserted records will not be seen by any of the clients that have a READ lock until they release their read locks. With INSERT DELAYED you can queue inserts into a local queue, until the locks are released, without having the client wait for the insert to complete. See section 6.4.3 INSERT DELAYED syntax.

``Atomic,'' in the sense that we mean it, is nothing magical. It only means that you can be sure that while each specific update is running, no other user can interfere with it, and there will never be an automatic rollback (which can happen on transaction based systems if you are not very careful). MySQL also guarantees that there will not be any dirty reads. You can find some example of how to write atomic updates in the commit-rollback section. See section 1.4.6 How to Cope Without COMMIT/ROLLBACK.

We have thought quite a bit about integrity and performance, and we believe that our atomic operations paradigm allows for both high reliability and extremely high performance, on the order of three to five times the speed of the fastest and most optimally tuned of transactional databases. We didn't leave out transactions because they are hard to do. The main reason we went with atomic operations as opposed to transactions is that by doing this we could apply many speed optimizations that would not otherwise have been possible.

Many of our users who have speed foremost in their minds are not at all concerned about transactions. For them transactions are not an issue. For those of our users who are concerned with or have wondered about transactions vis-a-vis MySQL, there is a ``MySQL way'' as we have outlined above. For those where safety is more important than speed, we recommend them to use the BDB, or InnoDB tables for all their critical data. See section 7 MySQL Table Types.

One final note: We are currently working on a safe replication schema that we believe to be better than any commercial replication system we know of. This system will work most reliably under the atomic operations, non-transactional, paradigm. Stay tuned.

1.4.4.4 Stored Procedures and Triggers

A stored procedure is a set of SQL commands that can be compiled and stored in the server. Once this has been done, clients don't need to keep reissuing the entire query but can refer to the stored procedure. This provides better performance because the query has to be parsed only once, and less information needs to be sent between the server and the client. You can also raise the conceptual level by having libraries of functions in the server.

A trigger is a stored procedure that is invoked when a particular event occurs. For example, you can install a stored procedure that is triggered each time a record is deleted from a transaction table and that automatically deletes the corresponding customer from a customer table when all his transactions are deleted.

The planned update language will be able to handle stored procedures, but without triggers. Triggers usually slow down everything, even queries for which they are not needed.

To see when MySQL might get stored procedures, see section 1.6 MySQL and the future (The TODO).

1.4.4.5 Foreign Keys

Note that foreign keys in SQL are not used to join tables, but are used mostly for checking referential integrity (foreign key constraints). If you want to get results from multiple tables from a SELECT statement, you do this by joining tables: 

SELECT * from table1,table2 where table1.id = table2.id;

See section 6.4.1.1 JOIN Syntax. See section 3.5.6 Using Foreign Keys.

The FOREIGN KEY syntax in MySQL exists only for compatibility with other SQL vendors' CREATE TABLE commands; it doesn't do anything. The FOREIGN KEY syntax without ON DELETE ... is mostly used for documentation purposes. Some ODBC applications may use this to produce automatic WHERE clauses, but this is usually easy to override. FOREIGN KEY is sometimes used as a constraint check, but this check is unnecessary in practice if rows are inserted into the tables in the right order. MySQL only supports these clauses because some applications require them to exist (regardless of whether or not they work).

In MySQL, you can work around the problem of ON DELETE ... not being implemented by adding the appropriate DELETE statement to an application when you delete records from a table that has a foreign key. In practice this is as quick (in some cases quicker) and much more portable than using foreign keys.

In the near future we will extend the FOREIGN KEY implementation so that at least the information will be saved in the table specification file and may be retrieved by mysqldump and ODBC. At a later stage we will implement the foreign key constraints for application that can't easily be coded to avoid them.

1.4.4.6 Reasons NOT to Use Foreign Keys constraints

There are so many problems with foreign key constraints that we don't know where to start:

The only nice aspect of FOREIGN KEY is that it gives ODBC and some other client programs the ability to see how a table is connected and to use this to show connection diagrams and to help in building applications.

MySQL will soon store FOREIGN KEY definitions so that a client can ask for and receive an answer about how the original connection was made. The current `.frm' file format does not have any place for it. At a later stage we will implement the foreign key constraints for application that can't easily be coded to avoid them.

1.4.4.7 Views

MySQL doesn't yet support views, but we plan to implement these to about 4.1.

Views are mostly useful for letting users access a set of relations as one table (in read-only mode). Many SQL databases don't allow one to update any rows in a view, but you have to do the updates in the separate tables.

As MySQL is mostly used in applications and on web system where the application writer has full control on the database usage, most of our users haven't regarded views to be very important. (At least no one has been interested enough in this to be prepared to finance the implementation of views).

One doesn't need views in MySQL to restrict access to columns as MySQL has a very sophisticated privilege system. See section 4.2 General Security Issues and the MySQL Access Privilege System.

1.4.4.8 `--' as the Start of a Comment

Some other SQL databases use `--' to start comments. MySQL has `#' as the start comment character, even if the mysql command-line tool removes all lines that start with `--'. You can also use the C comment style /* this is a comment */ with MySQL. See section 6.1.5 Comment Syntax.

MySQL Version 3.23.3 and above supports the `--' comment style only if the comment is followed by a space. This is because this degenerate comment style has caused many problems with automatically generated SQL queries that have used something like the following code, where we automatically insert the value of the payment for !payment!: 

UPDATE tbl_name SET credit=credit-!payment!

What do you think will happen when the value of payment is negative?

Because 1--1 is legal in SQL, we think it is terrible that `--' means start comment.

In MySQL Version 3.23 you can, however, use: 1-- This is a comment

The following discussion only concerns you if you are running a MySQL version earlier than Version 3.23:

If you have a SQL program in a text file that contains `--' comments you should use: 

shell> replace " --" " #" < text-file-with-funny-comments.sql \
         | mysql database

instead of the usual: 

shell> mysql database < text-file-with-funny-comments.sql

You can also edit the command file ``in place'' to change the `--' comments to `#' comments: 

shell> replace " --" " #" -- text-file-with-funny-comments.sql

Change them back with this command: 

shell> replace " #" " --" -- text-file-with-funny-comments.sql

1.4.5 What Standards Does MySQL Follow?

Entry level SQL92. ODBC levels 0-2.

1.4.6 How to Cope Without COMMIT/ROLLBACK

The following mostly applies only for ISAM, MyISAM, and HEAP tables. If you only use transaction-safe tables (BDB, or InnoDB tables) in an an update, you can do COMMIT and ROLLBACK also with MySQL. See section 6.7.1 BEGIN/COMMIT/ROLLBACK Syntax.

The problem with handling COMMIT-ROLLBACK efficiently with the above table types would require a completely different table layout than MySQL uses today. The table type would also need extra threads that do automatic cleanups on the tables, and the disk usage would be much higher. This would make these table types about 2-4 times slower than they are today.

For the moment, we prefer implementing the SQL server language (something like stored procedures). With this you would very seldom really need COMMIT-ROLLBACK. This would also give much better performance.

Loops that need transactions normally can be coded with the help of LOCK TABLES, and you don't need cursors when you can update records on the fly.

We at TcX had a greater need for a real fast database than a 100% general database. Whenever we find a way to implement these features without any speed loss, we will probably do it. For the moment, there are many more important things to do. Check the TODO for how we prioritize things at the moment. (Customers with higher levels of support can alter this, so things may be reprioritized.)

The current problem is actually ROLLBACK. Without ROLLBACK, you can do any kind of COMMIT action with LOCK TABLES. To support ROLLBACK with the above table types, MySQL would have to be changed to store all old records that were updated and revert everything back to the starting point if ROLLBACK was issued. For simple cases, this isn't that hard to do (the current isamlog could be used for this purpose), but it would be much more difficult to implement ROLLBACK for ALTER/DROP/CREATE TABLE.

To avoid using ROLLBACK, you can use the following strategy:

  1. Use LOCK TABLES ... to lock all the tables you want to access.
  2. Test conditions.
  3. Update if everything is okay.
  4. Use UNLOCK TABLES to release your locks.

This is usually a much faster method than using transactions with possible ROLLBACKs, although not always. The only situation this solution doesn't handle is when someone kills the threads in the middle of an update. In this case, all locks will be released but some of the updates may not have been executed.

You can also use functions to update records in a single operation. You can get a very efficient application by using the following techniques:

For example, when we are doing updates to some customer information, we update only the customer data that has changed and test only that none of the changed data, or data that depend on the changed data, has changed compared to the original row. The test for changed data is done with the WHERE clause in the UPDATE statement. If the record wasn't updated, we give the client a message: "Some of the data you have changed have been changed by another user". Then we show the old row versus the new row in a window, so the user can decide which version of the customer record he should use.

This gives us something that is similar to column locking but is actually even better, because we only update some of the columns, using values that are relative to their current values. This means that typical UPDATE statements look something like these: 

UPDATE tablename SET pay_back=pay_back+'relative change';

UPDATE customer
  SET
    customer_date='current_date',
    address='new address',
    phone='new phone',
    money_he_owes_us=money_he_owes_us+'new_money'
  WHERE
    customer_id=id AND address='old address' AND phone='old phone';

As you can see, this is very efficient and works even if another client has changed the values in the pay_back or money_he_owes_us columns.

In many cases, users have wanted ROLLBACK and/or LOCK TABLES for the purpose of managing unique identifiers for some tables. This can be handled much more efficiently by using an AUTO_INCREMENT column and either the SQL function LAST_INSERT_ID() or the C API function mysql_insert_id(). See section 8.4.3.126 mysql_insert_id().

At MySQL AB, we have never had any need for row-level locking because we have always been able to code around it. Some cases really need row locking, but they are very few. If you want row-level locking, you can use a flag column in the table and do something like this: 

UPDATE tbl_name SET row_flag=1 WHERE id=ID;

MySQL returns 1 for the number of affected rows if the row was found and row_flag wasn't already 1 in the original row.

You can think of it as MySQL changed the above query to: 

UPDATE tbl_name SET row_flag=1 WHERE id=ID and row_flag <> 1;

1.4.7 Known errors and design deficiencies in MySQL

The following problems are known and have a very high priority to get fixed:

The following problems are known and will be fixed in due time:

The following are known bugs in earlier versions of MySQL:

For platform-specific bugs, see the sections about compiling and porting.

1.5 How MySQL Compares to Other Databases

This section compares MySQL to other popular databases.

This section has been written by the MySQL developers, so it should be read with that in mind. There are no factual errors contained in this section that we know of. If you find something which you believe to be an error, please contact us about it at docs@mysql.com.

For a list of all supported limits, functions, and types, see the crash-me Web page at http://www.mysql.com/information/crash-me.php.

1.5.1 How MySQL Compares to mSQL

Performance
For a true comparison of speed, consult the growing MySQL benchmark suite. See section 5.1.4 The MySQL Benchmark Suite. Because there is no thread creation overhead, a small parser, few features, and simple security, mSQL should be quicker at: Because these operations are so simple, it is hard to be better at them when you have a higher startup overhead. After the connection is established, MySQL should perform much better. On the other hand, MySQL is much faster than mSQL (and most other SQL implementations) on the following:
SQL Features
Disk Space Efficiency
That is, how small can you make your tables? MySQL has very precise types, so you can create tables that take very little space. An example of a useful MySQL datatype is the MEDIUMINT that is 3 bytes long. If you have 100,000,000 records, saving even one byte per record is very important. mSQL2 has a more limited set of column types, so it is more difficult to get small tables.
Stability
This is harder to judge objectively. For a discussion of MySQL stability, see section 1.1.7 How Stable Is MySQL?. We have no experience with mSQL stability, so we cannot say anything about that.
Price
Another important issue is the license. MySQL has a more flexible license than mSQL, and is also less expensive than mSQL. Whichever product you choose to use, remember to at least consider paying for a license or e-mail support. (You are required to get a license if you include MySQL with a product that you sell, of course.)
Perl Interfaces
MySQL has basically the same interfaces to Perl as mSQL with some added features.
JDBC (Java)
MySQL currently has a lot of different JDBC drivers: The recommended driver is the mm driver. The Resin driver may also be good (at least the benchmarks looks good), but we haven't received that much information about this yet. We know that mSQL has a JDBC driver, but we have too little experience with it to compare.
Rate of Development
MySQL has a very small team of developers, but we are quite used to coding C and C++ very rapidly. Because threads, functions, GROUP BY, and so on are still not implemented in mSQL, it has a lot of catching up to do. To get some perspective on this, you can view the mSQL `HISTORY' file for the last year and compare it with the News section of the MySQL Reference Manual (see section F MySQL change history). It should be pretty obvious which one has developed most rapidly.
Utility Programs
Both mSQL and MySQL have many interesting third-party tools. Because it is very easy to port upward (from mSQL to MySQL), almost all the interesting applications that are available for mSQL are also available for MySQL. MySQL comes with a simple msql2mysql program that fixes differences in spelling between mSQL and MySQL for the most-used C API functions. For example, it changes instances of msqlConnect() to mysql_connect(). Converting a client program from mSQL to MySQL usually takes a couple of minutes.

1.5.1.1 How to Convert mSQL Tools for MySQL

According to our experience, it would just take a few hours to convert tools such as msql-tcl and msqljava that use the mSQL C API so that they work with the MySQL C API.

The conversion procedure is:

  1. Run the shell script msql2mysql on the source. This requires the replace program, which is distributed with MySQL.
  2. Compile.
  3. Fix all compiler errors.

Differences between the mSQL C API and the MySQL C API are:

1.5.1.2 How mSQL and MySQL Client/Server Communications Protocols Differ

There are enough differences that it is impossible (or at least not easy) to support both.

The most significant ways in which the MySQL protocol differs from the mSQL protocol are listed below:

1.5.1.3 How mSQL 2.0 SQL Syntax Differs from MySQL

Column types

MySQL
Has the following additional types (among others; see section 6.5.3 CREATE TABLE Syntax):
MySQL also supports the following additional type attributes:
mSQL2
mSQL column types correspond to the MySQL types shown below:
mSQL type Corresponding MySQL type
CHAR(len) CHAR(len)
TEXT(len) TEXT(len). len is the maximal length. And LIKE works.
INT INT. With many more options!
REAL REAL. Or FLOAT. Both 4- and 8-byte versions are available.
UINT INT UNSIGNED
DATE DATE. Uses ANSI SQL format rather than mSQL's own format.
TIME TIME
MONEY DECIMAL(12,2). A fixed-point value with two decimals.

Index Creation

MySQL
Indexes may be specified at table creation time with the CREATE TABLE statement.
mSQL
Indexes must be created after the table has been created, with separate CREATE INDEX statements.

To Insert a Unique Identifier into a Table

MySQL
Use AUTO_INCREMENT as a column type specifier. See section 8.4.3.126 mysql_insert_id().
mSQL
Create a SEQUENCE on a table and select the _seq column.

To Obtain a Unique Identifier for a Row

MySQL
Add a PRIMARY KEY or UNIQUE key to the table and use this. New in Version 3.23.11: If the PRIMARY or UNIQUE key consists of only one column and this is of type integer, one can also refer to it as _rowid.
mSQL
Use the _rowid column. Observe that _rowid may change over time depending on many factors.

To Get the Time a Column Was Last Modified

MySQL
Add a TIMESTAMP column to the table. This column is automatically set to the current date and time for INSERT or UPDATE statements if you don't give the column a value or if you give it a NULL value.
mSQL
Use the _timestamp column.

NULL Value Comparisons

MySQL
MySQL follows ANSI SQL, and a comparison with NULL is always NULL.
mSQL
In mSQL, NULL = NULL is TRUE. You must change =NULL to IS NULL and <>NULL to IS NOT NULL when porting old code from mSQL to MySQL.

String Comparisons

MySQL
Normally, string comparisons are performed in case-independent fashion with the sort order determined by the current character set (ISO-8859-1 Latin1 by default). If you don't like this, declare your columns with the BINARY attribute, which causes comparisons to be done according to the ASCII order used on the MySQL server host.
mSQL
All string comparisons are performed in case-sensitive fashion with sorting in ASCII order.

Case-insensitive Searching

MySQL
LIKE is a case-insensitive or case-sensitive operator, depending on the columns involved. If possible, MySQL uses indexes if the LIKE argument doesn't start with a wild-card character.
mSQL
Use CLIKE.

Handling of Trailing Spaces

MySQL
Strips all spaces at the end of CHAR and VARCHAR columns. Use a TEXT column if this behavior is not desired.
mSQL
Retains trailing space.

WHERE Clauses

MySQL
MySQL correctly prioritizes everything (AND is evaluated before OR). To get mSQL behavior in MySQL, use parentheses (as shown in an example below).
mSQL
Evaluates everything from left to right. This means that some logical calculations with more than three arguments cannot be expressed in any way. It also means you must change some queries when you upgrade to MySQL. You do this easily by adding parentheses. Suppose you have the following mSQL query: 
mysql> SELECT * FROM table WHERE a=1 AND b=2 OR a=3 AND b=4;
To make MySQL evaluate this the way that mSQL would, you must add parentheses: 
mysql> SELECT * FROM table WHERE (a=1 AND (b=2 OR (a=3 AND (b=4))));

Access Control

MySQL
Has tables to store grant (permission) options per user, host, and database. See section 4.2.5 How the Privilege System Works.
mSQL
Has a file `mSQL.acl' in which you can grant read/write privileges for users.

1.5.2 How MySQL Compares to PostgreSQL

When reading the following, please note that both products are continually evolving. We at MySQL AB and the PostgreSQL developers are both working on making our respective database as good as possible, so we are both a serious choice to any commercial database.

The following comparison is made by us at MySQL AB. We have tried to be as accurate and fair as possible, but because we don't have a full knowledge of all PostgreSQL features while we know MySQL througly, we may have got some things wrong. We will however correct these when they come to our attention.

We would first like to note that PostgreSQL and MySQL are both widely used products, but with different design goals, even if we are both striving to be ANSI SQL compatible. This means that for some applications MySQL is more suited, while for others PostgreSQL is more suited. When choosing which database to use, you should first check if the database's feature set satisfies your application. If you need raw speed, MySQL is probably your best choice. If you need some of the extra features that only PostgreSQL can offer, you should use PostgreSQL.

1.5.2.1 MySQL and PostgreSQL development strategies

When adding things to MySQL we take pride to do an optimal, definite solution. The code should be so good that we shouldn't have any need to change it in the foreseeable future. We also do not like to sacrifice speed for features but instead will do our utmost to find a solution that will give maximal throughput. This means that development will take a little longer, but the end result will be well worth this. This kind of development is only possible because all server code are checked by one of a few (currently two) persons before it's included in the MySQL server.

We at MySQL AB believe in frequent releases to be able to push out new features quickly to our users. Because of this we do a new small release about every three weeks, and a major branch every year. All releases are throughly tested with our testing tools on a lot of different platforms.

PostgreSQL is based on a kernel with lots of contributors. In this setup it makes sense to prioritize adding a lot of new features, instead of implementing them optimally, because one can always optimize things later if there arises a need for this.

Another big difference between MySQL and PostgreSQL is that nearly all of the code in the MySQL server are coded by developers that are employed by MySQL AB and are still working on the server code. The exceptions are the transaction engines, and the regexp library.

This is in sharp contrast to the PostgreSQL code where the majority of the code is coded by a big group of people with different backgrounds. It was only recently that the PostgreSQL developers announced that their current developer group had finally had time to take a look at all the code in the current PostgreSQL release.

Both of the above development methods has it's own merits and drawbacks. We here at MySQL AB think of course that our model is better because our model gives better code consistency, more optimal and reusable code, and in our opinion, fewer bugs. Because we are the authors of the MySQL server code, we are better able to coordinate new features and releases.

1.5.2.2 Featurewise Comparison of MySQL and PostgreSQL

On the crash-me page you can find a list of those database constructs and limits that one can detect automatically with a program. Note however that a lot of the numerical limits may be changed with startup options for respective database. The above web page is however extremely useful when you want to ensure that your applications works with many different databases or when you want to convert your application from one datbase to another.

MySQL offers the following advantages over PostgreSQL:

Drawbacks with MySQL compared to PostgreSQL:

PostgreSQL currently offers the following advantages over MySQL:

Note that because we know the MySQL road map, we have included in the following table the version when MySQL should support this feature. Unfortunately we couldn't do this for previous comparison, because we don't know the PostgreSQL roadmap.

Feature MySQL version
Subselects 4.1
Foreign keys 4.0 and 4.1
Views 4.2
Stored procedures 4.1
Extensible type system Not planned
Unions 4.0
Full join 4.0 or 4.1
Triggers 4.1
Constrainst 4.1
Cursors 4.1 or 4.2
Extensible index types like R-trees R-trees are planned for 4.2
Inherited tables Not planned

Other reasons to use PostgreSQL:

Drawbacks with PostgreSQL compared to MySQL:

For a complete list of drawbacks, you should also examine the first table in this section.

1.5.2.3 Benchmarking MySQL and PostgreSQL

The only open source benchmark that we know of that can be used to benchmark MySQL and PostgreSQL (and other databases) is our own. It can be found at http://www.mysql.com/information/benchmarks.html.

We have many times asked the PostgreSQL developers and some PostgreSQL users to help us extend this benchmark to make it the definitive benchmark for databases, but unfortunately we haven't gotten any feedback for this.

We the MySQL developers have, because of this, spent a lot of hours to get maximum performance from PostgreSQL for the benchmarks, but because we don't know PostgreSQL intimately, we are sure that there are things that we have missed. We have on the benchmark page documented exactly how we did run the benchmark so that it should be easy for anyone to repeat and verify our results.

The benchmarks are usually run with and without the --fast option. When run with --fast we are trying to use every trick the server can do to get the code to execute as fast as possible. The idea is that the normal run should show how the server would work in a default setup and the --fast run shows how the server would do if the application developer would use extensions in the server to make his application run faster.

When running with PostgreSQL and --fast we do a VACUUM() after every major table UPDATE and DROP TABLE to make the database in perfect shape for the following SELECTs. The time for VACUUM() is measured separately.

When running with PostgreSQL 7.1.1 we could, however, not run with --fast because during the INSERT test, the postmaster (the PostgreSQL deamon) died and the database was so corrupted that it was impossible to restart postmaster. After this happened twice, we decided to postpone the --fast test until next PostgreSQL release. The details about the machine we run the benchmark can be found on the benchmark page.

Before going to the other benchmarks we know of, we would like to give some background on benchmarks:

It's very easy to write a test that shows ANY database to be best database in the world, by just restricting the test to something the database is very good at and not test anything that the database is not good at. If one after this publishes the result with a single figure, things are even easier.

This would be like us measuring the speed of MySQL compared to PostgreSQL by looking at the summary time of the MySQL benchmarks on our web page. Based on this MySQL would be more than 40 times faster than PostgreSQL, something that is of course not true. We could make things even worse by just taking the test where PostgreSQL performs worst and claim that MySQL is more than 2000 times faster than PostgreSQL.

The case is that MySQL does a lot of optimizations that PostgreSQL doesn't do. This is of course also true the other way around. An SQL optimizer is a very complex thing, and a company could spend years on just making the optimizer faster and faster.

When looking at the benchmark results you should look for things that you do in your application and just use these results to decide which database would be best suited for your application. The benchmark results also shows things a particular database is not good at and should give you a notion about things to avoid and what you may have to do in other ways.

We know of two benchmark tests that claims that PostgreSQL performs better than MySQL. These both where multi-user tests, a test that we here at MySQL AB haven't had time to write and include in the benchmark suite, mainly because it's a big task to do this in a manner that is fair against all databases.

One is the benchmark paid for by Great Bridge, which you can read about at: http://www.greatbridge.com/about/press.php?content_id=4.

This is the probably worst benchmark we have ever seen anyone conduct. This was not only tuned to only test what PostgreSQL is absolutely best at, it was also totally unfair against every other database involved in the test.

NOTE: We know that not even some of the main PostgreSQL developers did like the way Great Bridge conducted the benchmark, so we don't blame them for the way the benchmark was made.

This benchmark has been condemned in a lot of postings and newsgroups so we will here just shortly repeat some things that where wrong with it.

Tim Perdue, a long time PostgreSQL fan and a reluctant MySQL user published a comparison on phpbuider.

When we became aware of the comparison, we phoned Tim Perdue about this because there were a lot of strange things in his results. For example, he claimed that MySQL had a problem with five users in his tests, when we know that there are users with similar machines as his that are using MySQL with 2000 simultaneous connections doing 400 queries per second. (In this case the limit was the web bandwidth, not the database.)

It sounded like he was using a Linux kernel that either had some problems with many threads, such as kernels before 2.4, which had a problem with many threads on multi-CPU machines. We have documented in this manual how to fix this and Tim should be aware of this problem.

The other possible problem could have been an old glibc library and that Tim didn't use a MySQL binary from our site, which is linked with a corrected glibc library, but had compiled a version of his own with. In any of the above cases, the symptom would have been exactly what Tim had measured.

We asked Tim if we could get access to his data so that we could repeat the benchmark and if he could check the MySQL version on the machine to find out what was wrong and he promised to come back to us about this. He has not done that yet.

Because of this we can't put any trust in this benchmark either :(

Over time things also changes and the above benchmarks are not that relevant anymore. MySQL now have a couple of different table handlers with different speed/concurrency tradeoffs. See section 7 MySQL Table Types. It would be interesting to see how the above tests would run with the different transactional table types in MySQL. PostgreSQL has of course also got new features since the test was made. As the above test are not publicly available there is no way for us to know how the database would preform in the same tests today.

Conclusion:

The only benchmarks that exist today that anyone can download and run against MySQL and PostgreSQL is the MySQL benchmarks. We here at MySQL believe that open source databases should be tested with open source tools! This is the only way to ensure that no one does tests that nobody can reproduce and use this to claim that a database is better than another. Without knowing all the facts it's impossible to answer the claims of the tester.

The thing we find strange is that every test we have seen about PostgreSQL, that is impossible to reproduce, claims that PostgreSQL is better in most cases while our tests, which anyone can reproduce, clearly shows otherwise. With this we don't want to say that PostgreSQL isn't good at many things (it is!) or that it isn't faster than MySQL under certain conditions. We would just like to see a fair test where they are very good so that we could get some friendly competition going!

For more information about our benchmarks suite See section 5.1.4 The MySQL Benchmark Suite.

We are working on an even better benchmark suite, including multi user tests, and a better documentation of what the individual tests really do and how to add more tests to the suite.

1.6 MySQL and the future (The TODO)

This appendix lists the features that we plan to implement in MySQL.

Everything in this list is approximately in the order it will be done. If you want to affect the priority order, please register a license or support us and tell us what you want to have done more quickly. See section 1.3 MySQL Licensing and Support.

The plan is that we in the future will support the full ANSI SQL99 standard, but with a lot of useful extensions. The challenge is to do this without sacrifying the speed or compromise the code.

1.6.1 Things that should be in 4.0

We plan to make MySQL Version 4.0 a ``quick'' release where we only add some new stuff to enable others to help us with developing new features into Version 4.1. The MySQL 4.0 version should only take us about a month to make after which we want to stabilize it and start working on Version 4.1. Version 4.0 should have the following new features:

The news section for 4.0 includes a list of the features we have already implemented in the 4.0 tree. See section F.1 Changes in release 4.0.x (Development; Alpha).

1.6.2 Things that must be done in the real near future

1.6.3 Things that have to be done sometime

Time is given according to amount of work, not real time.

1.6.4 Some things we don't have any plans to do

2 MySQL Installation

This chapter describes how to obtain and install MySQL:

2.1 Quick Standard Installation of MySQL

2.1.1 Installing MySQL on Linux

The recommended way to install MySQL on Linux is by using an RPM file. The MySQL RPMs are currently being built on a RedHat Version 6.2 system but should work on other versions of Linux that support rpm and use glibc.

If you have problems with an RPM file, for example, if you receive the error ``Sorry, the host 'xxxx' could not be looked up'', see section 2.6.1.1 Linux Notes for Binary Distributions.

The RPM files you may want to use are:

To see all files in an RPM package, run: 

shell> rpm -qpl MySQL-VERSION.i386.rpm

To perform a standard minimal installation, run: 

shell> rpm -i MySQL-VERSION.i386.rpm MySQL-client-VERSION.i386.rpm

To install just the client package, run: 

shell> rpm -i MySQL-client-VERSION.i386.rpm

The RPM places data in `/var/lib/mysql'. The RPM also creates the appropriate entries in `/etc/rc.d/' to start the server automatically at boot time. (This means that if you have performed a previous installation, you may want to make a copy of your previously installed MySQL startup file if you made any changes to it, so you don't lose your changes.)

After installing the RPM file(s), the mysqld daemon should be running and you should now be able to start using MySQL. See section 2.4 Post-installation Setup and Testing.

If something goes wrong, you can find more information in the binary installation chapter. See section M.1 Installing a MySQL Binary Distribution.

2.1.2 Installing MySQL on Windows

The following instructions apply to precompiled binary distributions. If you download a source distribution, you will have to compile and install it yourself.

If you don't have a copy of the MySQL distribution, you should first download one from http://www.mysql.com/downloads/mysql-3.23.html.

If you plan to connect to MySQL from some other program, you will probably also need the MyODBC driver. You can find this at the MyODBC download page (http://www.mysql.com/downloads/api-myodbc.html).

To install either distribution, unzip it in some empty directory and run the Setup.exe program.

By default, MySQL-Windows is configured to be installed in `C:\mysql'. If you want to install MySQL elsewhere, install it in `C:\mysql' first, then move the installation to where you want it. If you do move MySQL, you must indicate where everything is located by supplying a --basedir option when you start the server. For example, if you have moved the MySQL distribution to `D:\programs\mysql', you must start mysqld like this: 

C:\> D:\programs\mysql\bin\mysqld --basedir D:\programs\mysql

Use mysqld --help to display all the options that mysqld understands!

With all newer MySQL versions, you can also create a `C:\my.cnf' file that holds any default options for the MySQL server. Copy the file `\mysql\my-xxxxx.cnf' to `C:\my.cnf' and edit it to suit your setup. Note that you should specify all paths with `/' instead of `\'. If you use `\', you need to specify it twice, because `\' is the escape character in MySQL. See section 4.1.2 my.cnf Option Files.

Starting with MySQL 3.23.38, the Windows distribution includes both the normal and the MySQL-Max binaries. The main benefit of using the normal mysqld.exe binary is that it's a little faster and uses less resources.

Here is a list of the different MySQL servers you can use:

mysqld Compiled with full debugging and automatic memory allocation checking, symbolic links, BDB and InnoDB tables.
mysqld-opt Optimized binary with no support for transactional tables.
mysqld-nt Optimized binary for NT with support for named pipes. You can run this version on Win98, but in this case no named pipes are created and you must have TCP/IP installed.
mysqld-max Optimized binary with support for symbolic links, BDB and InnoDB tables.
mysqld-max-nt Like mysqld-max, but compiled with support for named pipes.

All of the above binaries are optimized for the Pentium Pro processor but should work on any Intel processor >= i386.

NOTE: If you want to use InnoDB tables, there are certain startup options that must be specified in your `my.ini' file! See section 7.6.2 InnoDB startup options.

2.2 General Installation Issues

2.2.1 How to Get MySQL

Check the MySQL home page for information about the current version and for downloading instructions.

Our main download mirror is located at:

http://download.sourceforge.net/mirrors/mysql/

If you are interested in becoming a MySQL mirror site, you may anonymously rsync with: rsync://download.sourceforge.net/mysql/. Please send e-mail to webmaster@mysql.com notifying us of your mirror to be added to the list below.

If you have problems downloading from our main site, try using one of the mirrors listed below.

Please report bad or out-of-date mirrors to webmaster@mysql.com.

Europe:

North America:

South America:

Asia:

Africa:

2.2.2 Operating Systems Supported by MySQL

We use GNU Autoconf, so it is possible to port MySQL to all modern systems with working Posix threads and a C++ compiler. (To compile only the client code, a C++ compiler is required but not threads.) We use and develop the software ourselves primarily on Sun Solaris (Versions 2.5 - 2.7) and SuSE Linux Version 7.x.

Note that for many operating systems, the native thread support works only in the latest versions. MySQL has been reported to compile successfully on the following operating system/thread package combinations:

Note that not all platforms are suited equally well for running MySQL. How well a certain platform is suited for a high-load mission critical MySQL server is determined by the following factors:

Based on the above criteria, the best platforms for running MySQL at this point are x86 with SuSE Linux 7.1, 2.4 kernel and ReiserFS (or any similar Linux distribution) and Sparc with Solaris 2.7 or 2.8. FreeBSD comes third, but we really hope it will join the top club once the thread library is improved. We also hope that at some point we will be able to include all other platforms on which MySQL compiles, runs ok, but not quite with the same level of stability and performance, into the top category. This will require some effort on our part in cooperation with the developers of the OS/library components MySQL depends upon. If you are interested in making one of those components better, are in a position to influence their development, and need more detailed instructions on what MySQL needs to run better, send an e-mail to internals@lists.mysql.com.

Please note that the comparison above is not to say that one OS is better or worse than the other in general. We are talking about choosing a particular OS for a dedicated purpose - running MySQL, and compare platforms in that regard only. With this in mind, the result of this comparison would be different if we included more issues into it. And in some cases, the reason one OS is better than the other could simply be that we have put forth more effort into testing on and optimizing for that particular platform. We are just stating our observations to help you make a decision on which platform to use MySQL on in your setup.

2.2.3 Which MySQL Version to Use

The first decision to make is whether you want to use the latest development release or the last stable release:

The second decision to make is whether you want to use a source distribution or a binary distribution. In most cases you should probably use a binary distribution, if one exists for your platform, as this generally will be easier to install than a source distribution.

In the following cases you probably will be better off with a source installation:

The MySQL naming scheme uses release numbers that consist of three numbers and a suffix. For example, a release name like mysql-3.21.17-beta is interpreted like this:

All versions of MySQL are run through our standard tests and benchmarks to ensure that they are relatively safe to use. Because the standard tests are extended over time to check for all previously found bugs, the test suite keeps getting better.

Note that all releases have been tested at least with:

An internal test suite
This is part of a production system for a customer. It has many tables with hundreds of megabytes of data.
The MySQL benchmark suite
This runs a range of common queries. It is also a test to see whether the latest batch of optimizations actually made the code faster. See section 5.1.4 The MySQL Benchmark Suite.
The crash-me test
This tries to determine what features the database supports and what its capabilities and limitations are. See section 5.1.4 The MySQL Benchmark Suite.

Another test is that we use the newest MySQL version in our internal production environment, on at least one machine. We have more than 100 gigabytes of data to work with.

2.2.4 Installation Layouts

This section describes the default layout of the directories created by installing binary and source distributions.

A binary distribution is installed by unpacking it at the installation location you choose (typically `/usr/local/mysql') and creates the following directories in that location:

Directory Contents of directory
`bin' Client programs and the mysqld server
`data' Log files, databases
`include' Include (header) files
`lib' Libraries
`scripts' mysql_install_db
`share/mysql' Error message files
`sql-bench' Benchmarks

A source distribution is installed after you configure and compile it. By default, the installation step installs files under `/usr/local', in the following subdirectories:

Directory Contents of directory
`bin' Client programs and scripts
`include/mysql' Include (header) files
`info' Documentation in Info format
`lib/mysql' Libraries
`libexec' The mysqld server
`share/mysql' Error message files
`sql-bench' Benchmarks and crash-me test
`var' Databases and log files

Within an installation directory, the layout of a source installation differs from that of a binary installation in the following ways:

You can create your own binary installation from a compiled source distribution by executing the script `scripts/make_binary_distribution'.

2.2.5 How and When Updates Are Released

MySQL is evolving quite rapidly here at MySQL AB and we want to share this with other MySQL users. We try to make a release when we have very useful features that others seem to have a need for.

We also try to help out users who request features that are easy to implement. We take note of what our licensed users want to have, and we especially take note of what our extended e-mail supported customers want and try to help them out.

No one has to download a new release. The News section will tell you if the new release has something you really want. See section F MySQL change history.

We use the following policy when updating MySQL:

The current stable release is Version 3.23; We have already moved active development to Version 4.0. Bugs will still be fixed in the stable version. We don't believe in a complete freeze, as this also leaves out bug fixes and things that ``must be done.'' ``Somewhat frozen'' means that we may add small things that ``almost surely will not affect anything that's already working.''

2.2.6 MySQL Binaries Compiled by MySQL AB

As a service, we at MySQL AB provide a set of binary distributions of MySQL that are compiled at our site or at sites where customers kindly have given us access to their machines.

These distributions are generated with scripts/make_binary_distribution and are configured with the following compilers and options:

SunOS 4.1.4 2 sun4c with gcc 2.7.2.1
CC=gcc CXX=gcc CXXFLAGS="-O3 -felide-constructors" ./configure --prefix=/usr/local/mysql --disable-shared --with-extra-charsets=complex --enable-assembler
SunOS 5.5.1 (and above) sun4u with egcs 1.0.3a or 2.90.27 or gcc 2.95.2 and newer
CC=gcc CFLAGS="-O3" CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex --enable-assembler
SunOS 5.6 i86pc with gcc 2.8.1
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex
Linux 2.0.33 i386 with pgcc 2.90.29 (egcs 1.0.3a)
CFLAGS="-O3 -mpentium -mstack-align-double" CXX=gcc CXXFLAGS="-O3 -mpentium -mstack-align-double -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --with-extra-charsets=complex
Linux 2.2.x with x686 with gcc 2.95.2
CFLAGS="-O3 -mpentiumpro" CXX=gcc CXXFLAGS="-O3 -mpentiumpro -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-extra-charset=complex
SCO 3.2v5.0.4 i386 with gcc 2.7-95q4
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
AIX 2 4 with gcc 2.7.2.2
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
OSF1 V4.0 564 alpha with gcc 2.8.1
CC=gcc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex
Irix 6.3 IP32 with gcc 2.8.0
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
BSDI BSD/OS 3.1 i386 with gcc 2.7.2.1
CC=gcc CXX=gcc CXXFLAGS=-O ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
BSDI BSD/OS 2.1 i386 with gcc 2.7.2
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex

Anyone who has more optimal options for any of the configurations listed above can always mail them to the developer's mailing list at internals@lists.mysql.com.

RPM distributions prior to MySQL Version 3.22 are user-contributed. Beginning with Version 3.22, the RPMs are generated by us at MySQL AB.

If you want to compile a debug version of MySQL, you should add --with-debug or --with-debug=full to the above configure lines and remove any -fomit-frame-pointer options.

2.3 Installing a MySQL Source Distribution

Before you proceed with the source installation, check first to see if our binary is available for your platform and if it will work for you. We put in a lot of effort into making sure that our binaries are built with the best possible options.

You need the following tools to build and install MySQL from source:

If you are using a recent version of gcc, recent enough to understand -fno-exceptions option, it is VERY IMPORTANT that you use it. Otherwise, you may compile a binary that crashes randomly. We also recommend that you use -felide-contructors and -fno-rtti along with -fno-exceptions. When in doubt, do the following: 


CFLAGS="-O3" CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static

On most systems this will give you a fast and stable binary.

If you run into problems, PLEASE ALWAYS USE mysqlbug when posting questions to mysql@lists.mysql.com. Even if the problem isn't a bug, mysqlbug gathers system information that will help others solve your problem. By not using mysqlbug, you lessen the likelihood of getting a solution to your problem! You will find mysqlbug in the `scripts' directory after you unpack the distribution. See section 1.2.22.3 How to Report Bugs or Problems.

2.3.1 Quick Installation Overview

The basic commands you must execute to install a MySQL source distribution are: 

shell> groupadd mysql
shell> useradd -g mysql mysql
shell> gunzip < mysql-VERSION.tar.gz | tar -xvf -
shell> cd mysql-VERSION
shell> ./configure --prefix=/usr/local/mysql
shell> make
shell> make install
shell> scripts/mysql_install_db
shell> chown -R root  /usr/local/mysql
shell> chown -R mysql /usr/local/mysql/var
shell> chgrp -R mysql /usr/local/mysql
shell> cp support-files/my-medium.cnf /etc/my.cnf
shell> /usr/local/mysql/bin/safe_mysqld --user=mysql &

If you want have support for InnoDB tables, you should edit the /etc/my.cnf file and remove the # character before the parameters that starts with innodb_.... See section 4.1.2 my.cnf Option Files. See section 7.6.2 InnoDB startup options.

If you start from a source RPM, then do the following: 

shell> rpm --rebuild MySQL-VERSION.src.rpm

This will make a binary RPM that you can install.

You can add new users using the bin/mysql_setpermission script if you install the DBI and Msql-Mysql-modules Perl modules.

A more detailed description follows.

To install a source distribution, follow the steps below, then proceed to section 2.4 Post-installation Setup and Testing, for post-installation initialization and testing:

  1. Pick the directory under which you want to unpack the distribution, and move into it.
  2. Obtain a distribution file from one of the sites listed in section 2.2.1 How to Get MySQL.
  3. If you are interested in using Berkeley DB tables with MySQL, you will need to obtain a patched version of the Berkeley DB source code. Please read the chapter on Berkeley DB tables before proceeding. See section 7.5 BDB or Berkeley_DB Tables. MySQL source distributions are provided as compressed tar archives and have names like `mysql-VERSION.tar.gz', where VERSION is a number like 3.23.48.
  4. Add a user and group for mysqld to run as: 
    shell> groupadd mysql
shell> useradd -g mysql mysql
    These commands add the mysql group, and the mysql user. The syntax for useradd and groupadd may differ slightly on different versions of Unix. They may also be called adduser and addgroup. You may wish to call the user and group something else instead of mysql.
  5. Unpack the distribution into the current directory: 
    shell> gunzip < /path/to/mysql-VERSION.tar.gz | tar xvf -
    This command creates a directory named `mysql-VERSION'.
  6. Change into the top-level directory of the unpacked distribution: 
    shell> cd mysql-VERSION
    Note that currently you must configure and build MySQL from this top-level directory. You can not build it in a different directory.
  7. Configure the release and compile everything: 
    shell> ./configure --prefix=/usr/local/mysql
shell> make
    When you run configure, you might want to specify some options. Run ./configure --help for a list of options. section 2.3.3 Typical configure Options, discusses some of the more useful options. If configure fails, and you are going to send mail to mysql@lists.mysql.com to ask for assistance, please include any lines from `config.log' that you think can help solve the problem. Also include the last couple of lines of output from configure if configure aborts. Post the bug report using the mysqlbug script. See section 1.2.22.3 How to Report Bugs or Problems. If the compile fails, see section 2.3.5 Problems Compiling?, for help with a number of common problems.
  8. Install everything: 
    shell> make install
    You might need to run this command as root.
  9. Create the MySQL grant tables (necessary only if you haven't installed MySQL before): 
    shell> scripts/mysql_install_db
    Note that MySQL versions older than Version 3.22.10 started the MySQL server when you run mysql_install_db. This is no longer true!
  10. Change ownership of binaries to root and ownership of the data directory to the user that you will run mysqld as: 
    shell> chown -R root  /usr/local/mysql
shell> chown -R mysql /usr/local/mysql/var
shell> chgrp -R mysql /usr/local/mysql
    The first command changes the owner attribute of the files to the root user, the second one changes the owner attribute of the data directory to the mysql user, and the third one changes the group attribute to the mysql group.
  11. If you want to install support for the Perl DBI/DBD interface, see section M.2 Perl Installation Comments.
  12. If you would like MySQL to start automatically when you boot your machine, you can copy support-files/mysql.server to the location where your system has its startup files. More information can be found in the support-files/mysql.server script itself and in section 2.4.3 Starting and Stopping MySQL Automatically.

After everything has been installed, you should initialize and test your distribution: 

shell> /usr/local/mysql/bin/safe_mysqld --user=mysql &

If that command fails immediately with mysqld daemon ended then you can find some information in the file `mysql-data-directory/'hostname'.err'. The likely reason is that you already have another mysqld server running. See section 4.1.4 Running Multiple MySQL Servers on the Same Machine.

See section 2.4 Post-installation Setup and Testing.

2.3.2 Applying Patches

Sometimes patches appear on the mailing list or are placed in the patches area of the MySQL Web site.

To apply a patch from the mailing list, save the message in which the patch appears in a file, change into the top-level directory of your MySQL source tree, and run these commands: 

shell> patch -p1 < patch-file-name
shell> rm config.cache
shell> make clean

Patches from the FTP site are distributed as plain text files or as files compressed with gzip. Apply a plain patch as shown above for mailing list patches. To apply a compressed patch, change into the top-level directory of your MySQL source tree and run these commands: 

shell> gunzip < patch-file-name.gz | patch -p1
shell> rm config.cache
shell> make clean

After applying a patch, follow the instructions for a normal source install, beginning with the ./configure step. After running the make install step, restart your MySQL server.

You may need to bring down any currently running server before you run make install. (Use mysqladmin shutdown to do this.) Some systems do not allow you to install a new version of a program if it replaces the version that is currently executing.

2.3.3 Typical configure Options

The configure script gives you a great deal of control over how you configure your MySQL distribution. Typically you do this using options on the configure command line. You can also affect configure using certain environment variables. See section H Environment Variables. For a list of options supported by configure, run this command: 

shell> ./configure --help

Some of the more commonly-used configure options are described below:

2.3.4 Installing from the Development Source Tree

CAUTION: You should read this section only if you are interested in helping us test our new code. If you just want to get MySQL up and running on your system, you should use a standard release distribution (either a source or binary distribution will do).

To obtain our most recent development source tree, use these instructions:

  1. Download BitKeeper from http://www.bitmover.com/cgi-bin/download.cgi. You will need Bitkeeper 2.0 or newer to access our repository.
  2. Follow the instructions to install it.
  3. After BitKeeper is installed, use this command if you want to clone the MySQL 3.23 branch: 
    shell> bk clone bk://work.mysql.com:7000 mysql
    To clone the 4.0 branch, use this command instead: 
    shell> bk clone bk://work.mysql.com:7001 mysql-4.0
    The initial download of the source tree may take a while, depending on the speed of your connection; be patient.
  4. You will need GNU autoconf, automake, libtool, and m4 to run the next set of commands. If you get some strange error during this stage, check that you really have libtool installed! 
    shell> cd mysql
shell> bk -r edit
shell> aclocal; autoheader; autoconf;  automake;
shell> ./configure  # Add your favorite options here
shell> make
    A collection of our standard configure scripts is located in the `BUILD/' subdirectory. If you are lazy, you can use `BUILD/compile-pentium-debug'. To compile on a different architecture, modify the script removing flags that are Pentium-specific.
  5. When the build is done, run make install. Be careful with this on a production machine; the command may overwrite your live release installation. If you have another installation of MySQL, we recommand that you run ./configure with different values for the prefix, tcp-port, and unix-socket-path options than those used for your production server.
  6. Play hard with your new installation and try to make the new features crash. Start by running make test. See section 9.3.2 MySQL Test Suite.
  7. If you have gotten to the make stage and the distribution does not compile, please report it to bugs@lists.mysql.com. If you have installed the latest versions of the required GNU tools, and they crash trying to process our configuration files, please report that also. However, if you execute aclocal and get a command not found error or a similar problem, do not report it. Instead, make sure all the necessary tools are installed and that your PATH variable is set correctly so your shell can find them.
  8. After the initial bk clone operation to get the source tree, you should run bk pull periodically to get the updates.
  9. You can examine the change history for the tree with all the diffs by using bk sccstool. If you see some funny diffs or code that you have a question about, do not hesitate to send e-mail to internals@lists.mysql.com. Also, if you think you have a better idea on how to do something, send an email to the same address with a patch. bk diffs will produce a patch for you after you have made changes to the source. If you do not have the time to code your idea, just send a description.
  10. BitKeeper has a nice help utility that you can access via bk helptool.

2.3.5 Problems Compiling?

All MySQL programs compile cleanly for us with no warnings on Solaris using gcc. On other systems, warnings may occur due to differences in system include files. See section 2.3.6 MIT-pthreads Notes for warnings that may occur when using MIT-pthreads. For other problems, check the list below.

The solution to many problems involves reconfiguring. If you do need to reconfigure, take note of the following:

To prevent old configuration information or object files from being used, run these commands before rerunning configure: 

shell> rm config.cache
shell> make clean

Alternatively, you can run make distclean.

The list below describes some of the problems compiling MySQL that have been found to occur most often:

2.3.6 MIT-pthreads Notes

This section describes some of the issues involved in using MIT-pthreads.

Note that on Linux you should NOT use MIT-pthreads but install LinuxThreads! See section 2.6.1 Linux Notes (All Linux Versions).

If your system does not provide native thread support, you will need to build MySQL using the MIT-pthreads package. This includes older FreeBSD systems, SunOS 4.x, Solaris 2.4 and earlier, and some others. See section 2.2.2 Operating Systems Supported by MySQL.

2.4 Post-installation Setup and Testing

Once you've installed MySQL (from either a binary or source distribution), you need to initialize the grant tables, start the server, and make sure that the server works okay. You may also wish to arrange for the server to be started and stopped automatically when your system starts up and shuts down.

Normally you install the grant tables and start the server like this for installation from a source distribution: 

shell> ./scripts/mysql_install_db
shell> cd mysql_installation_directory
shell> ./bin/safe_mysqld --user=mysql &

For a binary distribution (not RPM or pkg packages), do this: 

shell> cd mysql_installation_directory
shell> ./bin/mysql_install_db
shell> ./bin/safe_mysqld --user=mysql &

This creates the mysql database which will hold all database privileges, the test database which you can use to test MySQL and also privilege entries for the user that run mysql_install_db and a root user (without any passwords). This also starts the mysqld server.

mysql_install_db will not overwrite any old privilege tables, so it should be safe to run in any circumstances. If you don't want to have the test database you can remove it with mysqladmin -u root drop test.

Testing is most easily done from the top-level directory of the MySQL distribution. For a binary distribution, this is your installation directory (typically something like `/usr/local/mysql'). For a source distribution, this is the main directory of your MySQL source tree.

In the commands shown below in this section and in the following subsections, BINDIR is the path to the location in which programs like mysqladmin and safe_mysqld are installed. For a binary distribution, this is the `bin' directory within the distribution. For a source distribution, BINDIR is probably `/usr/local/bin', unless you specified an installation directory other than `/usr/local' when you ran configure. EXECDIR is the location in which the mysqld server is installed. For a binary distribution, this is the same as BINDIR. For a source distribution, EXECDIR is probably `/usr/local/libexec'.

Testing is described in detail below:

  1. If necessary, start the mysqld server and set up the initial MySQL grant tables containing the privileges that determine how users are allowed to connect to the server. This is normally done with the mysql_install_db script: 
    shell> scripts/mysql_install_db
    Typically, mysql_install_db needs to be run only the first time you install MySQL. Therefore, if you are upgrading an existing installation, you can skip this step. (However, mysql_install_db is quite safe to use and will not update any tables that already exist, so if you are unsure of what to do, you can always run mysql_install_db.) mysql_install_db creates six tables (user, db, host, tables_priv, columns_priv, and func) in the mysql database. A description of the initial privileges is given in section 4.3.4 Setting Up the Initial MySQL Privileges. Briefly, these privileges allow the MySQL root user to do anything, and allow anybody to create or use databases with a name of 'test' or starting with 'test_'. If you don't set up the grant tables, the following error will appear in the log file when you start the server: 
    mysqld: Can't find file: 'host.frm'
    The above may also happen with a binary MySQL distribution if you don't start MySQL by executing exactly ./bin/safe_mysqld! See section 4.7.2 safe_mysqld, the wrapper around mysqld. You might need to run mysql_install_db as root. However, if you prefer, you can run the MySQL server as an unprivileged (non-root) user, provided that user can read and write files in the database directory. Instructions for running MySQL as an unprivileged user are given in section A.3.2 How to Run MySQL As a Normal User. If you have problems with mysql_install_db, see section 2.4.1 Problems Running mysql_install_db. There are some alternatives to running the mysql_install_db script as it is provided in the MySQL distribution: For more information about these alternatives, see section 4.3.4 Setting Up the Initial MySQL Privileges.
  2. Start the MySQL server like this: 
    shell> cd mysql_installation_directory
shell> bin/safe_mysqld &
    If you have problems starting the server, see section 2.4.2 Problems Starting the MySQL Server.
  3. Use mysqladmin to verify that the server is running. The following commands provide a simple test to check that the server is up and responding to connections: 
    shell> BINDIR/mysqladmin version
shell> BINDIR/mysqladmin variables
    The output from mysqladmin version varies slightly depending on your platform and version of MySQL, but should be similar to that shown below: 
    shell> BINDIR/mysqladmin version
mysqladmin  Ver 8.14 Distrib 3.23.32, for linux on i586
Copyright (C) 2000 MySQL AB & MySQL Finland AB & TCX DataKonsult AB
This software comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to modify and redistribute it under the GPL license

Server version          3.23.32-debug
Protocol version        10
Connection              Localhost via Unix socket
TCP port                3306
UNIX socket             /tmp/mysql.sock
Uptime:                 16 sec

Threads: 1  Questions: 9  Slow queries: 0  Opens: 7  Flush tables: 2  Open tables: 0 Queries per second avg: 0.000  Memory in use: 132K  Max memory used: 16773K
    To get a feeling for what else you can do with BINDIR/mysqladmin, invoke it with the --help option.
  4. Verify that you can shut down the server: 
    shell> BINDIR/mysqladmin -u root shutdown
  5. Verify that you can restart the server. Do this using safe_mysqld or by invoking mysqld directly. For example: 
    shell> BINDIR/safe_mysqld --log &
    If safe_mysqld fails, try running it from the MySQL installation directory (if you are not already there). If that doesn't work, see section 2.4.2 Problems Starting the MySQL Server.
  6. Run some simple tests to verify that the server is working. The output should be similar to what is shown below: 
    shell> BINDIR/mysqlshow
+-----------+
| Databases |
+-----------+
| mysql     |
+-----------+

shell> BINDIR/mysqlshow mysql
Database: mysql
+--------------+
|    Tables    |
+--------------+
| columns_priv |
| db           |
| func         |
| host         |
| tables_priv  |
| user         |
+--------------+

shell> BINDIR/mysql -e "select host,db,user from db" mysql
+------+--------+------+
| host | db     | user |
+------+--------+------+
| %    | test   |      |
| %    | test_% |      |
+------+--------+------+
    There is also a benchmark suite in the `sql-bench' directory (under the MySQL installation directory) that you can use to compare how MySQL performs on different platforms. The `sql-bench/Results' directory contains the results from many runs against different databases and platforms. To run all tests, execute these commands: 
    shell> cd sql-bench
shell> run-all-tests
    If you don't have the `sql-bench' directory, you are probably using an RPM for a binary distribution. (Source distribution RPMs include the benchmark directory.) In this case, you must first install the benchmark suite before you can use it. Beginning with MySQL Version 3.22, there are benchmark RPM files named `mysql-bench-VERSION-i386.rpm' that contain benchmark code and data. If you have a source distribution, you can also run the tests in the `tests' subdirectory. For example, to run `auto_increment.tst', do this: 
    shell> BINDIR/mysql -vvf test < ./tests/auto_increment.tst
    The expected results are shown in the `./tests/auto_increment.res' file.

2.4.1 Problems Running mysql_install_db

The purpose of the mysql_install_db script is to generate new MySQL privilege tables. It will not affect any other data! It will also not do anything if you already have MySQL privilege tables installed!

If you want to re-create your privilege tables, you should take down the mysqld server, if it's running, and then do something like: 

mv mysql-data-directory/mysql mysql-data-directory/mysql-old
mysql_install_db

This section lists problems you might encounter when you run mysql_install_db:

mysql_install_db doesn't install the grant tables
You may find that mysql_install_db fails to install the grant tables and terminates after displaying the following messages: 
starting mysqld daemon with databases from XXXXXX
mysql daemon ended
In this case, you should examine the log file very carefully! The log should be located in the directory `XXXXXX' named by the error message, and should indicate why mysqld didn't start. If you don't understand what happened, include the log when you post a bug report using mysqlbug! See section 1.2.22.3 How to Report Bugs or Problems.
There is already a mysqld daemon running
In this case, you probably don't have to run mysql_install_db at all. You have to run mysql_install_db only once, when you install MySQL the first time.
Installing a second mysqld daemon doesn't work when one daemon is running
This can happen when you already have an existing MySQL installation, but want to put a new installation in a different place (for example, for testing, or perhaps you simply want to run two installations at the same time). Generally the problem that occurs when you try to run the second server is that it tries to use the same socket and port as the old one. In this case you will get the error message: Can't start server: Bind on TCP/IP port: Address already in use or Can't start server : Bind on unix socket.... See section 4.1.3 Installing Many Servers on the Same Machine.
You don't have write access to `/tmp'
If you don't have write access to create a socket file at the default place (in `/tmp') or permission to create temporary files in `/tmp,' you will get an error when running mysql_install_db or when starting or using mysqld. You can specify a different socket and temporary directory as follows: 
shell> TMPDIR=/some_tmp_dir/
shell> MYSQL_UNIX_PORT=/some_tmp_dir/mysqld.sock
shell> export TMPDIR MYSQL_UNIX_PORT
See section A.4.5 How to Protect or change the MySQL socket file `/tmp/mysql.sock'. `some_tmp_dir' should be the path to some directory for which you have write permission. See section H Environment Variables. After this you should be able to run mysql_install_db and start the server with these commands: 
shell> scripts/mysql_install_db
shell> BINDIR/safe_mysqld &
mysqld crashes immediately
If you are running RedHat Version 5.0 with a version of glibc older than 2.0.7-5, you should make sure you have installed all glibc patches! There is a lot of information about this in the MySQL mail archives. Links to the mail archives are available online at http://www.mysql.com/documentation/. Also, see section 2.6.1 Linux Notes (All Linux Versions). You can also start mysqld manually using the --skip-grant-tables option and add the privilege information yourself using mysql: 
shell> BINDIR/safe_mysqld --skip-grant-tables &
shell> BINDIR/mysql -u root mysql
From mysql, manually execute the SQL commands in mysql_install_db. Make sure you run mysqladmin flush-privileges or mysqladmin reload afterward to tell the server to reload the grant tables.

2.4.2 Problems Starting the MySQL Server

If you are going to use tables that support transactions (BDB, InnoDB), you should first create a my.cnf file and set startup options for the table types you plan to use. See section 7 MySQL Table Types.

Generally, you start the mysqld server in one of three ways:

When the mysqld daemon starts up, it changes directory to the data directory. This is where it expects to write log files and the pid (process ID) file, and where it expects to find databases.

The data directory location is hardwired in when the distribution is compiled. However, if mysqld expects to find the data directory somewhere other than where it really is on your system, it will not work properly. If you have problems with incorrect paths, you can find out what options mysqld allows and what the default path settings are by invoking mysqld with the --help option. You can override the defaults by specifying the correct pathnames as command-line arguments to mysqld. (These options can be used with safe_mysqld as well.)

Normally you should need to tell mysqld only the base directory under which MySQL is installed. You can do this with the --basedir option. You can also use --help to check the effect of changing path options (note that --help must be the final option of the mysqld command). For example: 

shell> EXECDIR/mysqld --basedir=/usr/local --help

Once you determine the path settings you want, start the server without the --help option.

Whichever method you use to start the server, if it fails to start up correctly, check the log file to see if you can find out why. Log files are located in the data directory (typically `/usr/local/mysql/data' for a binary distribution, `/usr/local/var' for a source distribution, `\mysql\data\mysql.err' on Windows.) Look in the data directory for files with names of the form `host_name.err' and `host_name.log' where host_name is the name of your server host. Then check the last few lines of these files: 

shell> tail host_name.err
shell> tail host_name.log

If you find something like the following in the log file: 

000729 14:50:10  bdb:  Recovery function for LSN 1 27595 failed
000729 14:50:10  bdb:  warning: ./test/t1.db: No such file or directory
000729 14:50:10  Can't init databases

This means that you didn't start mysqld with --bdb-no-recover and Berkeley DB found something wrong with its log files when it tried to recover your databases. To be able to continue, you should move away the old Berkeley DB log file from the database directory to some other place, where you can later examine these. The log files are named `log.0000000001', where the number will increase over time.

If you are running mysqld with BDB table support and mysqld core dumps at start this could be because of some problems with the BDB recover log. In this case you can try starting mysqld with --bdb-no-recover. If this helps, then you should remove all `log.*' files from the data directory and try starting mysqld again.

If you get the following error, it means that some other program (or another mysqld server) is already using the TCP/IP port or socket mysqld is trying to use: 

Can't start server: Bind on TCP/IP port: Address already in use
  or
Can't start server : Bind on unix socket...

Use ps to make sure that you don't have another mysqld server running. If you can't find another server running, you can try to execute the command telnet your-host-name tcp-ip-port-number and press RETURN a couple of times. If you don't get an error message like telnet: Unable to connect to remote host: Connection refused, something is using the TCP/IP port mysqld is trying to use. See section 2.4.1 Problems Running mysql_install_db and section 4.1.4 Running Multiple MySQL Servers on the Same Machine.

If mysqld is currently running, you can find out what path settings it is using by executing this command: 

shell> mysqladmin variables

or 

shell> mysqladmin -h 'your-host-name' variables

If safe_mysqld starts the server but you can't connect to it, you should make sure you have an entry in `/etc/hosts' that looks like this: 

127.0.0.1       localhost

This problem occurs only on systems that don't have a working thread library and for which MySQL must be configured to use MIT-pthreads.

If you can't get mysqld to start you can try to make a trace file to find the problem. See section G.1.2 Creating trace files.

If you are using InnoDB tables, refer to the InnoDB-specific startup options. See section 7.6.2 InnoDB startup options.

If you are using BDB (Berkeley DB) tables, you should familiarize yourself with the different BDB specific startup options. See section 7.5.3 BDB startup options.

2.4.3 Starting and Stopping MySQL Automatically

The mysql.server and safe_mysqld scripts can be used to start the server automatically at system startup time. mysql.server can also be used to stop the server.

The mysql.server script can be used to start or stop the server by invoking it with start or stop arguments: 

shell> mysql.server start
shell> mysql.server stop

mysql.server can be found in the `share/mysql' directory under the MySQL installation directory or in the `support-files' directory of the MySQL source tree.

Before mysql.server starts the server, it changes directory to the MySQL installation directory, then invokes safe_mysqld. You might need to edit mysql.server if you have a binary distribution that you've installed in a non-standard location. Modify it to cd into the proper directory before it runs safe_mysqld. If you want the server to run as some specific user, add an appropriate user line to the `/etc/my.cnf' file, as shown later in this section.

mysql.server stop brings down the server by sending a signal to it. You can take down the server manually by executing mysqladmin shutdown.

You might want to add these start and stop commands to the appropriate places in your `/etc/rc*' files when you start using MySQL for production applications. Note that if you modify mysql.server, then upgrade MySQL sometime, your modified version will be overwritten, so you should make a copy of your edited version that you can reinstall.

If your system uses `/etc/rc.local' to start external scripts, you should append the following to it: 

/bin/sh -c 'cd /usr/local/mysql ; ./bin/safe_mysqld --user=mysql &'

You can also add options for mysql.server in a global `/etc/my.cnf' file. A typical `/etc/my.cnf' file might look like this: 

[mysqld]
datadir=/usr/local/mysql/var
socket=/var/tmp/mysql.sock
port=3306
user=mysql

[mysql.server]
basedir=/usr/local/mysql

The mysql.server script understands the following options: datadir, basedir, and pid-file.

The following table shows which option groups each of the startup scripts read from option files:

Script Option groups
mysqld mysqld and server
mysql.server mysql.server, mysqld, and server
safe_mysqld mysql.server, mysqld, and server

See section 4.1.2 my.cnf Option Files.

2.5 Upgrading/Downgrading MySQL

You can always move the MySQL form and data files between different versions on the same architecture as long as you have the same base version of MySQL. The current base version is 3. If you change the character set when running MySQL (which may also change the sort order), you must run myisamchk -r -q on all tables. Otherwise your indexes may not be ordered correctly.

If you are afraid of new versions, you can always rename your old mysqld to something like mysqld-'old-version-number'. If your new mysqld then does something unexpected, you can simply shut it down and restart with your old mysqld!

When you do an upgrade you should also back up your old databases, of course.

If after an upgrade, you experience problems with recompiled client programs, like Commands out of sync or unexpected core dumps, you probably have used an old header or library file when compiling your programs. In this case you should check the date for your `mysql.h' file and `libmysqlclient.a' library to verify that they are from the new MySQL distribution. If not, please recompile your programs!

If you get some problems that the new mysqld server doesn't want to start or that you can't connect without a password, check that you don't have some old `my.cnf' file from your old installation! You can check this with: program-name --print-defaults. If this outputs anything other than the program name, you have an active my.cnf file that will affect things!

It is a good idea to rebuild and reinstall the Msql-Mysql-modules distribution whenever you install a new release of MySQL, particularly if you notice symptoms such as all your DBI scripts dumping core after you upgrade MySQL.

2.5.1 Upgrading From Version 3.22 to Version 3.23

MySQL Version 3.23 supports tables of the new MyISAM type and the old ISAM type. You don't have to convert your old tables to use these with Version 3.23. By default, all new tables will be created with type MyISAM (unless you start mysqld with the --default-table-type=isam option). You can change an ISAM table to a MyISAM table with ALTER TABLE table_name TYPE=MyISAM or the Perl script mysql_convert_table_format.

Version 3.22 and 3.21 clients will work without any problems with a Version 3.23 server.

The following lists tell what you have to watch out for when upgrading to Version 3.23:

2.5.2 Upgrading from Version 3.21 to Version 3.22

Nothing that affects compatibility has changed between Version 3.21 and 3.22. The only pitfall is that new tables that are created with DATE type columns will use the new way to store the date. You can't access these new fields from an old version of mysqld.

After installing MySQL Version 3.22, you should start the new server and then run the mysql_fix_privilege_tables script. This will add the new privileges that you need to use the GRANT command. If you forget this, you will get Access denied when you try to use ALTER TABLE, CREATE INDEX, or DROP INDEX. If your MySQL root user requires a password, you should give this as an argument to mysql_fix_privilege_tables.

The C API interface to mysql_real_connect() has changed. If you have an old client program that calls this function, you must place a 0 for the new db argument (or recode the client to send the db element for faster connections). You must also call mysql_init() before calling mysql_real_connect()! This change was done to allow the new mysql_options() function to save options in the MYSQL handler structure.

The mysqld variable key_buffer has changed names to key_buffer_size, but you can still use the old name in your startup files.

2.5.3 Upgrading from Version 3.20 to Version 3.21

If you are running a version older than Version 3.20.28 and want to switch to Version 3.21, you need to do the following:

You can start the mysqld Version 3.21 server with safe_mysqld --old-protocol to use it with clients from a Version 3.20 distribution. In this case, the new client function mysql_errno() will not return any server error, only CR_UNKNOWN_ERROR (but it works for client errors), and the server uses the old password() checking rather than the new one.

If you are NOT using the --old-protocol option to mysqld, you will need to make the following changes:

MySQL Version 3.20.28 and above can handle the new user table format without affecting clients. If you have a MySQL version earlier than Version 3.20.28, passwords will no longer work with it if you convert the user table. So to be safe, you should first upgrade to at least Version 3.20.28 and then upgrade to Version 3.21.

The new client code works with a 3.20.x mysqld server, so if you experience problems with 3.21.x, you can use the old 3.20.x server without having to recompile the clients again.

If you are not using the --old-protocol option to mysqld, old clients will issue the error message: 

ERROR: Protocol mismatch. Server Version = 10 Client Version = 9

The new Perl DBI/DBD interface also supports the old mysqlperl interface. The only change you have to make if you use mysqlperl is to change the arguments to the connect() function. The new arguments are: host, database, user, password (the user and password arguments have changed places). See section 8.2.2 The DBI Interface.

The following changes may affect queries in old applications:

2.5.4 Upgrading to Another Architecture

If you are using MySQL Version 3.23, you can copy the .frm, .MYI, and .MYD files between different architectures that support the same floating-point format. (MySQL takes care of any byte swapping issues.)

The MySQL ISAM data and index files (`.ISD' and `*.ISM', respectively) are architecture-dependent and in some cases OS-dependent. If you want to move your applications to another machine that has a different architecture or OS than your current machine, you should not try to move a database by simply copying the files to the other machine. Use mysqldump instead.

By default, mysqldump will create a file full of SQL statements. You can then transfer the file to the other machine and feed it as input to the mysql client.

Try mysqldump --help to see what options are available. If you are moving the data to a newer version of MySQL, you should use mysqldump --opt with the newer version to get a fast, compact dump.

The easiest (although not the fastest) way to move a database between two machines is to run the following commands on the machine on which the database is located: 

shell> mysqladmin -h 'other hostname' create db_name
shell> mysqldump --opt db_name \
        | mysql -h 'other hostname' db_name

If you want to copy a database from a remote machine over a slow network, you can use: 

shell> mysqladmin create db_name
shell> mysqldump -h 'other hostname' --opt --compress db_name \
        | mysql db_name

You can also store the result in a file, then transfer the file to the target machine and load the file into the database there. For example, you can dump a database to a file on the source machine like this: 

shell> mysqldump --quick db_name | gzip > db_name.contents.gz

(The file created in this example is compressed.) Transfer the file containing the database contents to the target machine and run these commands there: 

shell> mysqladmin create db_name
shell> gunzip < db_name.contents.gz | mysql db_name

You can also use mysqldump and mysqlimport to accomplish the database transfer. For big tables, this is much faster than simply using mysqldump. In the commands shown below, DUMPDIR represents the full pathname of the directory you use to store the output from mysqldump.

First, create the directory for the output files and dump the database: 

shell> mkdir DUMPDIR
shell> mysqldump --tab=DUMPDIR db_name

Then transfer the files in the DUMPDIR directory to some corresponding directory on the target machine and load the files into MySQL there: 

shell> mysqladmin create db_name           # create database
shell> cat DUMPDIR/*.sql | mysql db_name   # create tables in database
shell> mysqlimport db_name DUMPDIR/*.txt   # load data into tables

Also, don't forget to copy the mysql database, because that's where the grant tables (user, db, host) are stored. You may have to run commands as the MySQL root user on the new machine until you have the mysql database in place.

After you import the mysql database on the new machine, execute mysqladmin flush-privileges so that the server reloads the grant table information.

2.6 Operating System Specific Notes

2.6.1 Linux Notes (All Linux Versions)

The notes below regarding glibc apply only to the situation when you build MySQL yourself. If you are running Linux on an x86 machine, in most cases it is much better for you to just use our binary. We link our binaries against the best patched version of glibc we can come up with and with the best compiler options, in an attempt to make it suitable for a high-load server. So if you read the text below, and are in doubt about what you should do, try our binary first to see if it meets your needs, and worry about your own build only after you have discovered that our binary is not good enough. In that case, we would appreciate a note about it, so we can build a better binary next time. For a typical user, even for setups with a lot of concurrent connections and/or tables exceeding 2GB limit, our binary in most cases is the best choice.

MySQL uses LinuxThreads on Linux. If you are using an old Linux version that doesn't have glibc2, you must install LinuxThreads before trying to compile MySQL. You can get LinuxThreads at http://www.mysql.com/Downloads/Linux.

NOTE: We have seen some strange problems with Linux 2.2.14 and MySQL on SMP systems; If you have a SMP system, we recommend you to upgrade to Linux 2.4 ASAP! Your system will be faster and more stable by doing this!

Note that glibc versions before and including Version 2.1.1 have a fatal bug in pthread_mutex_timedwait handling, which is used when you do INSERT DELAYED. We recommend you to not use INSERT DELAYED before upgrading glibc.

If you plan to have 1000+ concurrent connections, you will need to make some changes to LinuxThreads, recompile it, and relink MySQL against the new `libpthread.a'. Increase PTHREAD_THREADS_MAX in `sysdeps/unix/sysv/linux/bits/local_lim.h' to 4096 and decrease STACK_SIZE in `linuxthreads/internals.h' to 256 KB. The paths are relative to the root of glibc Note that MySQL will not be stable with around 600-1000 connections if STACK_SIZE is the default of 2 MB.

If you have a problem with that MySQL can't open enough files, or connections, it may be that you haven't configured Linux to handle enough files.

In Linux 2.2 and forwards, you can check the number of allocated file handlers by doing: 

cat /proc/sys/fs/file-max
cat /proc/sys/fs/dquot-max 
cat /proc/sys/fs/super-max

If you have more than 16M of memory, you should add something like the following in your boot script (`/etc/rc/boot.local' on SuSE): 

echo 65536 > /proc/sys/fs/file-max
echo 8192 > /proc/sys/fs/dquot-max
echo 1024 > /proc/sys/fs/super-max

You can also run the above from the command line as root, but in this case your old limits will be used next time your computer reboots.

You should also add /etc/my.cnf: 

[safe_mysqld]
open-files-limit=8192

The above should allow MySQL to create up to 8192 connections + files.

The STACK_SIZE constant in LinuxThreads controls the spacing of thread stacks in the address space. It needs to be large enough so that there will be plenty of room for the stack of each individual thread, but small enough to keep the stack of some thread from running into the global mysqld data. Unfortunately, the Linux implementation of mmap(), as we have experimentally discovered, will successfully unmap an already mapped region if you ask it to map out an address already in use, zeroing out the data on the entire page, instead of returning an error. So, the safety of mysqld or any other threaded application depends on the "gentleman" behavior of the code that creates threads. The user must take measures to make sure the number of running threads at any time is sufficiently low for thread stacks to stay away from the global heap. With mysqld, you should enforce this "gentleman" behavior by setting a reasonable value for the max_connections variable.

If you build MySQL yourself and do not want to mess with patching LinuxThreads, you should set max_connections to a value no higher than 500. It should be even less if you have a large key buffer, large heap tables, or some other things that make mysqld allocate a lot of memory or if you are running a 2.2 kernel with a 2GB patch. If you are using our binary or RPM version 3.23.25 or later, you can safely set max_connections at 1500, assuming no large key buffer or heap tables with lots of data. The more you reduce STACK_SIZE in LinuxThreads the more threads you can safely create. We recommend the values between 128K and 256K.

If you use a lot of concurrent connections, you may suffer from a "feature" in the 2.2 kernel that penalizes a process for forking or cloning a child in an attempt to prevent a fork bomb attack. This will cause MySQL not to scale well as you increase the number of concurrent clients. On single CPU systems, we have seen this manifested in a very slow thread creation, which means it may take a long time to connect to MySQL (as long as 1 minute), and it may take just as long to shut it down. On multiple CPU systems, we have observed a gradual drop in query speed as the number of clients increases. In the process of trying to find a solution, we have received a kernel patch from one of our users, who claimed it made a lot of difference for his site. The patch is available here (http://www.mysql.com/Downloads/Patches/linux-fork.patch). We have now done rather extensive testing of this patch on both development and production systems. It has significantly improved MySQL performance without causing any problems and we now recommend it to our users who are still running high-load servers on 2.2 kernels. This issue has been fixed in the 2.4 kernel, so if you are not satisfied with the current performance of your system, rather than patching your 2.2 kernel, it might be easier to just upgrade to 2.4, which will also give you a nice SMP boost in addition to fixing this fairness bug.

We have tested MySQL on the 2.4 kernel on a 2 CPU machine and found MySQL scales MUCH better - there was virtually no slowdown on query throughput all the way up to 1000 clients, and MySQL scaling factor ( computed as the ratio of maximum throughput to the throughput with one client) was 180%. We have observed similar results on a 4-CPU system - virtually no slowdown as the number of clients was increased up to 1000, and 300% scaling factor. So for a high-load SMP server we would definitely recommend the 2.4 kernel at this point. We have discovered that it is essential to run mysqld process with the highest possible priority on the 2.4 kernel to achieve maximum performance. This can be done by adding renice -20 $$ command to safe_mysqld. In our testing on a 4-CPU machine, increasing the priority gave 60% increase in throughput with 400 clients.

We are currently also trying to collect more info on how well MySQL performs on 2.4 kernel on 4-way and 8-way systems. If you have access such a system and have done some benchmarks, please send a mail to docs@mysql.com with the results - we will include them in the manual.

There is another issue that greatly hurts MySQL performance, especially on SMP systems. The implementation of mutex in LinuxThreads in glibc-2.1 is very bad for programs with many threads that only hold the mutex for a short time. On an SMP system, ironic as it is, if you link MySQL against unmodified LinuxThreads, removing processors from the machine improves MySQL performance in many cases. We have made a patch available for glibc 2.1.3, linuxthreads-2.1-patch to correct this behavior.

With glibc-2.2.2 MySQL version 3.23.36 will use the adaptive mutex, which is much better than even the patched one in glibc-2.1.3. Be warned, however, that under some conditions, the current mutex code in glibc-2.2.2 overspins, which hurts MySQL performance. The chance of this condition can be reduced by renicing mysqld process to the highest priority. We have also been able to correct the overspin behavior with a patch, available here. It combines the correction of overspin, maximum number of threads, and stack spacing all in one. You will need to apply it in the linuxthreads directory with patch -p0 </tmp/linuxthreads-2.2.2.patch. We hope it will be included in some form in to the future releases of glibc-2.2. In any case, if you link against glibc-2.2.2 you still need to correct STACK_SIZE and PTHREAD_THREADS_MAX. We hope that the defaults will be corrected to some more acceptable values for high-load MySQL setup in the future, so that your own build can be reduced to ./configure; make; make install.

We recommend that you use the above patches to build a special static version of libpthread.a and use it only for statically linking against MySQL. We know that the patches are safe for MySQL and significantly improve its performance, but we cannot say anything about other applications. If you link other applications against the patched version of the library, or build a patched shared version and install it on your system, you are doing it at your own risk with regard to other applications that depend on LinuxThreads.

If you experience any strange problems during the installation of MySQL, or with some common utilties hanging, it is very likely that they are either library or compiler related. If this is the case, using our binary will resolve them.

One known problem with the binary distribution is that with older Linux systems that use libc (like RedHat 4.x or Slackware), you will get some non-fatal problems with hostname resolution. See section 2.6.1.1 Linux Notes for Binary Distributions.

When using LinuxThreads you will see a minimum of three processes running. These are in fact threads. There will be one thread for the LinuxThreads manager, one thread to handle connections, and one thread to handle alarms and signals.

Note that the Linux kernel and the LinuxThread library can by default only have 1024 threads. This means that you can only have up to 1021 connections to MySQL on an unpatched system. The page http://www.volano.com/linuxnotes.html contains information how to go around this limit.

If you see a dead mysqld daemon process with ps, this usually means that you have found a bug in MySQL or you have a corrupted table. See section A.4.1 What To Do If MySQL Keeps Crashing.

To get a core dump on Linux if mysqld dies with a SIGSEGV signal, you can start mysqld with the --core-file option. Note that you also probably need to raise the core file size by adding ulimit -c 1000000 to safe_mysqld or starting safe_mysqld with --core-file-sizes=1000000. See section 4.7.2 safe_mysqld, the wrapper around mysqld.

If you are linking your own MySQL client and get the error: 

ld.so.1: ./my: fatal: libmysqlclient.so.4: open failed: No such file or directory

When executing them, the problem can be avoided by one of the following methods:

If you are using the Fujitsu compiler (fcc / FCC) you will have some problems compiling MySQL because the Linux header files are very gcc oriented.

The following configure line should work with fcc/FCC: 

CC=fcc CFLAGS="-O -K fast -K lib -K omitfp -Kpreex -D_GNU_SOURCE -DCONST=const -DNO_STRTOLL_PROTO" CXX=FCC CXXFLAGS="-O -K fast -K lib  -K omitfp -K preex --no_exceptions --no_rtti -D_GNU_SOURCE -DCONST=const -Dalloca=__builtin_alloca -DNO_STRTOLL_PROTO '-D_EXTERN_INLINE=static __inline'" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-low-memory

2.6.1.1 Linux Notes for Binary Distributions

MySQL needs at least Linux Version 2.0.

The binary release is linked with -static, which means you do not normally need to worry about which version of the system libraries you have. You need not install LinuxThreads, either. A program linked with -static is slightly bigger than a dynamically linked program but also slightly faster (3-5%). One problem, however, is that you can't use user-definable functions (UDFs) with a statically linked program. If you are going to write or use UDF functions (this is something only for C or C++ programmers), you must compile MySQL yourself, using dynamic linking.

If you are using a libc-based system (instead of a glibc2 system), you will probably get some problems with hostname resolving and getpwnam() with the binary release. (This is because glibc unfortunately depends on some external libraries to resolve hostnames and getpwent(), even when compiled with -static). In this case you probably get the following error message when you run mysql_install_db: 

Sorry, the host 'xxxx' could not be looked up

or the following error when you try to run mysqld with the --user option: 

getpwnam: No such file or directory

You can solve this problem in one of the following ways:

The Linux-Intel binary and RPM releases of MySQL are configured for the highest possible speed. We are always trying to use the fastest stable compiler available.

MySQL Perl support requires Version Perl 5.004_03 or newer.

On some Linux 2.2 versions, you may get the error Resource temporarily unavailable when you do a lot of new connections to a mysqld server over TCP/IP.

The problem is that Linux has a delay between when you close a TCP/IP socket and until this is actually freed by the system. As there is only room for a finite number of TCP/IP slots, you will get the above error if you try to do too many new TCP/IP connections during a small time, like when you run the MySQL `test-connect' benchmark over TCP/IP.

We have mailed about this problem a couple of times to different Linux mailing lists but have never been able to resolve this properly.

The only known 'fix' to this problem is to use persistent connections in your clients or use sockets, if you are running the database server and clients on the same machine. We hope that the Linux 2.4 kernel will fix this problem in the future.

2.6.1.2 Linux x86 Notes

MySQL requires libc Version 5.4.12 or newer. It's known to work with libc 5.4.46. glibc Version 2.0.6 and later should also work. There have been some problems with the glibc RPMs from RedHat, so if you have problems, check whether or not there are any updates! The glibc 2.0.7-19 and 2.0.7-29 RPMs are known to work.

On some older Linux distributions, configure may produce an error like this: 

Syntax error in sched.h. Change _P to __P in the /usr/include/sched.h file.
See the Installation chapter in the Reference Manual.

Just do what the error message says and add an extra underscore to the _P macro that has only one underscore, then try again.

You may get some warnings when compiling; those shown below can be ignored: 

mysqld.cc -o objs-thread/mysqld.o
mysqld.cc: In function `void init_signals()':
mysqld.cc:315: warning: assignment of negative value `-1' to `long unsigned int'
mysqld.cc: In function `void * signal_hand(void *)':
mysqld.cc:346: warning: assignment of negative value `-1' to `long unsigned int'

In Debian GNU/Linux, if you want MySQL to start automatically when the system boots, do the following: 

shell> cp support-files/mysql.server /etc/init.d/mysql.server
shell> /usr/sbin/update-rc.d mysql.server defaults 99

mysql.server can be found in the `share/mysql' directory under the MySQL installation directory or in the `support-files' directory of the MySQL source tree.

If mysqld always core dumps when it starts up, the problem may be that you have an old `/lib/libc.a'. Try renaming it, then remove `sql/mysqld' and do a new make install and try again. This problem has been reported on some Slackware installations.

If you get the following error when linking mysqld, it means that your `libg++.a' is not installed correctly: 

/usr/lib/libc.a(putc.o): In function `_IO_putc':
putc.o(.text+0x0): multiple definition of `_IO_putc'

You can avoid using `libg++.a' by running configure like this: 

shell> CXX=gcc ./configure

2.6.1.3 Linux SPARC Notes

In some implementations, readdir_r() is broken. The symptom is that SHOW DATABASES always returns an empty set. This can be fixed by removing HAVE_READDIR_R from `config.h' after configuring and before compiling.

Some problems will require patching your Linux installation. The patch can be found at http://www.mysql.com/Downloads/patches/Linux-sparc-2.0.30.diff. This patch is against the Linux distribution `sparclinux-2.0.30.tar.gz' that is available at vger.rutgers.edu (a version of Linux that was never merged with the official 2.0.30). You must also install LinuxThreads Version 0.6 or newer.

2.6.1.4 Linux Alpha Notes

MySQL Version 3.23.12 is the first MySQL version that is tested on Linux-Alpha. If you plan to use MySQL on Linux-Alpha, you should ensure that you have this version or newer.

We have tested MySQL on Alpha with our benchmarks and test suite, and it appears to work nicely. The main thing we haven't yet had time to test is how things works with many concurrent users.

When we compiled the standard MySQL binary we are using SuSE 6.4, kernel 2.2.13-SMP, Compaq C compiler (V6.2-504) and Compaq C++ compiler (V6.3-005) on a Comaq DS20 machine with an Alpha EV6 processor.

You can find the above compilers at http://www.support.compaq.com/alpha-tools). By using these compilers, instead of gcc, we get about 9-14 % better performance with MySQL.

Note that the configure line optimized the binary for the current CPU; This means you can only use our binary if you have an Alpha EV6 processor. We also compile statically to avoid library problems. 

CC=ccc CFLAGS="-fast" CXX=cxx CXXFLAGS="-fast -noexceptions -nortti" ./configure --prefix=/usr/local/mysql --disable-shared --with-extra-charsets=complex --enable-thread-safe-client --with-mysqld-ldflags=-non_shared --with-client-ldflags=-non_shared

If you want to use egcs the following configure line worked for us: 

CFLAGS="-O3 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --disable-shared

Some known problems when running MySQL on Linux-Alpha:

2.6.1.5 Linux PowerPC Notes

MySQL should work on MkLinux with the newest glibc package (tested with glibc 2.0.7).

2.6.1.6 Linux MIPS Notes

To get MySQL to work on Qube2, (Linux Mips), you need the newest glibc libraries (glibc-2.0.7-29C2 is known to work). You must also use the egcs C++ compiler (egcs-1.0.2-9, gcc 2.95.2 or newer).

2.6.1.7 Linux IA64 Notes

To get MySQL to compile on Linux Ia64, we use the following compile line: Using gcc-2.96: 

CC=gcc CFLAGS="-O3 -fno-omit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fno-omit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql "--with-comment=Official MySQL binary" --with-extra-charsets=complex

On Ia64 the MySQL client binaries are using shared libraries. This means that if you install our binary distribution in some other place than `/usr/local/mysql' you need to either modify `/etc/ld.so.conf' or add the path to the directory where you have `libmysqlclient.so' to the LD_LIBRARY_PATH environment variable.

See section A.3.1 Problems When Linking with the MySQL Client Library.

2.6.2 Windows Notes

This section describes installation and use of MySQL on Windows. This information is also provided in the `README' file that comes with the MySQL Windows distribution.

2.6.2.1 Starting MySQL on Windows 95 or Windows 98

MySQL uses TCP/IP to connect a client to a server. (This will allow any machine on your network to connect to your MySQL server.) Because of this, you must install TCP/IP on your machine before starting MySQL. You can find TCP/IP on your Windows CD-ROM.

Note that if you are using an old Win95 release (for example OSR2), it's likely that you have an old Winsock package! MySQL requires Winsock 2! You can get the newest Winsock from http://www.microsoft.com/. Win98 has the new Winsock 2 library, so the above doesn't apply for Win98.

To start the mysqld server, you should start an MS-DOS window and type: 

C:\> C:\mysql\bin\mysqld

This will start mysqld in the background without a window.

You can kill the MySQL server by executing: 

C:\> C:\mysql\bin\mysqladmin -u root shutdown

Note that Win95 and Win98 don't support creation of named pipes. On Win95 and Win98, you can only use named pipes to connect to a remote MySQL server running on a Windows NT server host. (The MySQL server must also support named pipes, of course. For example, using mysqld-opt under NT will not allow named pipe connections. You should use either mysqld-nt or mysqld-max-nt.)

If mysqld doesn't start, please check the `\mysql\data\mysql.err' file to see if the server wrote any message there to indicate the cause of the problem. You can also try to start the server with mysqld --standalone; In this case, you may get some useful information on the screen that may help solve the problem.

The last option is to start mysqld with --standalone --debug. In this case mysqld will write a log file `C:\mysqld.trace' that should contain the reason why mysqld doesn't start. See section G.1.2 Creating trace files.

2.6.2.2 Starting MySQL on Windows NT or Windows 2000

The Win95/Win98 section also applies to MySQL on NT/Win2000, with the following differences:

To get MySQL to work with TCP/IP on NT, you must install service pack 3 (or newer)!

Note that everything in the following that applies for NT also applies for Win2000!

For NT/Win2000, the server name is mysqld-nt. Normally you should install MySQL as a service on NT/Win2000: 

C:\> C:\mysql\bin\mysqld-nt --install

or 

C:\> C:\mysql\bin\mysqld-max-nt --install

(Under Windows NT, you can actually install any of the server binaries as a service, but only those having names that end with -nt.exe provide support for named pipes.)

You can start and stop the MySQL service with these commands: 

C:\> NET START mysql
C:\> NET STOP mysql

Note that in this case you can't use any other options for mysqld-nt!

You can also run mysqld-nt as a stand-alone program on NT if you need to start mysqld-nt with any options! If you start mysqld-nt without options on NT, mysqld-nt tries to start itself as a service with the default service options. If you have stopped mysqld-nt, you have to start it with NET START mysql.

The service is installed with the name MySQL. Once installed, it must be started using the Services Control Manager (SCM) Utility found in the Control Panel, or by using the NET START MySQL command. If any options are desired, they must be specified as ``Startup parameters'' in the SCM utility before you start the MySQL service. Once running, mysqld-nt can be stopped using mysqladmin, or from the SCM utility or by using the command NET STOP MySQL. If you use SCM to stop mysqld-nt, there is a strange message from SCM about mysqld shutdown normally. When run as a service, mysqld-nt has no access to a console and so no messages can be seen.

On NT you can get the following service error messages:

Permission Denied Means that it cannot find mysqld-nt.exe.
Cannot Register Means that the path is incorrect.
Failed to install service. Means that the service is already installed or that the Service Control Manager is in bad state.

If you have problems installing mysqld-nt as a service, try starting it with the full path: 

C:\> C:\mysql\bin\mysqld-nt --install

If this doesn't work, you can get mysqld-nt to start properly by fixing the path in the registry!

If you don't want to start mysqld-nt as a service, you can start it as follows: 

C:\> C:\mysql\bin\mysqld-nt --standalone

or 

C:\> C:\mysql\bin\mysqld --standalone --debug

The last version gives you a debug trace in `C:\mysqld.trace'. See section G.1.2 Creating trace files.

2.6.2.3 Running MySQL on Windows

MySQL supports TCP/IP on all Windows platforms and named pipes on NT. The default is to use named pipes for local connections on NT and TCP/IP for all other cases if the client has TCP/IP installed. The host name specifies which protocol is used:

Host name
Protocol
NULL (none) On NT, try named pipes first; if that doesn't work, use TCP/IP. On Win95/Win98, TCP/IP is used.
. Named pipes
localhost TCP/IP to current host
hostname TCP/IP

You can force a MySQL client to use named pipes by specifying the --pipe option or by specifying . as the host name. Use the --socket option to specify the name of the pipe.

You can test whether or not MySQL is working by executing the following commands: 

C:\> C:\mysql\bin\mysqlshow
C:\> C:\mysql\bin\mysqlshow -u root mysql
C:\> C:\mysql\bin\mysqladmin version status proc
C:\> C:\mysql\bin\mysql test

If mysqld is slow to answer to connections on Win95/Win98, there is probably a problem with your DNS. In this case, start mysqld with --skip-name-resolve and use only localhost and IP numbers in the MySQL grant tables. You can also avoid DNS when connecting to a mysqld-nt MySQL server running on NT by using the --pipe argument to specify use of named pipes. This works for most MySQL clients.

There are two versions of the MySQL command-line tool:
mysql Compiled on native Windows, which offers very limited text editing capabilities.
mysqlc Compiled with the Cygnus GNU compiler and libraries, which offers readline editing.

If you want to use mysqlc.exe, you must copy `C:\mysql\lib\cygwinb19.dll' to your Windows system directory (`\windows\system' or similar place).

The default privileges on Windows give all local users full privileges to all databases without specifying a password. To make MySQL more secure, you should set a password for all users and remove the row in the mysql.user table that has Host='localhost' and User=''.

You should also add a password for the root user. The following example starts by removing the anonymous user that can be used by anyone to access the test database, then sets a root user password: 

C:\> C:\mysql\bin\mysql mysql
mysql> DELETE FROM user WHERE Host='localhost' AND User='';
mysql> QUIT
C:\> C:\mysql\bin\mysqladmin reload
C:\> C:\mysql\bin\mysqladmin -u root password your_password

After you've set the password, if you want to take down the mysqld server, you can do so using this command: 

C:\> mysqladmin --user=root --password=your_password shutdown

If you are using the old shareware version of MySQL Version 3.21 under Windows, the above command will fail with an error: parse error near 'SET OPTION password'. The fix is in to upgrade to the current MySQL version, which is freely available.

With the current MySQL versions you can easily add new users and change privileges with GRANT and REVOKE commands. See section 4.3.1 GRANT and REVOKE Syntax.

2.6.2.4 Connecting to a Remote MySQL from Windows with SSH

Here is a note about how to connect to get a secure connection to remote MySQL server with SSH (by David Carlson dcarlson@mplcomm.com):

You should now have an ODBC connection to MySQL, encrypted using SSH.

2.6.2.5 Splitting Data Across Different Disks on Windows

Beginning with MySQL Version 3.23.16, the mysqld-max and mysql-max-nt servers in the MySQL distribution are compiled with the -DUSE_SYMDIR option. This allows you to put a database on different disk by adding a symbolic link to it (in a manner similar to the way that symbolic links work on Unix).

On Windows, you make a symbolic link to a database by creating a file that contains the path to the destination directory and saving this in the `mysql_data' directory under the filename `database.sym'. Note that the symbolic link will be used only if the directory `mysql_data_dir\database' doesn't exist.

For example, if the MySQL data directory is `C:\mysql\data' and you want to have database foo located at `D:\data\foo', you should create the file `C:\mysql\data\foo.sym' that contains the text D:\data\foo\. After that, all tables created in the database foo will be created in `D:\data\foo'.

Note that because of the speed penalty you get when opening every table, we have not enabled this by default even if you have compiled MySQL with support for this. To enable symlinks you should put in your my.cnf or my.ini file the following entry: 

[mysqld]
use-symbolic-links

In MySQL 4.0 we will enable symlinks by default. Then you should instead use the skip-symlink option if you want to disable this.

2.6.2.6 Compiling MySQL Clients on Windows

In your source files, you should include `windows.h' before you include `mysql.h': 

#if defined(_WIN32) || defined(_WIN64)
#include <windows.h>
#endif
#include <mysql.h>

You can either link your code with the dynamic `libmysql.lib' library, which is just a wrapper to load in `libmysql.dll' on demand, or link with the static `mysqlclient.lib' library.

Note that as the mysqlclient libraries are compiled as threaded libraries, you should also compile your code to be multi-threaded!

2.6.2.7 MySQL-Windows Compared to Unix MySQL

MySQL-Windows has by now proven itself to be very stable. This version of MySQL has the same features as the corresponding Unix version with the following exceptions:

Win95 and threads
Win95 leaks about 200 bytes of main memory for each thread creation. Each connection in MySQL creates a new thread, so you shouldn't run mysqld for an extended time on Win95 if your server handles many connections! WinNT and Win98 don't suffer from this bug.
Concurrent reads
MySQL depends on the pread() and pwrite() calls to be able to mix INSERT and SELECT. Currently we use mutexes to emulate pread()/pwrite(). We will, in the long run, replace the file level interface with a virtual interface so that we can use the readfile()/writefile() interface on NT to get more speed. The current implementation limits the number of open files MySQL can use to 1024, which means that you will not be able to run as many concurrent threads on NT as on Unix.
Blocking read
MySQL uses a blocking read for each connection. This means that: We plan to fix this problem when our Windows developers have figured out a nice workaround.
UDF functions
For the moment, MySQL-Windows does not support user-definable functions.
DROP DATABASE
You can't drop a database that is in use by some thread.
Killing MySQL from the task manager
You can't kill MySQL from the task manager or with the shutdown utility in Win95. You must take it down with mysqladmin shutdown.
Case-insensitive names
Filenames are case insensitive on Windows, so database and table names are also case insensitive in MySQL for Windows. The only restriction is that database and table names must be specified using the same case throughout a given statement. See section 6.1.3 Case Sensitivity in Names.
The `\' directory character
Pathname components in Win95 are separated by the `\' character, which is also the escape character in MySQL. If you are using LOAD DATA INFILE or SELECT ... INTO OUTFILE, you must double the `\' character: 
mysql> LOAD DATA INFILE "C:\\tmp\\skr.txt" INTO TABLE skr;
mysql> SELECT * INTO OUTFILE 'C:\\tmp\\skr.txt' FROM skr;
Alternatively, use Unix style filenames with `/' characters: 
mysql> LOAD DATA INFILE "C:/tmp/skr.txt" INTO TABLE skr;
mysql> SELECT * INTO OUTFILE 'C:/tmp/skr.txt' FROM skr;
Can't open named pipe error
If you use a MySQL 3.22 version on NT with the newest mysql-clients you will get the following error: 
error 2017: can't open named pipe to host: . pipe...
This is because the release version of MySQL uses named pipes on NT by default. You can avoid this error by using the --host=localhost option to the new MySQL clients or create an option file `C:\my.cnf' that contains the following information: 
[client]
host = localhost
Access denied for user error
If you get the error Access denied for user: 'some-user@unknown' to database 'mysql' when accessing a MySQL server on the same machine, this means that MySQL can't resolve your host name properly. To fix this, you should create a file `\windows\hosts' with the following information: 
127.0.0.1       localhost
ALTER TABLE
While you are executing an ALTER TABLE statement, the table is locked from usage by other threads. This has to do with the fact that on Windows, you can't delete a file that is in use by another threads. (In the future, we may find some way to work around this problem.)
DROP TABLE on a table that is in use by a MERGE table will not work
The MERGE handler does its table mapping hidden from MySQL. Because Windows doesn't allow you to drop files that are open, you first must flush all MERGE tables (with FLUSH TABLES) or drop the MERGE table before dropping the table. We will fix this at the same time we introduce VIEWs.

Here are some open issues for anyone who might want to help us with the Windows release:

Other Windows-specific issues are described in the `README' file that comes with the MySQL-Windows distribution.

2.6.3 Solaris Notes

On Solaris, you may run into trouble even before you get the MySQL distribution unpacked! Solaris tar can't handle long file names, so you may see an error like this when you unpack MySQL: 

x mysql-3.22.12-beta/bench/Results/ATIS-mysql_odbc-NT_4.0-cmp-db2,informix,ms-sql,mysql,oracle,solid,sybase, 0 bytes, 0 tape blocks
tar: directory checksum error

In this case, you must use GNU tar (gtar) to unpack the distribution. You can find a precompiled copy for Solaris at http://www.mysql.com/Downloads/.

Sun native threads work only on Solaris 2.5 and higher. For Version 2.4 and earlier, MySQL will automatically use MIT-pthreads. See section 2.3.6 MIT-pthreads Notes.

If you get the following error from configure: 

checking for restartable system calls... configure: error can not run test
programs while cross compiling

This means that you have something wrong with your compiler installation! In this case you should upgrade your compiler to a newer version. You may also be able to solve this problem by inserting the following row into the `config.cache' file: 

ac_cv_sys_restartable_syscalls=${ac_cv_sys_restartable_syscalls='no'}

If you are using Solaris on a SPARC, the recommended compiler is gcc 2.95.2. You can find this at http://gcc.gnu.org/. Note that egcs 1.1.1 and gcc 2.8.1 don't work reliably on SPARC!

The recommended configure line when using gcc 2.95.2 is: 

CC=gcc CFLAGS="-O3" \
CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" \
./configure --prefix=/usr/local/mysql --with-low-memory --enable-assembler

If you have a ultra sparc, you can get 4 % more performance by adding "-mcpu=v8 -Wa,-xarch=v8plusa" to CFLAGS and CXXFLAGS.

If you have Sun Workshop (Fortre) 5.3 (or newer) compiler, you can run configure like this: 

CC=cc CFLAGS="-Xa -fast -xO4 -native -xstrconst -mt" \
CXX=CC CXXFLAGS="-noex -xO4 -mt" \
./configure --prefix=/usr/local/mysql --enable-assembler

In the MySQL benchmarks, we got a 6 % speedup on an Ultrasparc when using Sun Workshop 5.3 compared to using gcc with -mcpu flags.

If you get a problem with fdatasync or sched_yield, you can fix this by adding LIBS=-lrt to the configure line

The following paragraph is only relevant for older compilers than WorkShop 5.3:

You may also have to edit the configure script to change this line: 

#if !defined(__STDC__) || __STDC__ != 1

to this: 

#if !defined(__STDC__)

If you turn on __STDC__ with the -Xc option, the Sun compiler can't compile with the Solaris `pthread.h' header file. This is a Sun bug (broken compiler or broken include file).

If mysqld issues the error message shown below when you run it, you have tried to compile MySQL with the Sun compiler without enabling the multi-thread option (-mt): 

libc internal error: _rmutex_unlock: rmutex not held

Add -mt to CFLAGS and CXXFLAGS and try again.

If you get the following error when compiling MySQL with gcc, it means that your gcc is not configured for your version of Solaris: 

shell> gcc -O3 -g -O2 -DDBUG_OFF  -o thr_alarm ...
./thr_alarm.c: In function `signal_hand':
./thr_alarm.c:556: too many arguments to function `sigwait'

The proper thing to do in this case is to get the newest version of gcc and compile it with your current gcc compiler! At least for Solaris 2.5, almost all binary versions of gcc have old, unusable include files that will break all programs that use threads (and possibly other programs)!

Solaris doesn't provide static versions of all system libraries (libpthreads and libdl), so you can't compile MySQL with --static. If you try to do so, you will get the error: 

ld: fatal: library -ldl: not found

or

undefined reference to `dlopen'

or

cannot find -lrt

If too many processes try to connect very rapidly to mysqld, you will see this error in the MySQL log: 

Error in accept: Protocol error

You might try starting the server with the --set-variable back_log=50 option as a workaround for this. See section 4.1.1 mysqld Command-line Options.

If you are linking your own MySQL client, you might get the following error when you try to execute it: 

ld.so.1: ./my: fatal: libmysqlclient.so.#: open failed: No such file or directory

The problem can be avoided by one of the following methods:

When using the --with-libwrap configure option, you must also include the libraries that `libwrap.a' needs: 

--with-libwrap="/opt/NUtcpwrapper-7.6/lib/libwrap.a -lnsl -lsocket

If you have problems with configure trying to link with -lz and you don't have zlib installed, you have two options:

If you are using gcc and have problems with loading UDF functions into MySQL, try adding -lgcc to the link line for the UDF function.

If you would like MySQL to start automatically, you can copy `support-files/mysql.server' to `/etc/init.d' and create a symbolic link to it named `/etc/rc3.d/S99mysql.server'.

2.6.3.1 Solaris 2.7/2.8 Notes

You can normally use a Solaris 2.6 binary on Solaris 2.7 and 2.8. Most of the Solaris 2.6 issues also apply for Solaris 2.7 and 2.8.

Note that MySQL Version 3.23.4 and above should be able to autodetect new versions of Solaris and enable workarounds for the following problems!

Solaris 2.7 / 2.8 has some bugs in the include files. You may see the following error when you use gcc: 

/usr/include/widec.h:42: warning: `getwc' redefined
/usr/include/wchar.h:326: warning: this is the location of the previous
definition

If this occurs, you can do the following to fix the problem:

Copy /usr/include/widec.h to .../lib/gcc-lib/os/gcc-version/include and change line 41 from: 

#if     !defined(lint) && !defined(__lint)

to

#if     !defined(lint) && !defined(__lint) && !defined(getwc)

Alternatively, you can edit `/usr/include/widec.h' directly. Either way, after you make the fix, you should remove `config.cache' and run configure again!

If you get errors like this when you run make, it's because configure didn't detect the `curses.h' file (probably because of the error in `/usr/include/widec.h'): 

In file included from mysql.cc:50:
/usr/include/term.h:1060: syntax error before `,'
/usr/include/term.h:1081: syntax error before `;'

The solution to this is to do one of the following:

If you get a problem that your linker can't find -lz when linking your client program, the problem is probably that your `libz.so' file is installed in `/usr/local/lib'. You can fix this by one of the following methods:

2.6.3.2 Solaris x86 Notes

On Solaris 2.8 on x86, mysqld will core dump if you run 'strip' in.

If you are using gcc or egcs on Solaris x86 and you experience problems with core dumps under load, you should use the following configure command: 

CC=gcc CFLAGS="-O3 -fomit-frame-pointer -DHAVE_CURSES_H" \
CXX=gcc \
CXXFLAGS="-O3 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti -DHAVE_CURSES_H" \
./configure --prefix=/usr/local/mysql

This will avoid problems with the libstdc++ library and with C++ exceptions.

If this doesn't help, you should compile a debug version and run it with a trace file or under gdb. See section G.1.3 Debugging mysqld under gdb.

2.6.4 BSD Notes

2.6.4.1 FreeBSD Notes

FreeBSD 3.x is recommended for running MySQL since the thread package is much more integrated.

The easiest and therefor the preferred way to install is to use the mysql-server and mysql-client ports available on http://www.freebsd.org.

Using these gives you:

It is recommended you use MIT-pthreads on FreeBSD 2.x and native threads on Versions 3 and up. It is possible to run with native threads on some late 2.2.x versions but you may encounter problems shutting down mysqld.

The MySQL `Makefile's require GNU make (gmake) to work. If you want to compile MySQL you need to install GNU make first.

Be sure to have your name resolver setup correct. Otherwise you may experience resolver delays or failures when connecting to mysqld.

Make sure that the localhost entry in the `/etc/hosts' file is correct (otherwise you will have problems connecting to the database). The `/etc/hosts' file should start with a line: 

127.0.0.1       localhost localhost.your.domain

If you notice that configure will use MIT-pthreads, you should read the MIT-pthreads notes. See section 2.3.6 MIT-pthreads Notes.

If you get an error from make install that it can't find `/usr/include/pthreads', configure didn't detect that you need MIT-pthreads. This is fixed by executing these commands: 

shell> rm config.cache
shell> ./configure --with-mit-threads

FreeBSD is also known to have a very low default file handle limit. See section A.2.16 File Not Found. Uncomment the ulimit -n section in safe_mysqld or raise the limits for the mysqld user in /etc/login.conf (and rebuild it with cap_mkdb /etc/login.conf). Also be sure you set the appropriate class for this user in the password file if you are not using the default (use: chpass mysqld-user-name). See section 4.7.2 safe_mysqld, the wrapper around mysqld.

If you get problems with the current date in MySQL, setting the TZ variable will probably help. See section H Environment Variables.

To get a secure and stable system you should only use FreeBSD kernels that are marked -STABLE.

2.6.4.2 NetBSD notes

To compile on NetBSD you need GNU make. Otherwise the compile will crash when make tries to run lint on C++ files.

2.6.4.3 OpenBSD Notes

2.6.4.4 OpenBSD 2.5 Notes

On OpenBSD Version 2.5, you can compile MySQL with native threads with the following options: 

CFLAGS=-pthread CXXFLAGS=-pthread ./configure --with-mit-threads=no

2.6.4.5 OpenBSD 2.8 Notes

Our users have reported that OpenBSD 2.8 has a threading bug which causes problems with MySQL. The OpenBSD Developers have fixed the problem, but as of January 25th, 2001, it's only available in the ``-current'' branch. The symptoms of this threading bug are: slow response, high load, high CPU usage, and crashes.

2.6.4.6 BSD/OS Notes

2.6.4.7 BSD/OS Version 2.x Notes

If you get the following error when compiling MySQL, your ulimit value for virtual memory is too low: 

item_func.h: In method `Item_func_ge::Item_func_ge(const Item_func_ge &)':
item_func.h:28: virtual memory exhausted
make[2]: *** [item_func.o] Error 1

Try using ulimit -v 80000 and run make again. If this doesn't work and you are using bash, try switching to csh or sh; some BSDI users have reported problems with bash and ulimit.

If you are using gcc, you may also use have to use the --with-low-memory flag for configure to be able to compile `sql_yacc.cc'.

If you get problems with the current date in MySQL, setting the TZ variable will probably help. See section H Environment Variables.

2.6.4.8 BSD/OS Version 3.x Notes

Upgrade to BSD/OS Version 3.1. If that is not possible, install BSDIpatch M300-038.

Use the following command when configuring MySQL: 

shell> env CXX=shlicc++ CC=shlicc2 \
       ./configure \
           --prefix=/usr/local/mysql \
           --localstatedir=/var/mysql \
           --without-perl \
           --with-unix-socket-path=/var/mysql/mysql.sock

The following is also known to work: 

shell> env CC=gcc CXX=gcc CXXFLAGS=-O3 \
       ./configure \
           --prefix=/usr/local/mysql \
           --with-unix-socket-path=/var/mysql/mysql.sock

You can change the directory locations if you wish, or just use the defaults by not specifying any locations.

If you have problems with performance under heavy load, try using the --skip-thread-priority option to mysqld! This will run all threads with the same priority; on BSDI Version 3.1, this gives better performance (at least until BSDI fixes their thread scheduler).

If you get the error virtual memory exhausted while compiling, you should try using ulimit -v 80000 and run make again. If this doesn't work and you are using bash, try switching to csh or sh; some BSDI users have reported problems with bash and ulimit.

2.6.4.9 BSD/OS Version 4.x Notes

BSDI Version 4.x has some thread-related bugs. If you want to use MySQL on this, you should install all thread-related patches. At least M400-023 should be installed.

On some BSDI Version 4.x systems, you may get problems with shared libraries. The symptom is that you can't execute any client programs, for example, mysqladmin. In this case you need to reconfigure not to use shared libraries with the --disable-shared option to configure.

Some customers have had problems on BSDI 4.0.1 that the mysqld binary after a while can't open tables. This is because some library/system related bug causes mysqld to change current directory without asking for this!

The fix is to either upgrade to 3.23.34 or after running configure remove the line #define HAVE_REALPATH from config.h before running make.

Note that the above means that you can't symbolic link a database directories to another database directory or symbolic link a table to another database on BSDI! (Making a symbolic link to another disk is ok).

2.6.5 Mac OS X Notes

2.6.5.1 Mac OS X Public Beta

MySQL should work without any problems on Mac OS X Public Beta (Darwin). You don't need the pthread patches for this OS!

2.6.5.2 Mac OS X Server

Before trying to configure MySQL on Mac OS X server you must first install the pthread package from http://www.prnet.de/RegEx/mysql.html.

Our binary for Mac OS X is compiled on Rhapsody 5.5 with the following configure line: 

CC=gcc CFLAGS="-O2 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O2 -fomit-frame-pointer" ./configure --prefix=/usr/local/mysql "--with-comment=Official MySQL binary" --with-extra-charsets=complex  --disable-shared

You might want to also add aliases to your shell's resource file to access mysql and mysqladmin from the command line: 

alias mysql '/usr/local/mysql/bin/mysql'
alias mysqladmin '/usr/local/mysql/bin/mysqladmin'

2.6.6 Other Unix Notes

2.6.6.1 HP-UX Notes for Binary Distributions

Some of the binary distributions of MySQL for HP-UX is distributed as an HP depot file and as a tar file. To use the depot file you must be running at least HP-UX 10.x to have access to HP's software depot tools.

The HP version of MySQL was compiled on an HP 9000/8xx server under HP-UX 10.20, and uses MIT-pthreads. It is known to work well under this configuration. MySQL Version 3.22.26 and newer can also be built with HP's native thread package.

Other configurations that may work:

The following configurations almost definitely won't work:

To install the distribution, use one of the commands below, where /path/to/depot is the full pathname of the depot file:

The depot places binaries and libraries in `/opt/mysql' and data in `/var/opt/mysql'. The depot also creates the appropriate entries in `/etc/init.d' and `/etc/rc2.d' to start the server automatically at boot time. Obviously, this entails being root to install.

To install the HP-UX tar.gz distribution, you must have a copy of GNU tar.

2.6.6.2 HP-UX Version 10.20 Notes

There are a couple of small problems when compiling MySQL on HP-UX. We recommend that you use gcc instead of the HP-UX native compiler, because gcc produces better code!

We recommend using gcc 2.95 on HP-UX. Don't use high optimization flags (like -O6) as this may not be safe on HP-UX.

Note that MIT-pthreads can't be compiled with the HP-UX compiler because it can't compile .S (assembler) files.

The following configure line should work: 

CFLAGS="-DHPUX -I/opt/dce/include" CXXFLAGS="-DHPUX -I/opt/dce/include -felide-constructors -fno-exceptions -fno-rtti" CXX=gcc ./configure --with-pthread --with-named-thread-libs='-ldce' --prefix=/usr/local/mysql --disable-shared

If you are compiling gcc 2.95 yourself, you should NOT link it with the DCE libraries (libdce.a or libcma.a) if you want to compile MySQL with MIT-pthreads. If you mix the DCE and MIT-pthreads packages you will get a mysqld to which you cannot connect. Remove the DCE libraries while you compile gcc 2.95!

2.6.6.3 HP-UX Version 11.x Notes

For HP-UX Version 11.x we recommend MySQL Version 3.23.15 or later.

Because of some critical bugs in the standard HP-UX libraries, you should install the following patches before trying to run MySQL on HP-UX 11.0: 

PHKL_22840 Streams cumulative
PHNE_22397 ARPA cumulative

This will solve a problem that one gets EWOULDBLOCK from recv() and EBADF from accept() in threaded applications.

If you are using gcc 2.95.1 on an unpatched HP-UX 11.x system, you will get the error: 

In file included from /usr/include/unistd.h:11,
                 from ../include/global.h:125,
                 from mysql_priv.h:15,
                 from item.cc:19:
/usr/include/sys/unistd.h:184: declaration of C function ...
/usr/include/sys/pthread.h:440: previous declaration ...
In file included from item.h:306,
                 from mysql_priv.h:158,
                 from item.cc:19:

The problem is that HP-UX doesn't define pthreads_atfork() consistently. It has conflicting prototypes in `/usr/include/sys/unistd.h':184 and `/usr/include/sys/pthread.h':440 (details below).

One solution is to copy `/usr/include/sys/unistd.h' into `mysql/include' and edit `unistd.h' and change it to match the definition in `pthread.h'. Here's the diff: 

183,184c183,184
<      extern int pthread_atfork(void (*prepare)(), void (*parent)(),
<                                                void (*child)());
---
>      extern int pthread_atfork(void (*prepare)(void), void (*parent)(void),
>                                                void (*child)(void));

After this, the following configure line should work: 

CFLAGS="-fomit-frame-pointer -O3 -fpic" CXX=gcc CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti -O3" ./configure --prefix=/usr/local/mysql --disable-shared

Here is some information that a HP-UX Version 11.x user sent us about compiling MySQL with HP-UX:x compiler: 

 Environment:
      proper compilers.
         setenv CC cc
         setenv CXX aCC
      flags
         setenv CFLAGS -D_REENTRANT
         setenv CXXFLAGS -D_REENTRANT
         setenv CPPFLAGS -D_REENTRANT
     % aCC -V
     aCC: HP ANSI C++ B3910B X.03.14.06
     % cc -V /tmp/empty.c
     cpp.ansi: HP92453-01 A.11.02.00 HP C Preprocessor (ANSI)
     ccom: HP92453-01 A.11.01.00 HP C Compiler
     cc: "/tmp/empty.c", line 1: warning 501: Empty source file.

  configuration:
     ./configure  --with-pthread        \
     --prefix=/source-control/mysql     \
     --with-named-thread-libs=-lpthread \
     --with-low-memory

    added '#define _CTYPE_INCLUDED' to include/m_ctype.h. This
    symbol is the one defined in HP's /usr/include/ctype.h:

     /* Don't include std ctype.h when this is included */
     #define _CTYPE_H
     #define __CTYPE_INCLUDED
     #define _CTYPE_INCLUDED
     #define _CTYPE_USING   /* Don't put names in global namespace. */

If you get the following error from configure 

checking for cc option to accept ANSI C... no
configure: error: MySQL requires a ANSI C compiler (and a C++ compiler). Try gcc. See the Installation chapter in the Reference Manual.

Check that you don't have the path to the K&R compiler before the path to the HP-UX C and C++ compiler.

2.6.6.4 IBM-AIX notes

Automatic detection of xlC is missing from Autoconf, so a configure command something like this is needed when compiling MySQL (This example uses the IBM compiler): 

export CC="xlc_r -ma -O3 -qstrict -qoptimize=3 -qmaxmem=8192 "
export CXX="xlC_r -ma -O3 -qstrict -qoptimize=3 -qmaxmem=8192"
export CFLAGS="-I /usr/local/include"
export LDLFAGS="-L /usr/local/lib"
export CPPFLAGS=$CFLAGS
export CXXFLAGS=$CFLAGS

./configure --prefix=/usr/local \
		--localstatedir=/var/mysql \
		--sysconfdir=/etc/mysql \
		--sbindir='/usr/local/bin' \
		--libexecdir='/usr/local/bin' \
		--enable-thread-safe-client \
		--enable-large-files

Above are the options used to compile the MySQL distribution that can be found at http://www-frec.bull.com/.

If you change the -O3 to -O2 in the above configure line, you must also remove the -qstrict option (this is a limitation in the IBM C compiler).

If you are using gcc or egcs to compile MySQL, you MUST use the -fno-exceptions flag, as the exception handling in gcc/egcs is not thread safe! (This is tested with egcs 1.1.). There are also some known problems with IBM's assembler, which may cause it to generate bad code when used with gcc.

We recommend the following configure line with egcs and gcc 2.95 on AIX: 

CC="gcc -pipe -mcpu=power -Wa,-many" \
CXX="gcc -pipe -mcpu=power -Wa,-many" \
CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti" \
./configure --prefix=/usr/local/mysql --with-low-memory

The -Wa,-many is necessary for the compile to be successful. IBM is aware of this problem but is in to hurry to fix it because of the workaround available. We don't know if the -fno-exceptions is required with gcc 2.95, but as MySQL doesn't use exceptions and the above option generates faster code, we recommend that you should always use this option with egcs / gcc.

If you get a problem with assembler code try changing the -mcpu=xxx to match your cpu. Typically power2, power, or powerpc may need to be used, alternatively you might need to use 604 or 604e. I'm not positive but I would think using "power" would likely be safe most of the time, even on a power2 machine.

If you don't know what your cpu is then do a "uname -m", this will give you back a string that looks like "000514676700", with a format of xxyyyyyymmss where xx and ss are always 0's, yyyyyy is a unique system id and mm is the id of the CPU Planar. A chart of these values can be found at http://www.rs6000.ibm.com/doc_link/en_US/a_doc_lib/cmds/aixcmds5/uname.htm. This will give you a machine type and a machine model you can use to determine what type of cpu you have.

If you have problems with signals (MySQL dies unexpectedly under high load) you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with: 

shell> CFLAGS=-DDONT_USE_THR_ALARM CXX=gcc \
       CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti -DDONT_USE_THR_ALARM" \
       ./configure --prefix=/usr/local/mysql --with-debug --with-low-memory

This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are ``sleeping'' on a connection with mysqladmin kill or mysqladmin shutdown. Instead, the client will die when it issues its next command.

On some versions of AIX, linking with libbind.a makes getservbyname core dump. This is an AIX bug and should be reported to IBM.

For AIX 4.2.1 and gcc you have to do the following changes.

After configuring, edit `config.h' and `include/my_config.h' and change the line that says 

#define HAVE_SNPRINTF 1

to 

#undef HAVE_SNPRINTF

And finally, in `mysqld.cc' you need to add a prototype for initgoups. 

#ifdef _AIX41
extern "C" int initgroups(const char *,int);
#endif

2.6.6.5 SunOS 4 Notes

On SunOS 4, MIT-pthreads is needed to compile MySQL, which in turn means you will need GNU make.

Some SunOS 4 systems have problems with dynamic libraries and libtool. You can use the following configure line to avoid this problem: 

shell> ./configure --disable-shared --with-mysqld-ldflags=-all-static

When compiling readline, you may get warnings about duplicate defines. These may be ignored.

When compiling mysqld, there will be some implicit declaration of function warnings. These may be ignored.

2.6.6.6 Alpha-DEC-UNIX Notes (Tru64)

If you are using egcs 1.1.2 on Digital Unix, you should upgrade to gcc 2.95.2, as egcs on DEC has some serious bugs!

When compiling threaded programs under Digital Unix, the documentation recommends using the -pthread option for cc and cxx and the libraries -lmach -lexc (in addition to -lpthread). You should run configure something like this: 

CC="cc -pthread" CXX="cxx -pthread -O" \
./configure --with-named-thread-libs="-lpthread -lmach -lexc -lc"

When compiling mysqld, you may see a couple of warnings like this: 

mysqld.cc: In function void handle_connections()':
mysqld.cc:626: passing long unsigned int *' as argument 3 of
accept(int,sockadddr *, int *)'

You can safely ignore these warnings. They occur because configure can detect only errors, not warnings.

If you start the server directly from the command line, you may have problems with it dying when you log out. (When you log out, your outstanding processes receive a SIGHUP signal.) If so, try starting the server like this: 

shell> nohup mysqld [options] &

nohup causes the command following it to ignore any SIGHUP signal sent from the terminal. Alternatively, start the server by running safe_mysqld, which invokes mysqld using nohup for you. See section 4.7.2 safe_mysqld, the wrapper around mysqld.

If you get a problem when compiling mysys/get_opt.c, just remove the line #define _NO_PROTO from the start of that file!

If you are using Compac's CC compiler, the following configure line should work: 

CC="cc -pthread"
CFLAGS="-O4 -ansi_alias -ansi_args -fast -inline speed all -arch host"
CXX="cxx -pthread"
CXXFLAGS="-O4 -ansi_alias -ansi_args -fast -inline speed all -arch host"
export CC CFLAGS CXX CXXFLAGS
./configure \
--prefix=/usr/local/mysql \
--with-low-memory \
--enable-large-files \
--enable-shared=yes \
--with-named-thread-libs="-lpthread -lmach -lexc -lc"
gnumake

If you get a problem with libtool, when compiling with shared libraries as above, when linking mysql, you should be able to get around this by issuing: 

cd mysql
/bin/sh ../libtool --mode=link cxx -pthread  -O3 -DDBUG_OFF \
-O4 -ansi_alias -ansi_args -fast -inline speed \
-speculate all \ -arch host  -DUNDEF_HAVE_GETHOSTBYNAME_R \
-o mysql  mysql.o readline.o sql_string.o completion_hash.o \
../readline/libreadline.a -lcurses \
../libmysql/.libs/libmysqlclient.so  -lm
cd ..
gnumake
gnumake install
scripts/mysql_install_db

2.6.6.7 Alpha-DEC-OSF1 Notes

If you have problems compiling and have DEC CC and gcc installed, try running configure like this: 

CC=cc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 \
./configure --prefix=/usr/local/mysql

If you get problems with the `c_asm.h' file, you can create and use a 'dummy' `c_asm.h' file with: 

touch include/c_asm.h
CC=gcc CFLAGS=-I./include \
CXX=gcc CXXFLAGS=-O3 \
./configure --prefix=/usr/local/mysql

Note that the following problems with the ld program can be fixed by downloading the latest DEC (Compaq) patch kit from: http://ftp.support.compaq.com/public/unix/.

On OSF1 V4.0D and compiler "DEC C V5.6-071 on Digital Unix V4.0 (Rev. 878)" the compiler had some strange behavior (undefined asm symbols). /bin/ld also appears to be broken (problems with _exit undefined errors occuring while linking mysqld). On this system, we have managed to compile MySQL with the following configure line, after replacing /bin/ld with the version from OSF 4.0C: 

CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql

With the Digital compiler "C++ V6.1-029", the following should work: 

CC=cc -pthread
CFLAGS=-O4 -ansi_alias -ansi_args -fast -inline speed -speculate all -arch host
CXX=cxx -pthread
CXXFLAGS=-O4 -ansi_alias -ansi_args -fast -inline speed -speculate all -arch host -noexceptions -nortti
export CC CFLAGS CXX CXXFLAGS
./configure --prefix=/usr/mysql/mysql --with-mysqld-ldflags=-all-static --disable-shared --with-named-thread-libs="-lmach -lexc -lc"

In some versions of OSF1, the alloca() function is broken. Fix this by removing the line in `config.h' that defines 'HAVE_ALLOCA'.

The alloca() function also may have an incorrect prototype in /usr/include/alloca.h. This warning resulting from this can be ignored.

configure will use the following thread libraries automatically: --with-named-thread-libs="-lpthread -lmach -lexc -lc".

When using gcc, you can also try running configure like this: 

shell> CFLAGS=-D_PTHREAD_USE_D4 CXX=gcc CXXFLAGS=-O3 ./configure ....

If you have problems with signals (MySQL dies unexpectedly under high load), you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with: 

shell> CFLAGS=-DDONT_USE_THR_ALARM \
       CXXFLAGS=-DDONT_USE_THR_ALARM \
       ./configure ...

This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are ``sleeping'' on a connection with mysqladmin kill or mysqladmin shutdown. Instead, the client will die when it issues its next command.

With gcc 2.95.2, you will probably run into the following compile error: 

sql_acl.cc:1456: Internal compiler error in `scan_region', at except.c:2566
Please submit a full bug report.

To fix this you should change to the sql directory and do a ``cut and paste'' of the last gcc line, but change -O3 to -O0 (or add -O0 immediately after gcc if you don't have any -O option on your compile line.) After this is done you can just change back to the top-level directly and run make again.

2.6.6.8 SGI Irix Notes

If you are using Irix Version 6.5.3 or newer mysqld will only be able to create threads if you run it as a user with CAP_SCHED_MGT privileges (like root) or give the mysqld server this privilege with the following shell command: 

shell> chcap "CAP_SCHED_MGT+epi" /opt/mysql/libexec/mysqld

You may have to undefine some things in `config.h' after running configure and before compiling.

In some Irix implementations, the alloca() function is broken. If the mysqld server dies on some SELECT statements, remove the lines from `config.h' that define HAVE_ALLOC and HAVE_ALLOCA_H. If mysqladmin create doesn't work, remove the line from `config.h' that defines HAVE_READDIR_R. You may have to remove the HAVE_TERM_H line as well.

SGI recommends that you install all of the patches on this page as a set: http://support.sgi.com/surfzone/patches/patchset/6.2_indigo.rps.html

At the very minimum, you should install the latest kernel rollup, the latest rld rollup, and the latest libc rollup.

You definitely need all the POSIX patches on this page, for pthreads support:

http://support.sgi.com/surfzone/patches/patchset/6.2_posix.rps.html

If you get the something like the following error when compiling `mysql.cc': 

"/usr/include/curses.h", line 82: error(1084): invalid combination of type

Type the following in the top-level directory of your MySQL source tree: 

shell> extra/replace bool curses_bool < /usr/include/curses.h > include/curses.h
shell> make

There have also been reports of scheduling problems. If only one thread is running, things go slow. Avoid this by starting another client. This may lead to a 2-to-10-fold increase in execution speed thereafter for the other thread. This is a poorly understood problem with Irix threads; you may have to improvise to find solutions until this can be fixed.

If you are compiling with gcc, you can use the following configure command: 

CC=gcc CXX=gcc CXXFLAGS=-O3 \
./configure --prefix=/usr/local/mysql --enable-thread-safe-client --with-named-thread-libs=-lpthread

On Irix 6.5.11 with native Irix C and C++ compilers ver. 7.3.1.2, the following is reported to work 

CC=cc CXX=CC CFLAGS='-O3 -n32 -TARG:platform=IP22 -I/usr/local/include \
-L/usr/local/lib' CXXFLAGS='-O3 -n32 -TARG:platform=IP22 \
-I/usr/local/include -L/usr/local/lib' ./configure --prefix=/usr/local/mysql \
--with-berkeley-db --with-innodb \
--with-libwrap=/usr/local --with-named-curses-libs=/usr/local/lib/libncurses.a

2.6.6.9 SCO Notes

The current port is tested only on a ``sco3.2v5.0.4'' and ``sco3.2v5.0.5'' system. There has also been a lot of progress on a port to ``sco 3.2v4.2''.

For the moment the recommended compiler on OpenServer is gcc 2.95.2. With this you should be able to compile MySQL with just: 

CC=gcc CXX=gcc ./configure ... (options)
  1. For OpenServer 5.0.X you need to use GDS in Skunkware 95 (95q4c). This is necessary because GNU gcc 2.7.2 in Skunkware 97 does not have GNU as. You can also use egcs 1.1.2 or newer http://www.egcs.com/. If you are using egcs 1.1.2 you have to execute the following command: 
    shell> cp -p /usr/include/pthread/stdtypes.h /usr/local/lib/gcc-lib/i386-pc-sco3.2v5.0.5/egcs-2.91.66/include/pthread/
  2. You need the port of GCC 2.5.x for this product and the Development system. They are required on this version of SCO Unix. You cannot just use the GCC Dev system.
  3. You should get the FSU Pthreads package and install it first. This can be found at http://www.cs.wustl.edu/~schmidt/ACE_wrappers/FSU-threads.tar.gz. You can also get a precompiled package from http://www.mysql.com/Downloads/SCO/FSU-threads-3.5c.tar.gz.
  4. FSU Pthreads can be compiled with SCO Unix 4.2 with tcpip. Or OpenServer 3.0 or Open Desktop 3.0 (OS 3.0 ODT 3.0), with the SCO Development System installed using a good port of GCC 2.5.x ODT or OS 3.0 you will need a good port of GCC 2.5.x There are a lot of problems without a good port. The port for this product requires the SCO Unix Development system. Without it, you are missing the libraries and the linker that is needed.
  5. To build FSU Pthreads on your system, do the following:
    1. Run ./configure in the `threads/src' directory and select the SCO OpenServer option. This command copies `Makefile.SCO5' to `Makefile'.
    2. Run make.
    3. To install in the default `/usr/include' directory, login as root, then cd to the `thread/src' directory, and run make install.
  6. Remember to use GNU make when making MySQL.
  7. If you don't start safe_mysqld as root, you probably will get only the default 110 open files per process. mysqld will write a note about this in the log file.
  8. With SCO 3.2V5.0.5, you should use FSU Pthreads version 3.5c or newer. You should also use gcc 2.95.2 or newer! The following configure command should work: 
    shell> ./configure --prefix=/usr/local/mysql --disable-shared
  9. With SCO 3.2V4.2, you should use FSU Pthreads version 3.5c or newer. The following configure command should work: 
    shell> CFLAGS="-D_XOPEN_XPG4" CXX=gcc CXXFLAGS="-D_XOPEN_XPG4" \
       ./configure \
           --prefix=/usr/local/mysql \
           --with-named-thread-libs="-lgthreads -lsocket -lgen -lgthreads" \
           --with-named-curses-libs="-lcurses"
    You may get some problems with some include files. In this case, you can find new SCO-specific include files at http://www.mysql.com/Downloads/SCO/SCO-3.2v4.2-includes.tar.gz. You should unpack this file in the `include' directory of your MySQL source tree.

SCO development notes:

If you want to install DBI on SCO, you have to edit the `Makefile' in DBI-xxx and each subdirectory.

Note that the following assumes gcc 2.95.2 or newer: 

OLD:                                  NEW:
CC = cc                               CC = gcc
CCCDLFLAGS = -KPIC -W1,-Bexport       CCCDLFLAGS = -fpic
CCDLFLAGS = -wl,-Bexport              CCDLFLAGS =

LD = ld                               LD = gcc -G -fpic
LDDLFLAGS = -G -L/usr/local/lib       LDDLFLAGS = -L/usr/local/lib
LDFLAGS = -belf -L/usr/local/lib      LDFLAGS = -L/usr/local/lib

LD = ld                               LD = gcc -G -fpic
OPTIMISE = -Od                        OPTIMISE = -O1

OLD:
CCCFLAGS = -belf -dy -w0 -U M_XENIX -DPERL_SCO5 -I/usr/local/include

NEW:
CCFLAGS = -U M_XENIX -DPERL_SCO5 -I/usr/local/include

This is because the Perl dynaloader will not load the DBI modules if they were compiled with icc or cc.

Perl works best when compiled with cc.

2.6.6.10 SCO Unixware Version 7.0 Notes

You must use a version of MySQL at least as recent as Version 3.22.13 because that version fixes some portability problems under Unixware.

We have been able to compile MySQL with the following configure command on Unixware Version 7.0.1: 

CC=cc CXX=CC ./configure --prefix=/usr/local/mysql

If you want to use gcc, you must use gcc 2.95.2 or newer.

2.6.7 OS/2 Notes

MySQL uses quite a few open files. Because of this, you should add something like the following to your `CONFIG.SYS' file: 

SET EMXOPT=-c -n -h1024

If you don't do this, you will probably run into the following error: 

File 'xxxx' not found (Errcode: 24)

When using MySQL with OS/2 Warp 3, FixPack 29 or above is required. With OS/2 Warp 4, FixPack 4 or above is required. This is a requirement of the Pthreads library. MySQL must be installed in a partition that supports long filenames such as HPFS, FAT32, etc.

The `INSTALL.CMD' script must be run from OS/2's own `CMD.EXE' and may not work with replacement shells such as `4OS2.EXE'.

The `scripts/mysql-install-db' script has been renamed. It is now called `install.cmd' and is a REXX script, which will set up the default MySQL security settings and create the WorkPlace Shell icons for MySQL.

Dynamic module support is compiled in but not fully tested. Dynamic modules should be compiled using the Pthreads run-time library. 

gcc -Zdll -Zmt -Zcrtdll=pthrdrtl -I../include -I../regex -I.. \
    -o example udf_example.cc -L../lib -lmysqlclient udf_example.def
mv example.dll example.udf

Note: Due to limitations in OS/2, UDF module name stems must not exceed 8 characters. Modules are stored in the `/mysql2/udf' directory; the safe-mysqld.cmd script will put this directory in the BEGINLIBPATH environment variable. When using UDF modules, specified extensions are ignored -- it is assumed to be `.udf'. For example, in Unix, the shared module might be named `example.so' and you would load a function from it like this: 

mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "example.so";

Is OS/2, the module would be named `example.udf', but you would not specify the module extension: 

mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "example";

2.6.8 BeOS Notes

We are really interested in getting MySQL to work on BeOS, but unfortunately we don't have any person who knows BeOS or has time to do a port.

We are interested in finding someone to do a port, and we will help them with any technical questions they may have while doing the port.

We have previously talked with some BeOS developers that have said that MySQL is 80% ported to BeOS, but we haven't heard from them in a while.

2.6.9 Novell Netware Notes

We are really interested in getting MySQL to work on Netware, but unfortunately we don't have any person who knows Netware or has time to do a port.

We are interested in finding someone to do a port, and we will help them with any technical questions they may have while doing the port.

3 Introduction to MySQL: A MySQL Tutorial

This chapter provides a tutorial introduction to MySQL by showing how to use the mysql client program to create and use a simple database. mysql (sometimes referred to as the ``terminal monitor'' or just ``monitor'') is an interactive program that allows you to connect to a MySQL server, run queries, and view the results. mysql may also be used in batch mode: you place your queries in a file beforehand, then tell mysql to execute the contents of the file. Both ways of using mysql are covered here.

To see a list of options provided by mysql, invoke it with the --help option: 

shell> mysql --help

This chapter assumes that mysql is installed on your machine and that a MySQL server is available to which you can connect. If this is not true, contact your MySQL administrator. (If you are the administrator, you will need to consult other sections of this manual.)

This chapter describes the entire process of setting up and using a database. If you are interested only in accessing an already-existing database, you may want to skip over the sections that describe how to create the database and the tables it contains.

Because this chapter is tutorial in nature, many details are necessarily left out. Consult the relevant sections of the manual for more information on the topics covered here.

3.1 Connecting to and Disconnecting from the Server

To connect to the server, you'll usually need to provide a MySQL user name when you invoke mysql and, most likely, a password. If the server runs on a machine other than the one where you log in, you'll also need to specify a hostname. Contact your administrator to find out what connection parameters you should use to connect (that is, what host, user name, and password to use). Once you know the proper parameters, you should be able to connect like this: 

shell> mysql -h host -u user -p
Enter password: ********

The ******** represents your password; enter it when mysql displays the Enter password: prompt.

If that works, you should see some introductory information followed by a mysql> prompt: 

shell> mysql -h host -u user -p
Enter password: ********
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 459 to server version: 3.22.20a-log

Type 'help' for help.

mysql>

The prompt tells you that mysql is ready for you to enter commands.

Some MySQL installations allow users to connect as the anonymous (unnamed) user to the server running on the local host. If this is the case on your machine, you should be able to connect to that server by invoking mysql without any options: 

shell> mysql

After you have connected successfully, you can disconnect any time by typing QUIT at the mysql> prompt: 

mysql> QUIT
Bye

You can also disconnect by pressing Control-D.

Most examples in the following sections assume you are connected to the server. They indicate this by the mysql> prompt.

3.2 Entering Queries

Make sure you are connected to the server, as discussed in the previous section. Doing so will not in itself select any database to work with, but that's okay. At this point, it's more important to find out a little about how to issue queries than to jump right in creating tables, loading data into them, and retrieving data from them. This section describes the basic principles of entering commands, using several queries you can try out to familiarize yourself with how mysql works.

Here's a simple command that asks the server to tell you its version number and the current date. Type it in as shown below following the mysql> prompt and hit the RETURN key: 

mysql> SELECT VERSION(), CURRENT_DATE;
+--------------+--------------+
| version()    | CURRENT_DATE |
+--------------+--------------+
| 3.22.20a-log | 1999-03-19   |
+--------------+--------------+
1 row in set (0.01 sec)
mysql>

This query illustrates several things about mysql:

Keywords may be entered in any lettercase. The following queries are equivalent: 

mysql> SELECT VERSION(), CURRENT_DATE;
mysql> select version(), current_date;
mysql> SeLeCt vErSiOn(), current_DATE;

Here's another query. It demonstrates that you can use mysql as a simple calculator: 

mysql> SELECT SIN(PI()/4), (4+1)*5;
+-------------+---------+
| SIN(PI()/4) | (4+1)*5 |
+-------------+---------+
|    0.707107 |      25 |
+-------------+---------+

The commands shown thus far have been relatively short, single-line statements. You can even enter multiple statements on a single line. Just end each one with a semicolon: 

mysql> SELECT VERSION(); SELECT NOW();
+--------------+
| version()    |
+--------------+
| 3.22.20a-log |
+--------------+

+---------------------+
| NOW()               |
+---------------------+
| 1999-03-19 00:15:33 |
+---------------------+

A command need not be given all on a single line, so lengthy commands that require several lines are not a problem. mysql determines where your statement ends by looking for the terminating semicolon, not by looking for the end of the input line. (In other words, mysql accepts free-format input: it collects input lines but does not execute them until it sees the semicolon.)

Here's a simple multiple-line statement: 

mysql> SELECT
    -> USER()
    -> ,
    -> CURRENT_DATE;
+--------------------+--------------+
| USER()             | CURRENT_DATE |
+--------------------+--------------+
| joesmith@localhost | 1999-03-18   |
+--------------------+--------------+

In this example, notice how the prompt changes from mysql> to -> after you enter the first line of a multiple-line query. This is how mysql indicates that it hasn't seen a complete statement and is waiting for the rest. The prompt is your friend, because it provides valuable feedback. If you use that feedback, you will always be aware of what mysql is waiting for.

If you decide you don't want to execute a command that you are in the process of entering, cancel it by typing \c: 

mysql> SELECT
    -> USER()
    -> \c
mysql>

Here, too, notice the prompt. It switches back to mysql> after you type \c, providing feedback to indicate that mysql is ready for a new command.

The following table shows each of the prompts you may see and summarizes what they mean about the state that mysql is in:

Prompt Meaning
mysql> Ready for new command.
-> Waiting for next line of multiple-line command.
'> Waiting for next line, collecting a string that begins with a single quote (`'').
"> Waiting for next line, collecting a string that begins with a double quote (`"').

Multiple-line statements commonly occur by accident when you intend to issue a command on a single line, but forget the terminating semicolon. In this case, mysql waits for more input: 

mysql> SELECT USER()
    ->

If this happens to you (you think you've entered a statement but the only response is a -> prompt), most likely mysql is waiting for the semicolon. If you don't notice what the prompt is telling you, you might sit there for a while before realizing what you need to do. Enter a semicolon to complete the statement, and mysql will execute it: 

mysql> SELECT USER()
    -> ;
+--------------------+
| USER()             |
+--------------------+
| joesmith@localhost |
+--------------------+

The '> and "> prompts occur during string collection. In MySQL, you can write strings surrounded by either `'' or `"' characters (for example, 'hello' or "goodbye"), and mysql lets you enter strings that span multiple lines. When you see a '> or "> prompt, it means that you've entered a line containing a string that begins with a `'' or `"' quote character, but have not yet entered the matching quote that terminates the string. That's fine if you really are entering a multiple-line string, but how likely is that? Not very. More often, the '> and "> prompts indicate that you've inadvertantly left out a quote character. For example: 

mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30;
    ">

If you enter this SELECT statement, then hit RETURN and wait for the result, nothing will happen. Instead of wondering why this query takes so long, notice the clue provided by the "> prompt. It tells you that mysql expects to see the rest of an unterminated string. (Do you see the error in the statement? The string "Smith is missing the second quote.)

At this point, what do you do? The simplest thing is to cancel the command. However, you cannot just type \c in this case, because mysql interprets it as part of the string that it is collecting! Instead, enter the closing quote character (so mysql knows you've finished the string), then type \c: 

mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30;
    "> "\c
mysql>

The prompt changes back to mysql>, indicating that mysql is ready for a new command.

It's important to know what the '> and "> prompts signify, because if you mistakenly enter an unterminated string, any further lines you type will appear to be ignored by mysql -- including a line containing QUIT! This can be quite confusing, especially if you don't know that you need to supply the terminating quote before you can cancel the current command.

3.3 Creating and Using a Database

Now that you know how to enter commands, it's time to access a database.

Suppose you have several pets in your home (your menagerie) and you'd like to keep track of various types of information about them. You can do so by creating tables to hold your data and loading them with the desired information. Then you can answer different sorts of questions about your animals by retrieving data from the tables. This section shows you how to:

The menagerie database will be simple (deliberately), but it is not difficult to think of real-world situations in which a similar type of database might be used. For example, a database like this could be used by a farmer to keep track of livestock, or by a veterinarian to keep track of patient records. A menagerie distribution containing some of the queries and sample data used in the following sections can be obtained from the MySQL Web site. It's available in either compressed tar format or Zip format.

Use the SHOW statement to find out what databases currently exist on the server: 

mysql> SHOW DATABASES;
+----------+
| Database |
+----------+
| mysql    |
| test     |
| tmp      |
+----------+

The list of databases is probably different on your machine, but the mysql and test databases are likely to be among them. The mysql database is required because it describes user access privileges. The test database is often provided as a workspace for users to try things out.

If the test database exists, try to access it: 

mysql> USE test
Database changed

Note that USE, like QUIT, does not require a semicolon. (You can terminate such statements with a semicolon if you like; it does no harm.) The USE statement is special in another way, too: it must be given on a single line.

You can use the test database (if you have access to it) for the examples that follow, but anything you create in that database can be removed by anyone else with access to it. For this reason, you should probably ask your MySQL administrator for permission to use a database of your own. Suppose you want to call yours menagerie. The administrator needs to execute a command like this: 

mysql> GRANT ALL ON menagerie.* TO your_mysql_name;

where your_mysql_name is the MySQL user name assigned to you.

3.3.1 Creating and Selecting a Database

If the administrator creates your database for you when setting up your permissions, you can begin using it. Otherwise, you need to create it yourself: 

mysql> CREATE DATABASE menagerie;

Under Unix, database names are case sensitive (unlike SQL keywords), so you must always refer to your database as menagerie, not as Menagerie, MENAGERIE, or some other variant. This is also true for table names. (Under Windows, this restriction does not apply, although you must refer to databases and tables using the same lettercase throughout a given query.)

Creating a database does not select it for use; you must do that explicitly. To make menagerie the current database, use this command: 

mysql> USE menagerie
Database changed

Your database needs to be created only once, but you must select it for use each time you begin a mysql session. You can do this by issuing a USE statement as shown above. Alternatively, you can select the database on the command line when you invoke mysql. Just specify its name after any connection parameters that you might need to provide. For example: 

shell> mysql -h host -u user -p menagerie
Enter password: ********

Note that menagerie is not your password on the command just shown. If you want to supply your password on the command line after the -p option, you must do so with no intervening space (for example, as -pmypassword, not as -p mypassword). However, putting your password on the command line is not recommended, because doing so exposes it to snooping by other users logged in on your machine.

3.3.2 Creating a Table

Creating the database is the easy part, but at this point it's empty, as SHOW TABLES will tell you: 

mysql> SHOW TABLES;
Empty set (0.00 sec)

The harder part is deciding what the structure of your database should be: what tables you will need and what columns will be in each of them.

You'll want a table that contains a record for each of your pets. This can be called the pet table, and it should contain, as a bare minimum, each animal's name. Because the name by itself is not very interesting, the table should contain other information. For example, if more than one person in your family keeps pets, you might want to list each animal's owner. You might also want to record some basic descriptive information such as species and sex.

How about age? That might be of interest, but it's not a good thing to store in a database. Age changes as time passes, which means you'd have to update your records often. Instead, it's better to store a fixed value such as date of birth. Then, whenever you need age, you can calculate it as the difference between the current date and the birth date. MySQL provides functions for doing date arithmetic, so this is not difficult. Storing birth date rather than age has other advantages, too:

You can probably think of other types of information that would be useful in the pet table, but the ones identified so far are sufficient for now: name, owner, species, sex, birth, and death.

Use a CREATE TABLE statement to specify the layout of your table: 

mysql> CREATE TABLE pet (name VARCHAR(20), owner VARCHAR(20),
    -> species VARCHAR(20), sex CHAR(1), birth DATE, death DATE);

VARCHAR is a good choice for the name, owner, and species columns because the column values will vary in length. The lengths of those columns need not all be the same, and need not be 20. You can pick any length from 1 to 255, whatever seems most reasonable to you. (If you make a poor choice and it turns out later that you need a longer field, MySQL provides an ALTER TABLE statement.)

Animal sex can be represented in a variety of ways, for example, "m" and "f", or perhaps "male" and "female". It's simplest to use the single characters "m" and "f".

The use of the DATE data type for the birth and death columns is a fairly obvious choice.

Now that you have created a table, SHOW TABLES should produce some output: 

mysql> SHOW TABLES;
+---------------------+
| Tables in menagerie |
+---------------------+
| pet                 |
+---------------------+

To verify that your table was created the way you expected, use a DESCRIBE statement: 

mysql> DESCRIBE pet;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name    | varchar(20) | YES  |     | NULL    |       |
| owner   | varchar(20) | YES  |     | NULL    |       |
| species | varchar(20) | YES  |     | NULL    |       |
| sex     | char(1)     | YES  |     | NULL    |       |
| birth   | date        | YES  |     | NULL    |       |
| death   | date        | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+

You can use DESCRIBE any time, for example, if you forget the names of the columns in your table or what types they are.

3.3.3 Loading Data into a Table

After creating your table, you need to populate it. The LOAD DATA and INSERT statements are useful for this.

Suppose your pet records can be described as shown below. (Observe that MySQL expects dates in YYYY-MM-DD format; this may be different than what you are used to.)

name owner species sex birth death
Fluffy Harold cat f 1993-02-04
Claws Gwen cat m 1994-03-17
Buffy Harold dog f 1989-05-13
Fang Benny dog m 1990-08-27
Bowser Diane dog m 1998-08-31 1995-07-29
Chirpy Gwen bird f 1998-09-11
Whistler Gwen bird 1997-12-09
Slim Benny snake m 1996-04-29

Because you are beginning with an empty table, an easy way to populate it is to create a text file containing a row for each of your animals, then load the contents of the file into the table with a single statement.

You could create a text file `pet.txt' containing one record per line, with values separated by tabs, and given in the order in which the columns were listed in the CREATE TABLE statement. For missing values (such as unknown sexes or death dates for animals that are still living), you can use NULL values. To represent these in your text file, use \N. For example, the record for Whistler the bird would look like this (where the whitespace between values is a single tab character):

Whistler Gwen bird \N 1997-12-09 \N

To load the text file `pet.txt' into the pet table, use this command: 

mysql> LOAD DATA LOCAL INFILE "pet.txt" INTO TABLE pet;

You can specify the column value separator and end of line marker explicitly in the LOAD DATA statement if you wish, but the defaults are tab and linefeed. These are sufficient for the statement to read the file `pet.txt' properly.

When you want to add new records one at a time, the INSERT statement is useful. In its simplest form, you supply values for each column, in the order in which the columns were listed in the CREATE TABLE statement. Suppose Diane gets a new hamster named Puffball. You could add a new record using an INSERT statement like this: 

mysql> INSERT INTO pet
    -> VALUES ('Puffball','Diane','hamster','f','1999-03-30',NULL);

Note that string and date values are specified as quoted strings here. Also, with INSERT, you can insert NULL directly to represent a missing value. You do not use \N like you do with LOAD DATA.

From this example, you should be able to see that there would be a lot more typing involved to load your records initially using several INSERT statements rather than a single LOAD DATA statement.

3.3.4 Retrieving Information from a Table

The SELECT statement is used to pull information from a table. The general form of the statement is: 

SELECT what_to_select
FROM which_table
WHERE conditions_to_satisfy

what_to_select indicates what you want to see. This can be a list of columns, or * to indicate ``all columns.'' which_table indicates the table from which you want to retrieve data. The WHERE clause is optional. If it's present, conditions_to_satisfy specifies conditions that rows must satisfy to qualify for retrieval.

3.3.4.1 Selecting All Data

The simplest form of SELECT retrieves everything from a table: 

mysql> SELECT * FROM pet;
+----------+--------+---------+------+------------+------------+
| name     | owner  | species | sex  | birth      | death      |
+----------+--------+---------+------+------------+------------+
| Fluffy   | Harold | cat     | f    | 1993-02-04 | NULL       |
| Claws    | Gwen   | cat     | m    | 1994-03-17 | NULL       |
| Buffy    | Harold | dog     | f    | 1989-05-13 | NULL       |
| Fang     | Benny  | dog     | m    | 1990-08-27 | NULL       |
| Bowser   | Diane  | dog     | m    | 1998-08-31 | 1995-07-29 |
| Chirpy   | Gwen   | bird    | f    | 1998-09-11 | NULL       |
| Whistler | Gwen   | bird    | NULL | 1997-12-09 | NULL       |
| Slim     | Benny  | snake   | m    | 1996-04-29 | NULL       |
| Puffball | Diane  | hamster | f    | 1999-03-30 | NULL       |
+----------+--------+---------+------+------------+------------+

This form of SELECT is useful if you want to review your entire table, for instance, after you've just loaded it with your initial dataset. As it happens, the output just shown reveals an error in your data file: Bowser appears to have been born after he died! Consulting your original pedigree papers, you find that the correct birth year is 1989, not 1998.

There are are least a couple of ways to fix this:

As shown above, it is easy to retrieve an entire table. But typically you don't want to do that, particularly when the table becomes large. Instead, you're usually more interested in answering a particular question, in which case you specify some constraints on the information you want. Let's look at some selection queries in terms of questions about your pets that they answer.

3.3.4.2 Selecting Particular Rows

You can select only particular rows from your table. For example, if you want to verify the change that you made to Bowser's birth date, select Bowser's record like this: 

mysql> SELECT * FROM pet WHERE name = "Bowser";
+--------+-------+---------+------+------------+------------+
| name   | owner | species | sex  | birth      | death      |
+--------+-------+---------+------+------------+------------+
| Bowser | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
+--------+-------+---------+------+------------+------------+

The output confirms that the year is correctly recorded now as 1989, not 1998.

String comparisons are normally case insensitive, so you can specify the name as "bowser", "BOWSER", etc. The query result will be the same.

You can specify conditions on any column, not just name. For example, if you want to know which animals were born after 1998, test the birth column: 

mysql> SELECT * FROM pet WHERE birth >= "1998-1-1";
+----------+-------+---------+------+------------+-------+
| name     | owner | species | sex  | birth      | death |
+----------+-------+---------+------+------------+-------+
| Chirpy   | Gwen  | bird    | f    | 1998-09-11 | NULL  |
| Puffball | Diane | hamster | f    | 1999-03-30 | NULL  |
+----------+-------+---------+------+------------+-------+

You can combine conditions, for example, to locate female dogs: 

mysql> SELECT * FROM pet WHERE species = "dog" AND sex = "f";
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

The preceding query uses the AND logical operator. There is also an OR operator: 

mysql> SELECT * FROM pet WHERE species = "snake" OR species = "bird";
+----------+-------+---------+------+------------+-------+
| name     | owner | species | sex  | birth      | death |
+----------+-------+---------+------+------------+-------+
| Chirpy   | Gwen  | bird    | f    | 1998-09-11 | NULL  |
| Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL  |
| Slim     | Benny | snake   | m    | 1996-04-29 | NULL  |
+----------+-------+---------+------+------------+-------+

AND and OR may be intermixed. If you do that, it's a good idea to use parentheses to indicate how conditions should be grouped: 

mysql> SELECT * FROM pet WHERE (species = "cat" AND sex = "m")
    -> OR (species = "dog" AND sex = "f");
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

3.3.4.3 Selecting Particular Columns

If you don't want to see entire rows from your table, just name the columns in which you're interested, separated by commas. For example, if you want to know when your animals were born, select the name and birth columns: 

mysql> SELECT name, birth FROM pet;
+----------+------------+
| name     | birth      |
+----------+------------+
| Fluffy   | 1993-02-04 |
| Claws    | 1994-03-17 |
| Buffy    | 1989-05-13 |
| Fang     | 1990-08-27 |
| Bowser   | 1989-08-31 |
| Chirpy   | 1998-09-11 |
| Whistler | 1997-12-09 |
| Slim     | 1996-04-29 |
| Puffball | 1999-03-30 |
+----------+------------+

To find out who owns pets, use this query: 

mysql> SELECT owner FROM pet;
+--------+
| owner  |
+--------+
| Harold |
| Gwen   |
| Harold |
| Benny  |
| Diane  |
| Gwen   |
| Gwen   |
| Benny  |
| Diane  |
+--------+

However, notice that the query simply retrieves the owner field from each record, and some of them appear more than once. To minimize the output, retrieve each unique output record just once by adding the keyword DISTINCT: 

mysql> SELECT DISTINCT owner FROM pet;
+--------+
| owner  |
+--------+
| Benny  |
| Diane  |
| Gwen   |
| Harold |
+--------+

You can use a WHERE clause to combine row selection with column selection. For example, to get birth dates for dogs and cats only, use this query: 

mysql> SELECT name, species, birth FROM pet
    -> WHERE species = "dog" OR species = "cat";
+--------+---------+------------+
| name   | species | birth      |
+--------+---------+------------+
| Fluffy | cat     | 1993-02-04 |
| Claws  | cat     | 1994-03-17 |
| Buffy  | dog     | 1989-05-13 |
| Fang   | dog     | 1990-08-27 |
| Bowser | dog     | 1989-08-31 |
+--------+---------+------------+

3.3.4.4 Sorting Rows

You may have noticed in the preceding examples that the result rows are displayed in no particular order. However, it's often easier to examine query output when the rows are sorted in some meaningful way. To sort a result, use an ORDER BY clause.

Here are animal birthdays, sorted by date: 

mysql> SELECT name, birth FROM pet ORDER BY birth;
+----------+------------+
| name     | birth      |
+----------+------------+
| Buffy    | 1989-05-13 |
| Bowser   | 1989-08-31 |
| Fang     | 1990-08-27 |
| Fluffy   | 1993-02-04 |
| Claws    | 1994-03-17 |
| Slim     | 1996-04-29 |
| Whistler | 1997-12-09 |
| Chirpy   | 1998-09-11 |
| Puffball | 1999-03-30 |
+----------+------------+

To sort in reverse order, add the DESC (descending) keyword to the name of the column you are sorting by: 

mysql> SELECT name, birth FROM pet ORDER BY birth DESC;
+----------+------------+
| name     | birth      |
+----------+------------+
| Puffball | 1999-03-30 |
| Chirpy   | 1998-09-11 |
| Whistler | 1997-12-09 |
| Slim     | 1996-04-29 |
| Claws    | 1994-03-17 |
| Fluffy   | 1993-02-04 |
| Fang     | 1990-08-27 |
| Bowser   | 1989-08-31 |
| Buffy    | 1989-05-13 |
+----------+------------+

You can sort on multiple columns. For example, to sort by type of animal, then by birth date within animal type with youngest animals first, use the following query: 

mysql> SELECT name, species, birth FROM pet ORDER BY species, birth DESC;
+----------+---------+------------+
| name     | species | birth      |
+----------+---------+------------+
| Chirpy   | bird    | 1998-09-11 |
| Whistler | bird    | 1997-12-09 |
| Claws    | cat     | 1994-03-17 |
| Fluffy   | cat     | 1993-02-04 |
| Fang     | dog     | 1990-08-27 |
| Bowser   | dog     | 1989-08-31 |
| Buffy    | dog     | 1989-05-13 |
| Puffball | hamster | 1999-03-30 |
| Slim     | snake   | 1996-04-29 |
+----------+---------+------------+

Note that the DESC keyword applies only to the column name immediately preceding it (birth); species values are still sorted in ascending order.

3.3.4.5 Date Calculations

MySQL provides several functions that you can use to perform calculations on dates, for example, to calculate ages or extract parts of dates.

To determine how many years old each of your pets is, compute the difference in the year part of the current date and the birth date, then subtract one if the current date occurs earlier in the calendar year than the birth date. The following query shows, for each pet, the birth date, the current date, and the age in years. 

mysql> SELECT name, birth, CURRENT_DATE,
    -> (YEAR(CURRENT_DATE)-YEAR(birth))
    -> - (RIGHT(CURRENT_DATE,5)<RIGHT(birth,5))
    -> AS age
    -> FROM pet;
+----------+------------+--------------+------+
| name     | birth      | CURRENT_DATE | age  |
+----------+------------+--------------+------+
| Fluffy   | 1993-02-04 | 2001-08-29   |    8 |
| Claws    | 1994-03-17 | 2001-08-29   |    7 |
| Buffy    | 1989-05-13 | 2001-08-29   |   12 |
| Fang     | 1990-08-27 | 2001-08-29   |   11 |
| Bowser   | 1989-08-31 | 2001-08-29   |   11 |
| Chirpy   | 1998-09-11 | 2001-08-29   |    2 |
| Whistler | 1997-12-09 | 2001-08-29   |    3 |
| Slim     | 1996-04-29 | 2001-08-29   |    5 |
| Puffball | 1999-03-30 | 2001-08-29   |    2 |
+----------+------------+--------------+------+

Here, YEAR() pulls out the year part of a date and RIGHT() pulls off the rightmost five characters that represent the MM-DD (calendar year) part of the date. The part of the expression that compares the MM-DD values evaluates to 1 or 0, which adjusts the year difference down a year if CURRENT_DATE occurs earlier in the year than birth. The full expression is somewhat ungainly, so an alias (age) is used to make the output column label more meaningful.

The query works, but the result could be scanned more easily if the rows were presented in some order. This can be done by adding an ORDER BY name clause to sort the output by name: 

mysql> SELECT name, birth, CURRENT_DATE,
    -> (YEAR(CURRENT_DATE)-YEAR(birth))
    -> - (RIGHT(CURRENT_DATE,5)<RIGHT(birth,5))
    -> AS age
    -> FROM pet ORDER BY name;
+----------+------------+--------------+------+
| name     | birth      | CURRENT_DATE | age  |
+----------+------------+--------------+------+
| Bowser   | 1989-08-31 | 2001-08-29   |   11 |
| Buffy    | 1989-05-13 | 2001-08-29   |   12 |
| Chirpy   | 1998-09-11 | 2001-08-29   |    2 |
| Claws    | 1994-03-17 | 2001-08-29   |    7 |
| Fang     | 1990-08-27 | 2001-08-29   |   11 |
| Fluffy   | 1993-02-04 | 2001-08-29   |    8 |
| Puffball | 1999-03-30 | 2001-08-29   |    2 |
| Slim     | 1996-04-29 | 2001-08-29   |    5 |
| Whistler | 1997-12-09 | 2001-08-29   |    3 |
+----------+------------+--------------+------+

To sort the output by age rather than name, just use a different ORDER BY clause: 

mysql> SELECT name, birth, CURRENT_DATE,
    -> (YEAR(CURRENT_DATE)-YEAR(birth))
    -> - (RIGHT(CURRENT_DATE,5)<RIGHT(birth,5))
    -> AS age
    -> FROM pet ORDER BY age;
+----------+------------+--------------+------+
| name     | birth      | CURRENT_DATE | age  |
+----------+------------+--------------+------+
| Chirpy   | 1998-09-11 | 2001-08-29   |    2 |
| Puffball | 1999-03-30 | 2001-08-29   |    2 |
| Whistler | 1997-12-09 | 2001-08-29   |    3 |
| Slim     | 1996-04-29 | 2001-08-29   |    5 |
| Claws    | 1994-03-17 | 2001-08-29   |    7 |
| Fluffy   | 1993-02-04 | 2001-08-29   |    8 |
| Fang     | 1990-08-27 | 2001-08-29   |   11 |
| Bowser   | 1989-08-31 | 2001-08-29   |   11 |
| Buffy    | 1989-05-13 | 2001-08-29   |   12 |
+----------+------------+--------------+------+

A similar query can be used to determine age at death for animals that have died. You determine which animals these are by checking whether or not the death value is NULL. Then, for those with non-NULL values, compute the difference between the death and birth values: 

mysql> SELECT name, birth, death,
    -> (YEAR(death)-YEAR(birth)) - (RIGHT(death,5)<RIGHT(birth,5))
    -> AS age
    -> FROM pet WHERE death IS NOT NULL ORDER BY age;
+--------+------------+------------+------+
| name   | birth      | death      | age  |
+--------+------------+------------+------+
| Bowser | 1989-08-31 | 1995-07-29 |    5 |
+--------+------------+------------+------+

The query uses death IS NOT NULL rather than death != NULL because NULL is a special value. This is explained later. See section 3.3.4.6 Working with NULL Values.

What if you want to know which animals have birthdays next month? For this type of calculation, year and day are irrelevant; you simply want to extract the month part of the birth column. MySQL provides several date-part extraction functions, such as YEAR(), MONTH(), and DAYOFMONTH(). MONTH() is the appropriate function here. To see how it works, run a simple query that displays the value of both birth and MONTH(birth): 

mysql> SELECT name, birth, MONTH(birth) FROM pet;
+----------+------------+--------------+
| name     | birth      | MONTH(birth) |
+----------+------------+--------------+
| Fluffy   | 1993-02-04 |            2 |
| Claws    | 1994-03-17 |            3 |
| Buffy    | 1989-05-13 |            5 |
| Fang     | 1990-08-27 |            8 |
| Bowser   | 1989-08-31 |            8 |
| Chirpy   | 1998-09-11 |            9 |
| Whistler | 1997-12-09 |           12 |
| Slim     | 1996-04-29 |            4 |
| Puffball | 1999-03-30 |            3 |
+----------+------------+--------------+

Finding animals with birthdays in the upcoming month is easy, too. Suppose the current month is April. Then the month value is 4 and you look for animals born in May (month 5) like this: 

mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = 5;
+-------+------------+
| name  | birth      |
+-------+------------+
| Buffy | 1989-05-13 |
+-------+------------+

There is a small complication if the current month is December, of course. You don't just add one to the month number (12) and look for animals born in month 13, because there is no such month. Instead, you look for animals born in January (month 1).

You can even write the query so that it works no matter what the current month is. That way you don't have to use a particular month number in the query. DATE_ADD() allows you to add a time interval to a given date. If you add a month to the value of NOW(), then extract the month part with MONTH(), the result produces the month in which to look for birthdays: 

mysql> SELECT name, birth FROM pet
    -> WHERE MONTH(birth) = MONTH(DATE_ADD(NOW(), INTERVAL 1 MONTH));

A different way to accomplish the same task is to add 1 to get the next month after the current one (after using the modulo function (MOD) to wrap around the month value to 0 if it is currently 12): 

mysql> SELECT name, birth FROM pet
    -> WHERE MONTH(birth) = MOD(MONTH(NOW()), 12) + 1;

Note that MONTH returns a number between 1 and 12. And MOD(something,12) returns a number between 0 and 11. So the addition has to be after the MOD(), otherwise we would go from November (11) to January (1).

3.3.4.6 Working with NULL Values

The NULL value can be surprising until you get used to it. Conceptually, NULL means missing value or unknown value and it is treated somewhat differently than other values. To test for NULL, you cannot use the arithmetic comparison operators such as =, <, or !=. To demonstrate this for yourself, try the following query: 

mysql> SELECT 1 = NULL, 1 != NULL, 1 < NULL, 1 > NULL;
+----------+-----------+----------+----------+
| 1 = NULL | 1 != NULL | 1 < NULL | 1 > NULL |
+----------+-----------+----------+----------+
|     NULL |      NULL |     NULL |     NULL |
+----------+-----------+----------+----------+

Clearly you get no meaningful results from these comparisons. Use the IS NULL and IS NOT NULL operators instead: 

mysql> SELECT 1 IS NULL, 1 IS NOT NULL;
+-----------+---------------+
| 1 IS NULL | 1 IS NOT NULL |
+-----------+---------------+
|         0 |             1 |
+-----------+---------------+

In MySQL, 0 or NULL means false and anything else means true. The default truth value from a boolean operation is 1.

This special treatment of NULL is why, in the previous section, it was necessary to determine which animals are no longer alive using death IS NOT NULL instead of death != NULL.

3.3.4.7 Pattern Matching

MySQL provides standard SQL pattern matching as well as a form of pattern matching based on extended regular expressions similar to those used by Unix utilities such as vi, grep, and sed.

SQL pattern matching allows you to use `_' to match any single character and `%' to match an arbitrary number of characters (including zero characters). In MySQL, SQL patterns are case insensitive by default. Some examples are shown below. Note that you do not use = or != when you use SQL patterns; use the LIKE or NOT LIKE comparison operators instead.

To find names beginning with `b': 

mysql> SELECT * FROM pet WHERE name LIKE "b%";
+--------+--------+---------+------+------------+------------+
| name   | owner  | species | sex  | birth      | death      |
+--------+--------+---------+------+------------+------------+
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL       |
| Bowser | Diane  | dog     | m    | 1989-08-31 | 1995-07-29 |
+--------+--------+---------+------+------------+------------+

To find names ending with `fy': 

mysql> SELECT * FROM pet WHERE name LIKE "%fy";
+--------+--------+---------+------+------------+-------+
| name   | owner  | species | sex  | birth      | death |
+--------+--------+---------+------+------------+-------+
| Fluffy | Harold | cat     | f    | 1993-02-04 | NULL  |
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL  |
+--------+--------+---------+------+------------+-------+

To find names containing a `w': 

mysql> SELECT * FROM pet WHERE name LIKE "%w%";
+----------+-------+---------+------+------------+------------+
| name     | owner | species | sex  | birth      | death      |
+----------+-------+---------+------+------------+------------+
| Claws    | Gwen  | cat     | m    | 1994-03-17 | NULL       |
| Bowser   | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
| Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL       |
+----------+-------+---------+------+------------+------------+

To find names containing exactly five characters, use the `_' pattern character: 

mysql> SELECT * FROM pet WHERE name LIKE "_____";
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

The other type of pattern matching provided by MySQL uses extended regular expressions. When you test for a match for this type of pattern, use the REGEXP and NOT REGEXP operators (or RLIKE and NOT RLIKE, which are synonyms).

Some characteristics of extended regular expressions are:

To demonstrate how extended regular expressions work, the LIKE queries shown above are rewritten below to use REGEXP.

To find names beginning with `b', use `^' to match the beginning of the name: 

mysql> SELECT * FROM pet WHERE name REGEXP "^b";
+--------+--------+---------+------+------------+------------+
| name   | owner  | species | sex  | birth      | death      |
+--------+--------+---------+------+------------+------------+
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL       |
| Bowser | Diane  | dog     | m    | 1989-08-31 | 1995-07-29 |
+--------+--------+---------+------+------------+------------+

Prior to MySQL Version 3.23.4, REGEXP is case sensitive, and the previous query will return no rows. To match either lowercase or uppercase `b', use this query instead: 

mysql> SELECT * FROM pet WHERE name REGEXP "^[bB]";

From MySQL 3.23.4 on, to force a REGEXP comparison to be case sensitive, use the BINARY keyword to make one of the strings a binary string. This query will match only lowercase `b' at the beginning of a name: 

mysql> SELECT * FROM pet WHERE name REGEXP BINARY "^b";

To find names ending with `fy', use `$' to match the end of the name: 

mysql> SELECT * FROM pet WHERE name REGEXP "fy$";
+--------+--------+---------+------+------------+-------+
| name   | owner  | species | sex  | birth      | death |
+--------+--------+---------+------+------------+-------+
| Fluffy | Harold | cat     | f    | 1993-02-04 | NULL  |
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL  |
+--------+--------+---------+------+------------+-------+

To find names containing a lowercase or uppercase `w', use this query: 

mysql> SELECT * FROM pet WHERE name REGEXP "w";
+----------+-------+---------+------+------------+------------+
| name     | owner | species | sex  | birth      | death      |
+----------+-------+---------+------+------------+------------+
| Claws    | Gwen  | cat     | m    | 1994-03-17 | NULL       |
| Bowser   | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
| Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL       |
+----------+-------+---------+------+------------+------------+

Because a regular expression pattern matches if it occurs anywhere in the value, it is not necessary in the previous query to put a wild card on either side of the pattern to get it to match the entire value like it would be if you used a SQL pattern.

To find names containing exactly five characters, use `^' and `$' to match the beginning and end of the name, and five instances of `.' in between: 

mysql> SELECT * FROM pet WHERE name REGEXP "^.....$";
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

You could also write the previous query using the `{n}' ``repeat-n-times'' operator: 

mysql> SELECT * FROM pet WHERE name REGEXP "^.{5}$";
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

3.3.4.8 Counting Rows

Databases are often used to answer the question, ``How often does a certain type of data occur in a table?'' For example, you might want to know how many pets you have, or how many pets each owner has, or you might want to perform various kinds of censuses on your animals.

Counting the total number of animals you have is the same question as ``How many rows are in the pet table?'' because there is one record per pet. The COUNT() function counts the number of non-NULL results, so the query to count your animals looks like this: 

mysql> SELECT COUNT(*) FROM pet;
+----------+
| COUNT(*) |
+----------+
|        9 |
+----------+

Earlier, you retrieved the names of the people who owned pets. You can use COUNT() if you want to find out how many pets each owner has: 

mysql> SELECT owner, COUNT(*) FROM pet GROUP BY owner;
+--------+----------+
| owner  | COUNT(*) |
+--------+----------+
| Benny  |        2 |
| Diane  |        2 |
| Gwen   |        3 |
| Harold |        2 |
+--------+----------+

Note the use of GROUP BY to group together all records for each owner. Without it, all you get is an error message: 

mysql> SELECT owner, COUNT(owner) FROM pet;
ERROR 1140 at line 1: Mixing of GROUP columns (MIN(),MAX(),COUNT()...)
with no GROUP columns is illegal if there is no GROUP BY clause

COUNT() and GROUP BY are useful for characterizing your data in various ways. The following examples show different ways to perform animal census operations.

Number of animals per species: 

mysql> SELECT species, COUNT(*) FROM pet GROUP BY species;
+---------+----------+
| species | COUNT(*) |
+---------+----------+
| bird    |        2 |
| cat     |        2 |
| dog     |        3 |
| hamster |        1 |
| snake   |        1 |
+---------+----------+

Number of animals per sex: 

mysql> SELECT sex, COUNT(*) FROM pet GROUP BY sex;
+------+----------+
| sex  | COUNT(*) |
+------+----------+
| NULL |        1 |
| f    |        4 |
| m    |        4 |
+------+----------+

(In this output, NULL indicates sex unknown.)

Number of animals per combination of species and sex: 

mysql> SELECT species, sex, COUNT(*) FROM pet GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| bird    | NULL |        1 |
| bird    | f    |        1 |
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
| hamster | f    |        1 |
| snake   | m    |        1 |
+---------+------+----------+

You need not retrieve an entire table when you use COUNT(). For example, the previous query, when performed just on dogs and cats, looks like this: 

mysql> SELECT species, sex, COUNT(*) FROM pet
    -> WHERE species = "dog" OR species = "cat"
    -> GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
+---------+------+----------+

Or, if you wanted the number of animals per sex only for known-sex animals: 

mysql> SELECT species, sex, COUNT(*) FROM pet
    -> WHERE sex IS NOT NULL
    -> GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| bird    | f    |        1 |
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
| hamster | f    |        1 |
| snake   | m    |        1 |
+---------+------+----------+

3.3.4.9 Using More Than one Table

The pet table keeps track of which pets you have. If you want to record other information about them, such as events in their lives like visits to the vet or when litters are born, you need another table. What should this table look like? It needs:

Given these considerations, the CREATE TABLE statement for the event table might look like this: 

mysql> CREATE TABLE event (name VARCHAR(20), date DATE,
    -> type VARCHAR(15), remark VARCHAR(255));

As with the pet table, it's easiest to load the initial records by creating a tab-delimited text file containing the information:

Fluffy 1995-05-15 litter 4 kittens, 3 female, 1 male
Buffy 1993-06-23 litter 5 puppies, 2 female, 3 male
Buffy 1994-06-19 litter 3 puppies, 3 female
Chirpy 1999-03-21 vet needed beak straightened
Slim 1997-08-03 vet broken rib
Bowser 1991-10-12 kennel
Fang 1991-10-12 kennel
Fang 1998-08-28 birthday Gave him a new chew toy
Claws 1998-03-17 birthday Gave him a new flea collar
Whistler 1998-12-09 birthday First birthday

Load the records like this: 

mysql> LOAD DATA LOCAL INFILE "event.txt" INTO TABLE event;

Based on what you've learned from the queries you've run on the pet table, you should be able to perform retrievals on the records in the event table; the principles are the same. But when is the event table by itself insufficient to answer questions you might ask?

Suppose you want to find out the ages of each pet when they had their litters. The event table indicates when this occurred, but to calculate the age of the mother, you need her birth date. Because that is stored in the pet table, you need both tables for the query: 

mysql> SELECT pet.name, (TO_DAYS(date) - TO_DAYS(birth))/365 AS age, remark
    -> FROM pet, event
    -> WHERE pet.name = event.name AND type = "litter";
+--------+------+-----------------------------+
| name   | age  | remark                      |
+--------+------+-----------------------------+
| Fluffy | 2.27 | 4 kittens, 3 female, 1 male |
| Buffy  | 4.12 | 5 puppies, 2 female, 3 male |
| Buffy  | 5.10 | 3 puppies, 3 female         |
+--------+------+-----------------------------+

There are several things to note about this query:

You need not have two different tables to perform a join. Sometimes it is useful to join a table to itself, if you want to compare records in a table to other records in that same table. For example, to find breeding pairs among your pets, you can join the pet table with itself to pair up males and females of like species: 

mysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species
    -> FROM pet AS p1, pet AS p2
    -> WHERE p1.species = p2.species AND p1.sex = "f" AND p2.sex = "m";
+--------+------+--------+------+---------+
| name   | sex  | name   | sex  | species |
+--------+------+--------+------+---------+
| Fluffy | f    | Claws  | m    | cat     |
| Buffy  | f    | Fang   | m    | dog     |
| Buffy  | f    | Bowser | m    | dog     |
+--------+------+--------+------+---------+

In this query, we specify aliases for the table name in order to refer to the columns and keep straight which instance of the table each column reference is associated with.

3.4 Getting Information About Databases and Tables

What if you forget the name of a database or table, or what the structure of a given table is (for example, what its columns are called)? MySQL addresses this problem through several statements that provide information about the databases and tables it supports.

You have already seen SHOW DATABASES, which lists the databases managed by the server. To find out which database is currently selected, use the DATABASE() function: 

mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| menagerie  |
+------------+

If you haven't selected any database yet, the result is blank.

To find out what tables the current database contains (for example, when you're not sure about the name of a table), use this command: 

mysql> SHOW TABLES;
+---------------------+
| Tables in menagerie |
+---------------------+
| event               |
| pet                 |
+---------------------+

If you want to find out about the structure of a table, the DESCRIBE command is useful; it displays information about each of a table's columns: 

mysql> DESCRIBE pet;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name    | varchar(20) | YES  |     | NULL    |       |
| owner   | varchar(20) | YES  |     | NULL    |       |
| species | varchar(20) | YES  |     | NULL    |       |
| sex     | char(1)     | YES  |     | NULL    |       |
| birth   | date        | YES  |     | NULL    |       |
| death   | date        | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+

Field indicates the column name, Type is the data type for the column, Null indicates whether or not the column can contain NULL values, Key indicates whether or not the column is indexed, and Default specifies the column's default value.

If you have indexes on a table, SHOW INDEX FROM tbl_name produces information about them.

3.5 Examples of Common Queries

Here are examples of how to solve some common problems with MySQL.

Some of the examples use the table shop to hold the price of each article (item number) for certain traders (dealers). Supposing that each trader has a single fixed price per article, then (article, dealer) is a primary key for the records.

Start the command line tool mysql and select a database: 

mysql your-database-name

(In most MySQL installations, you can use the database-name 'test').

You can create the example table as: 

CREATE TABLE shop (
 article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
 dealer  CHAR(20)                 DEFAULT ''     NOT NULL,
 price   DOUBLE(16,2)             DEFAULT '0.00' NOT NULL,
 PRIMARY KEY(article, dealer));

INSERT INTO shop VALUES
(1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45),(3,'C',1.69),
(3,'D',1.25),(4,'D',19.95);

Okay, so the example data is: 

mysql> SELECT * FROM shop;

+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0001 | A      |  3.45 |
|    0001 | B      |  3.99 |
|    0002 | A      | 10.99 |
|    0003 | B      |  1.45 |
|    0003 | C      |  1.69 |
|    0003 | D      |  1.25 |
|    0004 | D      | 19.95 |
+---------+--------+-------+

3.5.1 The Maximum Value for a Column

``What's the highest item number?'' 

SELECT MAX(article) AS article FROM shop

+---------+
| article |
+---------+
|       4 |
+---------+

3.5.2 The Row Holding the Maximum of a Certain Column

``Find number, dealer, and price of the most expensive article.''

In ANSI SQL this is easily done with a sub-query: 

SELECT article, dealer, price
FROM   shop
WHERE  price=(SELECT MAX(price) FROM shop)

In MySQL (which does not yet have sub-selects), just do it in two steps:

  1. Get the maximum price value from the table with a SELECT statement.
  2. Using this value compile the actual query: 
    SELECT article, dealer, price
FROM   shop
WHERE  price=19.95

Another solution is to sort all rows descending by price and only get the first row using the MySQL specific LIMIT clause: 

SELECT article, dealer, price
FROM   shop
ORDER BY price DESC
LIMIT 1

NOTE: If there are several most expensive articles (for example, each 19.95) the LIMIT solution shows only one of them!

3.5.3 Maximum of Column per Group

``What's the highest price per article?'' 

SELECT article, MAX(price) AS price
FROM   shop
GROUP BY article

+---------+-------+
| article | price |
+---------+-------+
|    0001 |  3.99 |
|    0002 | 10.99 |
|    0003 |  1.69 |
|    0004 | 19.95 |
+---------+-------+

3.5.4 The Rows Holding the Group-wise Maximum of a Certain Field

``For each article, find the dealer(s) with the most expensive price.''

In ANSI SQL, I'd do it with a sub-query like this: 

SELECT article, dealer, price
FROM   shop s1
WHERE  price=(SELECT MAX(s2.price)
              FROM shop s2
              WHERE s1.article = s2.article);

In MySQL it's best do it in several steps:

  1. Get the list of (article,maxprice).
  2. For each article get the corresponding rows that have the stored maximum price.

This can easily be done with a temporary table: 

CREATE TEMPORARY TABLE tmp (
        article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
        price   DOUBLE(16,2)             DEFAULT '0.00' NOT NULL);

LOCK TABLES shop read;

INSERT INTO tmp SELECT article, MAX(price) FROM shop GROUP BY article;

SELECT shop.article, dealer, shop.price FROM shop, tmp
WHERE shop.article=tmp.article AND shop.price=tmp.price;

UNLOCK TABLES;

DROP TABLE tmp;

If you don't use a TEMPORARY table, you must also lock the 'tmp' table.

``Can it be done with a single query?''

Yes, but only by using a quite inefficient trick that I call the ``MAX-CONCAT trick'': 

SELECT article,
       SUBSTRING( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 7) AS dealer,
  0.00+LEFT(      MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 6) AS price
FROM   shop
GROUP BY article;

+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0001 | B      |  3.99 |
|    0002 | A      | 10.99 |
|    0003 | C      |  1.69 |
|    0004 | D      | 19.95 |
+---------+--------+-------+

The last example can, of course, be made a bit more efficient by doing the splitting of the concatenated column in the client.

3.5.5 Using user variables

You can use MySQL user variables to remember results without having to store them in temporary variables in the client. See section 6.1.4 User Variables.

For example, to find the articles with the highest and lowest price you can do: 

select @min_price:=min(price),@max_price:=max(price) from shop;
select * from shop where price=@min_price or price=@max_price;

+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0003 | D      |  1.25 |
|    0004 | D      | 19.95 |
+---------+--------+-------+

3.5.6 Using Foreign Keys

You don't need foreign keys to join 2 tables.

The only thing MySQL doesn't do is CHECK to make sure that the keys you use really exist in the table(s) you're referencing and it doesn't automatically delete rows from table with a foreign key definition. If you use your keys like normal, it'll work just fine: 

CREATE TABLE persons (
    id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
    name CHAR(60) NOT NULL,
    PRIMARY KEY (id)
);

CREATE TABLE shirts (
    id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
    style ENUM('t-shirt', 'polo', 'dress') NOT NULL,
    color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL,
    owner SMALLINT UNSIGNED NOT NULL REFERENCES persons,
    PRIMARY KEY (id)
);

INSERT INTO persons VALUES (NULL, 'Antonio Paz');

INSERT INTO shirts VALUES
(NULL, 'polo', 'blue', LAST_INSERT_ID()),
(NULL, 'dress', 'white', LAST_INSERT_ID()),
(NULL, 't-shirt', 'blue', LAST_INSERT_ID());

INSERT INTO persons VALUES (NULL, 'Lilliana Angelovska');

INSERT INTO shirts VALUES
(NULL, 'dress', 'orange', LAST_INSERT_ID()),
(NULL, 'polo', 'red', LAST_INSERT_ID()),
(NULL, 'dress', 'blue', LAST_INSERT_ID()),
(NULL, 't-shirt', 'white', LAST_INSERT_ID());

SELECT * FROM persons;
+----+---------------------+
| id | name                |
+----+---------------------+
|  1 | Antonio Paz         |
|  2 | Lilliana Angelovska |
+----+---------------------+

SELECT * FROM shirts;
+----+---------+--------+-------+
| id | style   | color  | owner |
+----+---------+--------+-------+
|  1 | polo    | blue   |     1 |
|  2 | dress   | white  |     1 |
|  3 | t-shirt | blue   |     1 |
|  4 | dress   | orange |     2 |
|  5 | polo    | red    |     2 |
|  6 | dress   | blue   |     2 |
|  7 | t-shirt | white  |     2 |
+----+---------+--------+-------+

SELECT s.* FROM persons p, shirts s
 WHERE p.name LIKE 'Lilliana%'
   AND s.owner = p.id
   AND s.color <> 'white';

+----+-------+--------+-------+
| id | style | color  | owner |
+----+-------+--------+-------+
|  4 | dress | orange |     2 |
|  5 | polo  | red    |     2 |
|  6 | dress | blue   |     2 |
+----+-------+--------+-------+

3.5.7 Searching on Two Keys

MySQL doesn't yet optimize when you search on two different keys combined with OR (Searching on one key with different OR parts is optimized quite good): 

SELECT field1_index, field2_index FROM test_table WHERE field1_index = '1'
OR  field2_index = '1'

The reason is that we haven't yet had time to come up with an efficient way to handle this in the general case. (The AND handling is, in comparison, now completely general and works very well).

For the moment you can solve this very efficiently by using a TEMPORARY table. This type of optimization is also very good if you are using very complicated queries where the SQL server does the optimizations in the wrong order. 

CREATE TEMPORARY TABLE tmp
SELECT field1_index, field2_index FROM test_table WHERE field1_index = '1';
INSERT INTO tmp
SELECT field1_index, field2_index FROM test_table WHERE field2_index = '1';
SELECT * from tmp;
DROP TABLE tmp;

The above way to solve this query is in effect an UNION of two queries.

3.5.8 Calculating visits per day

The following shows an idea of how you can use the bit group functions to calculate the number of days per month a user has visited a web page. 

CREATE TABLE t1 (year YEAR(4), month INT(2) UNSIGNED ZEROFILL, day INT(2) UNSIGNED ZEROFILL);                                                            
INSERT INTO t1 VALUES(2000,1,1),(2000,1,20),(2000,1,30),(2000,2,2),(2000,2,23),(2000,2,23);

SELECT year,month,BIT_COUNT(BIT_OR(1<<day)) AS days FROM t1 GROUP BY year,month;

Which returns:

+------+-------+------+
| year | month | days |
+------+-------+------+
| 2000 |    01 |    3 |
| 2000 |    02 |    2 |
+------+-------+------+

The above calculates how many different days was used for a given year/month combination, with automatic removal of duplicate entries.

3.6 Using mysql in Batch Mode

In the previous sections, you used mysql interactively to enter queries and view the results. You can also run mysql in batch mode. To do this, put the commands you want to run in a file, then tell mysql to read its input from the file: 

shell> mysql < batch-file

If you need to specify connection parameters on the command line, the command might look like this: 

shell> mysql -h host -u user -p < batch-file
Enter password: ********

When you use mysql this way, you are creating a script file, then executing the script.

Why use a script? Here are a few reasons:

The default output format is different (more concise) when you run mysql in batch mode than when you use it interactively. For example, the output of SELECT DISTINCT species FROM pet looks like this when run interactively: 

+---------+
| species |
+---------+
| bird    |
| cat     |
| dog     |
| hamster |
| snake   |
+---------+

But like this when run in batch mode: 

species
bird
cat
dog
hamster
snake

If you want to get the interactive output format in batch mode, use mysql -t. To echo to the output the commands that are executed, use mysql -vvv.

3.7 Queries from Twin Project

At Analytikerna and Lentus, we have been doing the systems and field work for a big research project. This project is a collaboration between the Institute of Environmental Medicine at Karolinska Institutet Stockholm and the Section on Clinical Research in Aging and Psychology at the University of Southern California.

The project involves a screening part where all twins in Sweden older than 65 years are interviewed by telephone. Twins who meet certain criteria are passed on to the next stage. In this latter stage, twins who want to participate are visited by a doctor/nurse team. Some of the examinations include physical and neuropsychological examination, laboratory testing, neuroimaging, psychological status assessment, and family history collection. In addition, data are collected on medical and environmental risk factors.

More information about Twin studies can be found at: 

http://www.imm.ki.se/TWIN/TWINUKW.HTM

The latter part of the project is administered with a Web interface written using Perl and MySQL.

Each night all data from the interviews are moved into a MySQL database.

3.7.1 Find all Non-distributed Twins

The following query is used to determine who goes into the second part of the project: 

select
        concat(p1.id, p1.tvab) + 0 as tvid,
        concat(p1.christian_name, " ", p1.surname) as Name,
        p1.postal_code as Code,
        p1.city as City,
        pg.abrev as Area,
        if(td.participation = "Aborted", "A", " ") as A,
        p1.dead as dead1,
        l.event as event1,
        td.suspect as tsuspect1,
        id.suspect as isuspect1,
        td.severe as tsevere1,
        id.severe as isevere1,
        p2.dead as dead2,
        l2.event as event2,
        h2.nurse as nurse2,
        h2.doctor as doctor2,
        td2.suspect as tsuspect2,
        id2.suspect as isuspect2,
        td2.severe as tsevere2,
        id2.severe as isevere2,
        l.finish_date
from
        twin_project as tp
        /* For Twin 1 */
        left join twin_data as td on tp.id = td.id and tp.tvab = td.tvab
        left join informant_data as id on tp.id = id.id and tp.tvab = id.tvab
        left join harmony as h on tp.id = h.id and tp.tvab = h.tvab
        left join lentus as l on tp.id = l.id and tp.tvab = l.tvab
        /* For Twin 2 */
        left join twin_data as td2 on p2.id = td2.id and p2.tvab = td2.tvab
        left join informant_data as id2 on p2.id = id2.id and p2.tvab = id2.tvab
        left join harmony as h2 on p2.id = h2.id and p2.tvab = h2.tvab
        left join lentus as l2 on p2.id = l2.id and p2.tvab = l2.tvab,
        person_data as p1,
        person_data as p2,
        postal_groups as pg
where
        /* p1 gets main twin and p2 gets his/her twin. */
        /* ptvab is a field inverted from tvab */
        p1.id = tp.id and p1.tvab = tp.tvab and
        p2.id = p1.id and p2.ptvab = p1.tvab and
        /* Just the sceening survey */
        tp.survey_no = 5 and
        /* Skip if partner died before 65 but allow emigration (dead=9) */
        (p2.dead = 0 or p2.dead = 9 or
         (p2.dead = 1 and
          (p2.death_date = 0 or
           (((to_days(p2.death_date) - to_days(p2.birthday)) / 365)
            >= 65))))
        and
        (
        /* Twin is suspect */
        (td.future_contact = 'Yes' and td.suspect = 2) or
        /* Twin is suspect - Informant is Blessed */
        (td.future_contact = 'Yes' and td.suspect = 1 and id.suspect = 1) or
        /* No twin - Informant is Blessed */
        (ISNULL(td.suspect) and id.suspect = 1 and id.future_contact = 'Yes') or
        /* Twin broken off - Informant is Blessed */
        (td.participation = 'Aborted'
         and id.suspect = 1 and id.future_contact = 'Yes') or
        /* Twin broken off - No inform - Have partner */
        (td.participation = 'Aborted' and ISNULL(id.suspect) and p2.dead = 0))
        and
        l.event = 'Finished'
        /* Get at area code */
        and substring(p1.postal_code, 1, 2) = pg.code
        /* Not already distributed */
        and (h.nurse is NULL or h.nurse=00 or h.doctor=00)
        /* Has not refused or been aborted */
        and not (h.status = 'Refused' or h.status = 'Aborted'
        or h.status = 'Died' or h.status = 'Other')
order by
        tvid;

Some explanations:

concat(p1.id, p1.tvab) + 0 as tvid
We want to sort on the concatenated id and tvab in numerical order. Adding 0 to the result causes MySQL to treat the result as a number.
column id
This identifies a pair of twins. It is a key in all tables.
column tvab
This identifies a twin in a pair. It has a value of 1 or 2.
column ptvab
This is an inverse of tvab. When tvab is 1 this is 2, and vice versa. It exists to save typing and to make it easier for MySQL to optimize the query.

This query demonstrates, among other things, how to do lookups on a table from the same table with a join (p1 and p2). In the example, this is used to check whether a twin's partner died before the age of 65. If so, the row is not returned.

All of the above exist in all tables with twin-related information. We have a key on both id,tvab (all tables), and id,ptvab (person_data) to make queries faster.

On our production machine (A 200MHz UltraSPARC), this query returns about 150-200 rows and takes less than one second.

The current number of records in the tables used above:
Table Rows
person_data 71074
lentus 5291
twin_project 5286
twin_data 2012
informant_data 663
harmony 381
postal_groups 100

3.7.2 Show a Table on Twin Pair Status

Each interview ends with a status code called event. The query shown below is used to display a table over all twin pairs combined by event. This indicates in how many pairs both twins are finished, in how many pairs one twin is finished and the other refused, and so on. 

select
        t1.event,
        t2.event,
        count(*)
from
        lentus as t1,
        lentus as t2,
        twin_project as tp
where
        /* We are looking at one pair at a time */
        t1.id = tp.id
        and t1.tvab=tp.tvab
        and t1.id = t2.id
        /* Just the sceening survey */
        and tp.survey_no = 5
        /* This makes each pair only appear once */
        and t1.tvab='1' and t2.tvab='2'
group by
        t1.event, t2.event;

3.8 Using MySQL with Apache

The Contrib section includes programs that let you authenticate your users from a MySQL database and also let you log your log files into a MySQL table. See section D Contributed Programs.

You can change the Apache logging format to be easily readable by MySQL by putting the following into the Apache configuration file: 

LogFormat \
        "\"%h\",%{%Y%m%d%H%M%S}t,%>s,\"%b\",\"%{Content-Type}o\",  \
        \"%U\",\"%{Referer}i\",\"%{User-Agent}i\""

In MySQL you can do something like this: 

LOAD DATA INFILE '/local/access_log' INTO TABLE table_name
FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' ESCAPED BY '\\'

4 MySQL Database Administration

4.1 Configuring MySQL

4.1.1 mysqld Command-line Options

mysqld accepts the following command-line options:

--ansi
Use ANSI SQL syntax instead of MySQL syntax. See section 1.4.3 Running MySQL in ANSI Mode.
-b, --basedir=path
Path to installation directory. All paths are usually resolved relative to this.
--big-tables
Allow big result sets by saving all temporary sets on file. It solves most 'table full' errors, but also slows down the queries where in-memory tables would suffice. Since Version 3.23.2, MySQL is able to solve it automatically by using memory for small temporary tables and switching to disk tables where necessary.
--bind-address=IP
IP address to bind to.
--character-sets-dir=path
Directory where character sets are. See section 4.6.1 The Character Set Used for Data and Sorting.
--chroot=path
Chroot mysqld daemon during startup. Recommended security measure. It will somewhat limit LOAD DATA INFILE and SELECT ... INTO OUTFILE though.
--core-file
Write a core file if mysqld dies. For some systems you must also specify --core-file-size to safe_mysqld. See section 4.7.2 safe_mysqld, the wrapper around mysqld.
-h, --datadir=path
Path to the database root.
--default-character-set=charset
Set the default character set. See section 4.6.1 The Character Set Used for Data and Sorting.
--default-table-type=type
Set the default table type for tables. See section 7 MySQL Table Types.
--debug[...]=
If MySQL is configured with --with-debug, you can use this option to get a trace file of what mysqld is doing. See section G.1.2 Creating trace files.
--delay-key-write-for-all-tables
Don't flush key buffers between writes for any MyISAM table. See section 5.5.2 Tuning Server Parameters.
--enable-locking
Enable system locking. Note that if you use this option on a system which a not fully working lockd() (as on Linux) you will easily get mysqld to deadlock.
-T, --exit-info
This is a bit mask of different flags one can use for debugging the mysqld server; One should not use this option if one doesn't know exactly what it does!
--flush
Flush all changes to disk after each SQL command. Normally MySQL only does a write of all changes to disk after each SQL command and lets the operating system handle the syncing to disk. See section A.4.1 What To Do If MySQL Keeps Crashing.
-?, --help
Display short help and exit.
--init-file=file
Read SQL commands from this file at startup.
-L, --language=...
Client error messages in given language. May be given as a full path. See section 4.6.2 Non-English Error Messages.
-l, --log[=file]
Log connections and queries to file. See section 4.9.2 The General Query Log.
--log-isam[=file]
Log all ISAM/MyISAM changes to file (only used when debugging ISAM/MyISAM).
--log-slow-queries[=file]
Log all queries that have taken more than long_query_time seconds to execute to file. See section 4.9.5 The Slow Query Log.
--log-update[=file]
Log updates to file.# where # is a unique number if not given. See section 4.9.3 The Update Log.
--log-long-format
Log some extra information to update log. If you are using --log-slow-queries then queries that are not using indexes are logged to the slow query log.
--low-priority-updates
Table-modifying operations (INSERT/DELETE/UPDATE) will have lower priority than selects. It can also be done via {INSERT | REPLACE | UPDATE | DELETE} LOW_PRIORITY ... to lower the priority of only one query, or by SET OPTION SQL_LOW_PRIORITY_UPDATES=1 to change the priority in one thread. See section 5.3.2 Table Locking Issues.
--memlock
Lock the mysqld process in memory. This works only if your system supports the mlockall() system call (like Solaris). This may help if you have a problem where the operating system is causing mysqld to swap on disk.
--myisam-recover [=option[,option...]]] where option is any combination
of DEFAULT, BACKUP, FORCE or QUICK. You can also set this explicitely to "" if you want to disable this option. If this option is used, mysqld will on open check if the table is marked as crashed or if if the table wasn't closed properly. (The last option only works if you are running with --skip-locking). If this is the case mysqld will run check on the table. If the table was corrupted, mysqld will attempt to repair it. The following options affects how the repair works.
DEFAULT The same as not giving any option to --myisam-recover.
BACKUP If the data table was changed during recover, save a backup of the `table_name.MYD' data file as `table_name-datetime.BAK'.
FORCE Run recover even if we will loose more than one row from the .MYD file.
QUICK Don't check the rows in the table if there isn't any delete blocks.
Before a table is automatically repaired, MySQL will add a note about this in the error log. If you want to be able to recover from most things without user intervention, you should use the options BACKUP,FORCE. This will force a repair of a table even if some rows would be deleted, but it will keep the old data file as a backup so that you can later examine what happened.
--pid-file=path
Path to pid file used by safe_mysqld.
-P, --port=...
Port number to listen for TCP/IP connections.
-o, --old-protocol
Use the 3.20 protocol for compatibility with some very old clients. See section 2.5.3 Upgrading from Version 3.20 to Version 3.21.
--one-thread
Only use one thread (for debugging under Linux). See section G.1 Debugging a MySQL server.
-O, --set-variable var=option
Give a variable a value. --help lists variables. You can find a full description for all variables in the SHOW VARIABLES section in this manual. See section 4.5.5.4 SHOW VARIABLES. The tuning server parameters section includes information of how to optimize these. See section 5.5.2 Tuning Server Parameters.
--safe-mode
Skip some optimize stages. Implies --skip-delay-key-write.
--safe-show-database
Don't show databases for which the user doesn't have any privileges.
--safe-user-create
If this is enabled, a user can't create new users with the GRANT command, if the user doesn't have INSERT privilege to the mysql.user table or any column in this table.
--skip-concurrent-insert
Turn off the ability to select and insert at the same time on MyISAM tables. (This is only to be used if you think you have found a bug in this feature).
--skip-delay-key-write
Ignore the delay_key_write option for all tables. See section 5.5.2 Tuning Server Parameters.
--skip-grant-tables
This option causes the server not to use the privilege system at all. This gives everyone full access to all databases! (You can tell a running server to start using the grant tables again by executing mysqladmin flush-privileges or mysqladmin reload.)
--skip-host-cache
Never use host name cache for faster name-ip resolution, but query DNS server on every connect instead. See section 5.5.5 How MySQL uses DNS.
--skip-locking
Don't use system locking. To use isamchk or myisamchk you must shut down the server. See section 1.1.7 How Stable Is MySQL?. Note that in MySQL Version 3.23 you can use REPAIR and CHECK to repair/check MyISAM tables.
--skip-name-resolve
Hostnames are not resolved. All Host column values in the grant tables must be IP numbers or localhost. See section 5.5.5 How MySQL uses DNS.
--skip-networking
Don't listen for TCP/IP connections at all. All interaction with mysqld must be made via Unix sockets. This option is highly recommended for systems where only local requests are allowed. See section 5.5.5 How MySQL uses DNS.
--skip-new
Don't use new, possible wrong routines. Implies --skip-delay-key-write. This will also set default table type to ISAM. See section 7.3 ISAM Tables.
--skip-symlink
Don't delete or rename files that a symlinked file in the data directory points to.
--skip-safemalloc
If MySQL is configured with --with-debug=full, all programs will check the memory for overruns for every memory allocation and memory freeing. As this checking is very slow, you can avoid this, when you don't need memory checking, by using this option.
--skip-show-database
Don't allow 'SHOW DATABASE' commands, unless the user has process privilege.
--skip-stack-trace
Don't write stack traces. This option is useful when you are running mysqld under a debugger. See section G.1 Debugging a MySQL server.
--skip-thread-priority
Disable using thread priorities for faster response time.
--socket=path
Socket file to use for local connections instead of default /tmp/mysql.sock.
--sql-mode=option[,option[,option...]]
Option can be any combination of: REAL_AS_FLOAT, PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, SERIALIZE, ONLY_FULL_GROUP_BY. It can also be empty ("") if you want to reset this. By specifying all of the above options is same as using --ansi. With this option one can turn on only needed SQL modes. See section 1.4.3 Running MySQL in ANSI Mode.
transaction-isolation= { READ-UNCOMMITTED | READ-COMMITTED | REPEATABLE-READ | SERIALIZABLE }
Sets the default transaction isolation level. See section 6.7.3 SET TRANSACTION Syntax.
-t, --tmpdir=path
Path for temporary files. It may be useful if your default /tmp directory resides on a partition too small to hold temporary tables.
-u, --user=user_name
Run mysqld daemon as user user_name. This option is mandatory when starting mysqld as root.
-V, --version
Output version information and exit.
-W, --warnings
Print out warnings like Aborted connection... to the .err file. See section A.2.9 Communication Errors / Aborted Connection.

4.1.2 my.cnf Option Files

MySQL can, since Version 3.22, read default startup options for the server and for clients from option files.

MySQL reads default options from the following files on Unix:

Filename Purpose
/etc/my.cnf Global options
DATADIR/my.cnf Server-specific options
defaults-extra-file The file specified with --defaults-extra-file=#
~/.my.cnf User-specific options

DATADIR is the MySQL data directory (typically `/usr/local/mysql/data' for a binary installation or `/usr/local/var' for a source installation). Note that this is the directory that was specified at configuration time, not the one specified with --datadir when mysqld starts up! (--datadir has no effect on where the server looks for option files, because it looks for them before it processes any command-line arguments.)

MySQL reads default options from the following files on Windows:

Filename Purpose
windows-system-directory\my.ini Global options
C:\my.cnf Global options
C:\mysql\data\my.cnf Server-specific options

Note that on Windows, you should specify all paths with / instead of \. If you use \, you need to specify this twice, as \ is the escape character in MySQL.

MySQL tries to read option files in the order listed above. If multiple option files exist, an option specified in a file read later takes precedence over the same option specified in a file read earlier. Options specified on the command line take precedence over options specified in any option file. Some options can be specified using environment variables. Options specified on the command line or in option files take precedence over environment variable values. See section H Environment Variables.

The following programs support option files: mysql, mysqladmin, mysqld, mysqldump, mysqlimport, mysql.server, myisamchk, and myisampack.

You can use option files to specify any long option that a program supports! Run the program with --help to get a list of available options.

An option file can contain lines of the following forms:

#comment
Comment lines start with `#' or `;'. Empty lines are ignored.
[group]
group is the name of the program or group for which you want to set options. After a group line, any option or set-variable lines apply to the named group until the end of the option file or another group line is given.
option
This is equivalent to --option on the command line.
option=value
This is equivalent to --option=value on the command line.
set-variable = variable=value
This is equivalent to --set-variable variable=value on the command line. This syntax must be used to set a mysqld variable.

The client group allows you to specify options that apply to all MySQL clients (not mysqld). This is the perfect group to use to specify the password you use to connect to the server. (But make sure the option file is readable and writable only by yourself.)

Note that for options and values, all leading and trailing blanks are automatically deleted. You may use the escape sequences `\b', `\t', `\n', `\r', `\\', and `\s' in your value string (`\s' == blank).

Here is a typical global option file: 

[client]
port=3306
socket=/tmp/mysql.sock

[mysqld]
port=3306
socket=/tmp/mysql.sock
set-variable = key_buffer_size=16M
set-variable = max_allowed_packet=1M

[mysqldump]
quick

Here is typical user option file: 

[client]
# The following password will be sent to all standard MySQL clients
password=my_password

[mysql]
no-auto-rehash
set-variable = connect_timeout=2

[mysqlhotcopy]
interactive-timeout

If you have a source distribution, you will find sample configuration files named `my-xxxx.cnf' in the `support-files' directory. If you have a binary distribution, look in the `DIR/support-files' directory, where DIR is the pathname to the MySQL installation directory (typically `/usr/local/mysql'). Currently there are sample configuration files for small, medium, large, and very large systems. You can copy `my-xxxx.cnf' to your home directory (rename the copy to `.my.cnf') to experiment with this.

All MySQL clients that support option files support the following options:

--no-defaults Don't read any option files.
--print-defaults Print the program name and all options that it will get.
--defaults-file=full-path-to-default-file Only use the given configuration file.
--defaults-extra-file=full-path-to-default-file Read this configuration file after the global configuration file but before the user configuration file.

Note that the above options must be first on the command line to work! --print-defaults may however be used directly after the --defaults-xxx-file commands.

Note for developers: Option file handling is implemented simply by processing all matching options (that is, options in the appropriate group) before any command-line arguments. This works nicely for programs that use the last instance of an option that is specified multiple times. If you have an old program that handles multiply-specified options this way but doesn't read option files, you need add only two lines to give it that capability. Check the source code of any of the standard MySQL clients to see how to do this.

In shell scripts you can use the `my_print_defaults' command to parse the config files: 


shell> my_print_defaults client mysql
--port=3306
--socket=/tmp/mysql.sock
--no-auto-rehash

The above output contains all options for the groups 'client' and 'mysql'.

4.1.3 Installing Many Servers on the Same Machine

In some cases you may want to have many different mysqld daemons (servers) running on the same machine. You may for example want to run a new version of MySQL for testing together with an old version that is in production. Another case is when you want to give different users access to different mysqld servers that they manage themselves.

One way to get a new server running is by starting it with a different socket and port as follows: 

shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock
shell> MYSQL_TCP_PORT=3307
shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT
shell> scripts/mysql_install_db
shell> bin/safe_mysqld &

The environment variables appendix includes a list of other environment variables you can use to affect mysqld. See section H Environment Variables.

The above is the quick and dirty way that one commonly uses for testing. The nice thing with this is that all connections you do in the above shell will automatically be directed to the new running server!

If you need to do this more permanently, you should create an option file for each server. See section 4.1.2 my.cnf Option Files. In your startup script that is executed at boot time (mysql.server?) you should specify for both servers:

safe_mysqld --default-file=path-to-option-file

At least the following options should be different per server:

port=#
socket=path
pid-file=path

The following options should be different, if they are used:

log=path
log-bin=path
log-update=path
log-isam=path
bdb-logdir=path

If you want more performance, you can also specify the following differently:

tmpdir=path
bdb-tmpdir=path

See section 4.1.1 mysqld Command-line Options.

If you are installing binary MySQL versions (.tar files) and start them with ./bin/safe_mysqld then in most cases the only option you need to add/change is the socket and port argument to safe_mysqld.

4.1.4 Running Multiple MySQL Servers on the Same Machine

There are circumstances when you might want to run multiple servers on the same machine. For example, you might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you might be an Internet service provider that wants to provide independent MySQL installations for different customers.

If you want to run multiple servers, the easiest way is to compile the servers with different TCP/IP ports and socket files so they are not both listening to the same TCP/IP port or socket file. See section 4.7.3 mysqld_multi, program for managing multiple MySQL servers.

Assume an existing server is configured for the default port number and socket file. Then configure the new server with a configure command something like this: 

shell> ./configure  --with-tcp-port=port_number \
             --with-unix-socket-path=file_name \
             --prefix=/usr/local/mysql-3.22.9

Here port_number and file_name should be different than the default port number and socket file pathname, and the --prefix value should specify an installation directory different than the one under which the existing MySQL installation is located.

You can check the socket used by any currently executing MySQL server with this command: 

shell> mysqladmin -h hostname --port=port_number variables

Note that if you specify ``localhost'' as a hostname, mysqladmin will default to using Unix sockets instead of TCP/IP.

If you have a MySQL server running on the port you used, you will get a list of some of the most important configurable variables in MySQL, including the socket name.

You don't have to recompile a new MySQL server just to start with a different port and socket. You can change the port and socket to be used by specifying them at run time as options to safe_mysqld: 

shell> /path/to/safe_mysqld --socket=file_name --port=port_number

mysqld_multi can also take safe_mysqld (or mysqld) as an argument and pass the options from a configuration file to safe_mysqld and further to mysqld.

If you run the new server on the same database directory as another server with logging enabled, you should also specify the name of the log files to safe_mysqld with --log, --log-update, or --log-slow-queries. Otherwise, both servers may be trying to write to the same log file.

WARNING: Normally you should never have two servers that update data in the same database! If your OS doesn't support fault-free system locking, this may lead to unpleasant surprises!

If you want to use another database directory for the second server, you can use the --datadir=path option to safe_mysqld.

NOTE also that starting several MySQL servers (mysqlds) in different machines and letting them access one data directory over NFS is generally a BAD IDEA! The problem is that the NFS will become the bottleneck with the speed. It is not meant for such use. And last but not least, you would still have to come up with a solution how to make sure that two or more mysqlds are not interfering with each other. At the moment there is no platform that would 100% reliable do the file locking (lockd daemon usually) in every situation. Yet there would be one more possible risk with NFS; it would make the work even more complicated for lockd daemon to handle. So make it easy for your self and forget about the idea. The working solution is to have one computer with an operating system that efficiently handles threads and have several CPUs in it.

When you want to connect to a MySQL server that is running with a different port than the port that is compiled into your client, you can use one of the following methods:

4.2 General Security Issues and the MySQL Access Privilege System

MySQL has an advanced but non-standard security/privilege system. This section describes how it works.

4.2.1 General Security Guidelines

Anyone using MySQL on a computer connected to the Internet should read this section to avoid the most common security mistakes.

In discussing security, we emphasize the necessity of fully protecting the entire server host (not simply the MySQL server) against all types of applicable attacks: eavesdropping, altering, playback, and denial of service. We do not cover all aspects of availability and fault tolerance here.

MySQL uses security based on Access Control Lists (ACLs) for all connections, queries, and other operations that a user may attempt to perform. There is also some support for SSL-encrypted connections between MySQL clients and servers. Many of the concepts discussed here are not specific to MySQL at all; the same general ideas apply to almost all applications.

When running MySQL, follow these guidelines whenever possible:

4.2.2 How to Make MySQL Secure Against Crackers

When you connect to a MySQL server, you normally should use a password. The password is not transmitted in clear text over the connection, however the encryption algorithm is not very strong, and with some effort a clever attacker can crack the password if he is able to sniff the traffic between the client and the server. If the connection between the client and the server goes through an untrusted network, you should use an SSH tunnel to encrypt the communication.

All other information is transferred as text that can be read by anyone who is able to watch the connection. If you are concerned about this, you can use the compressed protocol (in MySQL Version 3.22 and above) to make things much harder. To make things even more secure you should use ssh. You can find an open source ssh client at http://www.openssh.org, and a commercial ssh client at http://www.ssh.com. With this, you can get an encrypted TCP/IP connection between a MySQL server and a MySQL client.

To make a MySQL system secure, you should strongly consider the following suggestions:

4.2.3 Startup Options for mysqld Concerning Security

The following mysqld options affect security:

--safe-show-database
With this option, SHOW DATABASES returns only those databases for which the user has some kind of privilege.
--safe-user-create
If this is enabled, an user can't create new users with the GRANT command, if the user doesn't have INSERT privilege to the mysql.user table. If you want to give a user access to just create new users with those privileges that the user has right to grant, you should give the user the following privilege: 
GRANT INSERT(user) on mysql.user to 'user''hostname';
This will ensure that the user can't change any privilege columns directly, but has to use the GRANT command to give privileges to other users.
--skip-grant-tables
This option causes the server not to use the privilege system at all. This gives everyone full access to all databases! (You can tell a running server to start using the grant tables again by executing mysqladmin flush-privileges or mysqladmin reload.)
--skip-name-resolve
Hostnames are not resolved. All Host column values in the grant tables must be IP numbers or localhost.
--skip-networking
Don't allow TCP/IP connections over the network. All connections to mysqld must be made via Unix sockets. This option is unsuitable for systems that use MIT-pthreads, because the MIT-pthreads package doesn't support Unix sockets.
--skip-show-database
With this option, the SHOW DATABASES statement doesn't return anything.

4.2.4 What the Privilege System Does

The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with privileges on a database such as select, insert, update and delete.

Additional functionality includes the ability to have an anonymous user and to grant privileges for MySQL-specific functions such as LOAD DATA INFILE and administrative operations.

4.2.5 How the Privilege System Works

The MySQL privilege system ensures that all users may do exactly the things that they are supposed to be allowed to do. When you connect to a MySQL server, your identity is determined by the host from which you connect and the user name you specify. The system grants privileges according to your identity and what you want to do.

MySQL considers both your hostname and user name in identifying you because there is little reason to assume that a given user name belongs to the same person everywhere on the Internet. For example, the user bill who connects from whitehouse.gov need not be the same person as the user bill who connects from microsoft.com. MySQL handles this by allowing you to distinguish users on different hosts that happen to have the same name: you can grant bill one set of privileges for connections from whitehouse.gov, and a different set of privileges for connections from microsoft.com.

MySQL access control involves two stages:

The server uses the user, db, and host tables in the mysql database at both stages of access control. The fields in these grant tables are shown below:

Table name user db host
Scope fields Host Host Host
User Db Db
Password User
Privilege fields Select_priv Select_priv Select_priv
Insert_priv Insert_priv Insert_priv
Update_priv Update_priv Update_priv
Delete_priv Delete_priv Delete_priv
Index_priv Index_priv Index_priv
Alter_priv Alter_priv Alter_priv
Create_priv Create_priv Create_priv
Drop_priv Drop_priv Drop_priv
Grant_priv Grant_priv Grant_priv
References_priv
Reload_priv
Shutdown_priv
Process_priv
File_priv

For the second stage of access control (request verification), the server may, if the request involves tables, additionally consult the tables_priv and columns_priv tables. The fields in these tables are shown below:

Table name tables_priv columns_priv
Scope fields Host Host
Db Db
User User
Table_name Table_name
Column_name
Privilege fields Table_priv Column_priv
Column_priv
Other fields Timestamp Timestamp
Grantor

Each grant table contains scope fields and privilege fields.

Scope fields determine the scope of each entry in the tables, that is, the context in which the entry applies. For example, a user table entry with Host and User values of 'thomas.loc.gov' and 'bob' would be used for authenticating connections made to the server by bob from the host thomas.loc.gov. Similarly, a db table entry with Host, User, and Db fields of 'thomas.loc.gov', 'bob' and 'reports' would be used when bob connects from the host thomas.loc.gov to access the reports database. The tables_priv and columns_priv tables contain scope fields indicating tables or table/column combinations to which each entry applies.

For access-checking purposes, comparisons of Host values are case insensitive. User, Password, Db, and Table_name values are case sensitive. Column_name values are case insensitive in MySQL Version 3.22.12 or later.

Privilege fields indicate the privileges granted by a table entry, that is, what operations can be performed. The server combines the information in the various grant tables to form a complete description of a user's privileges. The rules used to do this are described in section 4.2.9 Access Control, Stage 2: Request Verification.

Scope fields are strings, declared as shown below; the default value for each is the empty string:

Field name Type
Host CHAR(60)
User CHAR(16)
Password CHAR(16)
Db CHAR(64) (CHAR(60) for the tables_priv and columns_priv tables)
Table_name CHAR(60)
Column_name CHAR(60)

In the user, db and host tables, all privilege fields are declared as ENUM('N','Y') -- each can have a value of 'N' or 'Y', and the default value is 'N'.

In the tables_priv and columns_priv tables, the privilege fields are declared as SET fields:

Table name Field name Possible set elements
tables_priv Table_priv 'Select', 'Insert', 'Update', 'Delete', 'Create', 'Drop', 'Grant', 'References', 'Index', 'Alter'
tables_priv Column_priv 'Select', 'Insert', 'Update', 'References'
columns_priv Column_priv 'Select', 'Insert', 'Update', 'References'

Briefly, the server uses the grant tables like this:

Note that administrative privileges (reload, shutdown, etc.) are specified only in the user table. This is because administrative operations are operations on the server itself and are not database-specific, so there is no reason to list such privileges in the other grant tables. In fact, only the user table need be consulted to determine whether or not you can perform an administrative operation.

The file privilege is specified only in the user table, too. It is not an administrative privilege as such, but your ability to read or write files on the server host is independent of the database you are accessing.

The mysqld server reads the contents of the grant tables once, when it starts up. Changes to the grant tables take effect as indicated in section 4.3.3 When Privilege Changes Take Effect.

When you modify the contents of the grant tables, it is a good idea to make sure that your changes set up privileges the way you want. For help in diagnosing problems, see section 4.2.10 Causes of Access denied Errors. For advice on security issues, see section 4.2.2 How to Make MySQL Secure Against Crackers.

A useful diagnostic tool is the mysqlaccess script, which Yves Carlier has provided for the MySQL distribution. Invoke mysqlaccess with the --help option to find out how it works. Note that mysqlaccess checks access using only the user, db and host tables. It does not check table- or column-level privileges.

4.2.6 Privileges Provided by MySQL

Information about user privileges is stored in the user, db, host, tables_priv, and columns_priv tables in the mysql database (that is, in the database named mysql). The MySQL server reads the contents of these tables when it starts up and under the circumstances indicated in section 4.3.3 When Privilege Changes Take Effect.

The names used in this manual to refer to the privileges provided by MySQL are shown below, along with the table column name associated with each privilege in the grant tables and the context in which the privilege applies:

Privilege Column Context
select Select_priv tables
insert Insert_priv tables
update Update_priv tables
delete Delete_priv tables
index Index_priv tables
alter Alter_priv tables
create Create_priv databases, tables, or indexes
drop Drop_priv databases or tables
grant Grant_priv databases or tables
references References_priv databases or tables
reload Reload_priv server administration
shutdown Shutdown_priv server administration
process Process_priv server administration
file File_priv file access on server

The select, insert, update, and delete privileges allow you to perform operations on rows in existing tables in a database.

SELECT statements require the select privilege only if they actually retrieve rows from a table. You can execute certain SELECT statements even without permission to access any of the databases on the server. For example, you could use the mysql client as a simple calculator: 

mysql> SELECT 1+1;
mysql> SELECT PI()*2;

The index privilege allows you to create or drop (remove) indexes.

The alter privilege allows you to use ALTER TABLE.

The create and drop privileges allow you to create new databases and tables, or to drop (remove) existing databases and tables.

Note that if you grant the drop privilege for the mysql database to a user, that user can drop the database in which the MySQL access privileges are stored!

The grant privilege allows you to give to other users those privileges you yourself possess.

The file privilege gives you permission to read and write files on the server using the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements. Any user to whom this privilege is granted can read or write any file that the MySQL server can read or write.

The remaining privileges are used for administrative operations, which are performed using the mysqladmin program. The table below shows which mysqladmin commands each administrative privilege allows you to execute:

Privilege Commands permitted to privilege holders
reload reload, refresh, flush-privileges, flush-hosts, flush-logs, and flush-tables
shutdown shutdown
process processlist, kill

The reload command tells the server to re-read the grant tables. The refresh command flushes all tables and opens and closes the log files. flush-privileges is a synonym for reload. The other flush-* commands perform functions similar to refresh but are more limited in scope, and may be preferable in some instances. For example, if you want to flush just the log files, flush-logs is a better choice than refresh.

The shutdown command shuts down the server.

The processlist command displays information about the threads executing within the server. The kill command kills server threads. You can always display or kill your own threads, but you need the process privilege to display or kill threads initiated by other users. See section 4.5.4 KILL Syntax.

It is a good idea in general to grant privileges only to those users who need them, but you should exercise particular caution in granting certain privileges:

There are some things that you cannot do with the MySQL privilege system:

4.2.7 Connecting to the MySQL Server

MySQL client programs generally require that you specify connection parameters when you want to access a MySQL server: the host you want to connect to, your user name, and your password. For example, the mysql client can be started like this (optional arguments are enclosed between `[' and `]'): 

shell> mysql [-h host_name] [-u user_name] [-pyour_pass]

Alternate forms of the -h, -u, and -p options are --host=host_name, --user=user_name, and --password=your_pass. Note that there is no space between -p or --password= and the password following it.

NOTE: Specifying a password on the command line is not secure! Any user on your system may then find out your password by typing a command like: ps auxww. See section 4.1.2 my.cnf Option Files.

mysql uses default values for connection parameters that are missing from the command line:

Thus, for a Unix user joe, the following commands are equivalent: 

shell> mysql -h localhost -u joe
shell> mysql -h localhost
shell> mysql -u joe
shell> mysql

Other MySQL clients behave similarly.

On Unix systems, you can specify different default values to be used when you make a connection, so that you need not enter them on the command line each time you invoke a client program. This can be done in a couple of ways:

4.2.8 Access Control, Stage 1: Connection Verification

When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on your identity and whether or not you can verify your identity by supplying the correct password. If not, the server denies access to you completely. Otherwise, the server accepts the connection, then enters Stage 2 and waits for requests.

Your identity is based on two pieces of information:

Identity checking is performed using the three user table scope fields (Host, User, and Password). The server accepts the connection only if a user table entry matches your hostname and user name, and you supply the correct password.

Values in the user table scope fields may be specified as follows:

Non-blank Password values represent encrypted passwords. MySQL does not store passwords in plaintext form for anyone to see. Rather, the password supplied by a user who is attempting to connect is encrypted (using the PASSWORD() function). The encrypted password is then used when the client/server is checking if the password is correct (This is done without the encrypted password ever traveling over the connection.) Note that from MySQL's point of view the encrypted password is the REAL password, so you should not give anyone access to it! In particular, don't give normal users read access to the tables in the mysql database!

The examples below show how various combinations of Host and User values in user table entries apply to incoming connections:

Host value User value Connections matched by entry
'thomas.loc.gov' 'fred' fred, connecting from thomas.loc.gov
'thomas.loc.gov' '' Any user, connecting from thomas.loc.gov
'%' 'fred' fred, connecting from any host
'%' '' Any user, connecting from any host
'%.loc.gov' 'fred' fred, connecting from any host in the loc.gov domain
'x.y.%' 'fred' fred, connecting from x.y.net, x.y.com,x.y.edu, etc. (this is probably not useful)
'144.155.166.177' 'fred' fred, connecting from the host with IP address 144.155.166.177
'144.155.166.%' 'fred' fred, connecting from any host in the 144.155.166 class C subnet
'144.155.166.0/255.255.255.0' 'fred' Same as previous example

Because you can use IP wild-card values in the Host field (for example, '144.155.166.%' to match every host on a subnet), there is the possibility that someone might try to exploit this capability by naming a host 144.155.166.somewhere.com. To foil such attempts, MySQL disallows matching on hostnames that start with digits and a dot. Thus, if you have a host named something like 1.2.foo.com, its name will never match the Host column of the grant tables. Only an IP number can match an IP wild-card value.

An incoming connection may be matched by more than one entry in the user table. For example, a connection from thomas.loc.gov by fred would be matched by several of the entries just shown above. How does the server choose which entry to use if more than one matches? The server resolves this question by sorting the user table after reading it at startup time, then looking through the entries in sorted order when a user attempts to connect. The first matching entry is the one that is used.

user table sorting works as follows. Suppose the user table looks like this: 

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| %         | root     | ...
| %         | jeffrey  | ...
| localhost | root     | ...
| localhost |          | ...
+-----------+----------+-

When the server reads in the table, it orders the entries with the most-specific Host values first ('%' in the Host column means ``any host'' and is least specific). Entries with the same Host value are ordered with the most-specific User values first (a blank User value means ``any user'' and is least specific). The resulting sorted user table looks like this: 

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| localhost | root     | ...
| localhost |          | ...
| %         | jeffrey  | ...
| %         | root     | ...
+-----------+----------+-

When a connection is attempted, the server looks through the sorted entries and uses the first match found. For a connection from localhost by jeffrey, the entries with 'localhost' in the Host column match first. Of those, the entry with the blank user name matches both the connecting hostname and user name. (The '%'/'jeffrey' entry would have matched, too, but it is not the first match in the table.)

Here is another example. Suppose the user table looks like this: 

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| %              | jeffrey  | ...
| thomas.loc.gov |          | ...
+----------------+----------+-

The sorted table looks like this: 

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| thomas.loc.gov |          | ...
| %              | jeffrey  | ...
+----------------+----------+-

A connection from thomas.loc.gov by jeffrey is matched by the first entry, whereas a connection from whitehouse.gov by jeffrey is matched by the second.

A common misconception is to think that for a given user name, all entries that explicitly name that user will be used first when the server attempts to find a match for the connection. This is simply not true. The previous example illustrates this, where a connection from thomas.loc.gov by jeffrey is first matched not by the entry containing 'jeffrey' as the User field value, but by the entry with no user name!

If you have problems connecting to the server, print out the user table and sort it by hand to see where the first match is being made.

4.2.9 Access Control, Stage 2: Request Verification

Once you establish a connection, the server enters Stage 2. For each request that comes in on the connection, the server checks whether you have sufficient privileges to perform it, based on the type of operation you wish to perform. This is where the privilege fields in the grant tables come into play. These privileges can come from any of the user, db, host, tables_priv, or columns_priv tables. The grant tables are manipulated with GRANT and REVOKE commands. See section 4.3.1 GRANT and REVOKE Syntax. (You may find it helpful to refer to section 4.2.5 How the Privilege System Works, which lists the fields present in each of the grant tables.)

The user table grants privileges that are assigned to you on a global basis and that apply no matter what the current database is. For example, if the user table grants you the delete privilege, you can delete rows from any database on the server host! In other words, user table privileges are superuser privileges. It is wise to grant privileges in the user table only to superusers such as server or database administrators. For other users, you should leave the privileges in the user table set to 'N' and grant privileges on a database-specific basis only, using the db and host tables.

The db and host tables grant database-specific privileges. Values in the scope fields may be specified as follows:

The db and host tables are read in and sorted when the server starts up (at the same time that it reads the user table). The db table is sorted on the Host, Db, and User scope fields, and the host table is sorted on the Host and Db scope fields. As with the user table, sorting puts the most-specific values first and least-specific values last, and when the server looks for matching entries, it uses the first match that it finds.

The tables_priv and columns_priv tables grant table- and column-specific privileges. Values in the scope fields may be specified as follows:

The tables_priv and columns_priv tables are sorted on the Host, Db, and User fields. This is similar to db table sorting, although the sorting is simpler because only the Host field may contain wild cards.

The request verification process is described below. (If you are familiar with the access-checking source code, you will notice that the description here differs slightly from the algorithm used in the code. The description is equivalent to what the code actually does; it differs only to make the explanation simpler.)

For administrative requests (shutdown, reload, etc.), the server checks only the user table entry, because that is the only table that specifies administrative privileges. Access is granted if the entry allows the requested operation and denied otherwise. For example, if you want to execute mysqladmin shutdown but your user table entry doesn't grant the shutdown privilege to you, access is denied without even checking the db or host tables. (They contain no Shutdown_priv column, so there is no need to do so.)

For database-related requests (insert, update, etc.), the server first checks the user's global (superuser) privileges by looking in the user table entry. If the entry allows the requested operation, access is granted. If the global privileges in the user table are insufficient, the server determines the user's database-specific privileges by checking the db and host tables:

  1. The server looks in the db table for a match on the Host, Db, and User fields. The Host and User fields are matched to the connecting user's hostname and MySQL user name. The Db field is matched to the database the user wants to access. If there is no entry for the Host and User, access is denied.
  2. If there is a matching db table entry and its Host field is not blank, that entry defines the user's database-specific privileges.
  3. If the matching db table entry's Host field is blank, it signifies that the host table enumerates which hosts should be allowed access to the database. In this case, a further lookup is done in the host table to find a match on the Host and Db fields. If no host table entry matches, access is denied. If there is a match, the user's database-specific privileges are computed as the intersection (not the union!) of the privileges in the db and host table entries, that is, the privileges that are 'Y' in both entries. (This way you can grant general privileges in the db table entry and then selectively restrict them on a host-by-host basis using the host table entries.)

After determining the database-specific privileges granted by the db and host table entries, the server adds them to the global privileges granted by the user table. If the result allows the requested operation, access is granted. Otherwise, the server checks the user's table and column privileges in the tables_priv and columns_priv tables and adds those to the user's privileges. Access is allowed or denied based on the result.

Expressed in boolean terms, the preceding description of how a user's privileges are calculated may be summarized like this: 

global privileges
OR (database privileges AND host privileges)
OR table privileges
OR column privileges

It may not be apparent why, if the global user entry privileges are initially found to be insufficient for the requested operation, the server adds those privileges to the database-, table-, and column-specific privileges later. The reason is that a request might require more than one type of privilege. For example, if you execute an INSERT ... SELECT statement, you need both insert and select privileges. Your privileges might be such that the user table entry grants one privilege and the db table entry grants the other. In this case, you have the necessary privileges to perform the request, but the server cannot tell that from either table by itself; the privileges granted by the entries in both tables must be combined.

The host table can be used to maintain a list of secure servers.

At TcX, the host table contains a list of all machines on the local network. These are granted all privileges.

You can also use the host table to indicate hosts that are not secure. Suppose you have a machine public.your.domain that is located in a public area that you do not consider secure. You can allow access to all hosts on your network except that machine by using host table entries like this: 

+--------------------+----+-
| Host               | Db | ...
+--------------------+----+-
| public.your.domain | %  | ... (all privileges set to 'N')
| %.your.domain      | %  | ... (all privileges set to 'Y')
+--------------------+----+-

Naturally, you should always test your entries in the grant tables (for example, using mysqlaccess) to make sure your access privileges are actually set up the way you think they are.

4.2.10 Causes of Access denied Errors

If you encounter Access denied errors when you try to connect to the MySQL server, the list below indicates some courses of action you can take to correct the problem:

4.3 MySQL User Account Management

4.3.1 GRANT and REVOKE Syntax

GRANT priv_type [(column_list)] [, priv_type [(column_list)] ...]
    ON {tbl_name | * | *.* | db_name.*}
    TO user_name [IDENTIFIED BY 'password']
        [, user_name [IDENTIFIED BY 'password'] ...]
    [WITH GRANT OPTION]

REVOKE priv_type [(column_list)] [, priv_type [(column_list)] ...]
    ON {tbl_name | * | *.* | db_name.*}
    FROM user_name [, user_name ...]

GRANT is implemented in MySQL Version 3.22.11 or later. For earlier MySQL versions, the GRANT statement does nothing.

The GRANT and REVOKE commands allow system administrators to create users and grant and revoke rights to MySQL users at four privilege levels:

Global level
Global privileges apply to all databases on a given server. These privileges are stored in the mysql.user table.
Database level
Database privileges apply to all tables in a given database. These privileges are stored in the mysql.db and mysql.host tables.
Table level
Table privileges apply to all columns in a given table. These privileges are stored in the mysql.tables_priv table.
Column level
Column privileges apply to single columns in a given table. These privileges are stored in the mysql.columns_priv table.

If you give a grant for a users that doesn't exists, that user is created. For examples of how GRANT works, see section 4.3.5 Adding New Users to MySQL.

For the GRANT and REVOKE statements, priv_type may be specified as any of the following: 

ALL PRIVILEGES      FILE                RELOAD
ALTER               INDEX               SELECT
CREATE              INSERT              SHUTDOWN
DELETE              PROCESS             UPDATE
DROP                REFERENCES          USAGE

ALL is a synonym for ALL PRIVILEGES. REFERENCES is not yet implemented. USAGE is currently a synonym for ``no privileges.'' It can be used when you want to create a user that has no privileges.

To revoke the grant privilege from a user, use a priv_type value of GRANT OPTION: 

REVOKE GRANT OPTION ON ... FROM ...;

The only priv_type values you can specify for a table are SELECT, INSERT, UPDATE, DELETE, CREATE, DROP, GRANT, INDEX, and ALTER.

The only priv_type values you can specify for a column (that is, when you use a column_list clause) are SELECT, INSERT, and UPDATE.

You can set global privileges by using ON *.* syntax. You can set database privileges by using ON db_name.* syntax. If you specify ON * and you have a current database, you will set the privileges for that database. (WARNING: If you specify ON * and you don't have a current database, you will affect the global privileges!)

In order to accommodate granting rights to users from arbitrary hosts, MySQL supports specifying the user_name value in the form user@host. If you want to specify a user string containing special characters (such as `-'), or a host string containing special characters or wild-card characters (such as `%'), you can quote the user or host name (for example, 'test-user'@'test-hostname').

You can specify wild cards in the hostname. For example, user@"%.loc.gov" applies to user for any host in the loc.gov domain, and user@"144.155.166.%" applies to user for any host in the 144.155.166 class C subnet.

The simple form user is a synonym for user@"%". NOTE: If you allow anonymous users to connect to the MySQL server (which is the default), you should also add all local users as user@localhost because otherwise the anonymous user entry for the local host in the mysql.user table will be used when the user tries to log into the MySQL server from the local machine! Anonymous users are defined by inserting entries with User='' into the mysql.user table. You can verify if this applies to you by executing this query: 

mysql> SELECT Host,User FROM mysql.user WHERE User='';

For the moment, GRANT only supports host, table, database, and column names up to 60 characters long. A user name can be up to 16 characters.

The privileges for a table or column are formed from the logical OR of the privileges at each of the four privilege levels. For example, if the mysql.user table specifies that a user has a global select privilege, this can't be denied by an entry at the database, table, or column level.

The privileges for a column can be calculated as follows: 

global privileges
OR (database privileges AND host privileges)
OR table privileges
OR column privileges

In most cases, you grant rights to a user at only one of the privilege levels, so life isn't normally as complicated as above. The details of the privilege-checking procedure are presented in section 4.2 General Security Issues and the MySQL Access Privilege System.

If you grant privileges for a user/hostname combination that does not exist in the mysql.user table, an entry is added and remains there until deleted with a DELETE command. In other words, GRANT may create user table entries, but REVOKE will not remove them; you must do that explicitly using DELETE.

In MySQL Version 3.22.12 or later, if a new user is created or if you have global grant privileges, the user's password will be set to the password specified by the IDENTIFIED BY clause, if one is given. If the user already had a password, it is replaced by the new one.

WARNING: If you create a new user but do not specify an IDENTIFIED BY clause, the user has no password. This is insecure.

Passwords can also be set with the SET PASSWORD command. See section 5.5.6 SET Syntax.

If you grant privileges for a database, an entry in the mysql.db table is created if needed. When all privileges for the database have been removed with REVOKE, this entry is deleted.

If a user doesn't have any privileges on a table, the table is not displayed when the user requests a list of tables (for example, with a SHOW TABLES statement).

The WITH GRANT OPTION clause gives the user the ability to give to other users any privileges the user has at the specified privilege level. You should be careful to whom you give the grant privilege, as two users with different privileges may be able to join privileges!

You cannot grant another user a privilege you don't have yourself; the grant privilege allows you to give away only those privileges you possess.

Be aware that when you grant a user the grant privilege at a particular privilege level, any privileges the user already possesses (or is given in the future!) at that level are also grantable by that user. Suppose you grant a user the insert privilege on a database. If you then grant the select privilege on the database and specify WITH GRANT OPTION, the user can give away not only the select privilege, but also insert. If you then grant the update privilege to the user on the database, the user can give away the insert, select and update.

You should not grant alter privileges to a normal user. If you do that, the user can try to subvert the privilege system by renaming tables!

Note that if you are using table or column privileges for even one user, the server examines table and column privileges for all users and this will slow down MySQL a bit.

When mysqld starts, all privileges are read into memory. Database, table, and column privileges take effect at once, and user-level privileges take effect the next time the user connects. Modifications to the grant tables that you perform using GRANT or REVOKE are noticed by the server immediately. If you modify the grant tables manually (using INSERT, UPDATE, etc.), you should execute a FLUSH PRIVILEGES statement or run mysqladmin flush-privileges to tell the server to reload the grant tables. See section 4.3.3 When Privilege Changes Take Effect.

The biggest differences between the ANSI SQL and MySQL versions of GRANT are:

4.3.2 MySQL User Names and Passwords

There are several distinctions between the way user names and passwords are used by MySQL and the way they are used by Unix or Windows:

MySQL users and their privileges are normally created with the GRANT command. See section 4.3.1 GRANT and REVOKE Syntax.

When you login to a MySQL server with a command line client you should specify the password with --password=your-password. See section 4.2.7 Connecting to the MySQL Server. 

mysql --user=monty --password=guess database_name

If you want the client to prompt for a password, you should use --password without any argument 

mysql --user=monty --password database_name

or the short form: 

mysql -u monty -p database_name

Note that in the last example the password is NOT 'database_name'.

If you want to use the -p option to supply a password you should do so like this: 

mysql -u monty -pguess database_name

On some systems, the library call that MySQL uses to prompt for a password will automatically cut the password to 8 characters. Internally MySQL doesn't have any limit for the length of the password.

4.3.3 When Privilege Changes Take Effect

When mysqld starts, all grant table contents are read into memory and become effective at that point.

Modifications to the grant tables that you perform using GRANT, REVOKE, or SET PASSWORD are noticed by the server immediately.

If you modify the grant tables manually (using INSERT, UPDATE, etc.), you should execute a FLUSH PRIVILEGES statement or run mysqladmin flush-privileges or mysqladmin reload to tell the server to reload the grant tables. Otherwise your changes will have no effect until you restart the server. If you change the grant tables manually but forget to reload the privileges, you will be wondering why your changes don't seem to make any difference!

When the server notices that the grant tables have been changed, existing client connections are affected as follows:

Global privilege changes and password changes take effect the next time the client connects.

4.3.4 Setting Up the Initial MySQL Privileges

After installing MySQL, you set up the initial access privileges by running scripts/mysql_install_db. See section 2.3.1 Quick Installation Overview. The mysql_install_db script starts up the mysqld server, then initializes the grant tables to contain the following set of privileges:

NOTE: The default privileges are different for Windows. See section 2.6.2.3 Running MySQL on Windows.

Because your installation is initially wide open, one of the first things you should do is specify a password for the MySQL root user. You can do this as follows (note that you specify the password using the PASSWORD() function): 

shell> mysql -u root mysql
mysql> UPDATE user SET Password=PASSWORD('new_password')
           WHERE user='root';
mysql> FLUSH PRIVILEGES;

You can, in MySQL Version 3.22 and above, use the SET PASSWORD statement: 

shell> mysql -u root mysql
mysql> SET PASSWORD FOR root=PASSWORD('new_password');

Another way to set the password is by using the mysqladmin command: 

shell> mysqladmin -u root password new_password

Only users with write/update access to the mysql database can change the password for others users. All normal users (not anonymous ones) can only change their own password with either of the above commands or with SET PASSWORD=PASSWORD('new password').

Note that if you update the password in the user table directly using the first method, you must tell the server to re-read the grant tables (with FLUSH PRIVILEGES), because the change will go unnoticed otherwise.

Once the root password has been set, thereafter you must supply that password when you connect to the server as root.

You may wish to leave the root password blank so that you don't need to specify it while you perform additional setup or testing. However, be sure to set it before using your installation for any real production work.

See the scripts/mysql_install_db script to see how it sets up the default privileges. You can use this as a basis to see how to add other users.

If you want the initial privileges to be different than those just described above, you can modify mysql_install_db before you run it.

To re-create the grant tables completely, remove all the `.frm', `.MYI', and `.MYD' files in the directory containing the mysql database. (This is the directory named `mysql' under the database directory, which is listed when you run mysqld --help.) Then run the mysql_install_db script, possibly after editing it first to have the privileges you want.

NOTE: For MySQL versions older than Version 3.22.10, you should NOT delete the `.frm' files. If you accidentally do this, you should copy them back from your MySQL distribution before running mysql_install_db.

4.3.5 Adding New Users to MySQL

You can add users two different ways: by using GRANT statements or by manipulating the MySQL grant tables directly. The preferred method is to use GRANT statements, because they are more concise and less error-prone. See section 4.3.1 GRANT and REVOKE Syntax.

There is also a lot of contributed programs like phpmyadmin that can be used to create and administrate users. See section D Contributed Programs.

The examples below show how to use the mysql client to set up new users. These examples assume that privileges are set up according to the defaults described in the previous section. This means that to make changes, you must be on the same machine where mysqld is running, you must connect as the MySQL root user, and the root user must have the insert privilege for the mysql database and the reload administrative privilege. Also, if you have changed the root user password, you must specify it for the mysql commands below.

You can add new users by issuing GRANT statements: 

shell> mysql --user=root mysql
mysql> GRANT ALL PRIVILEGES ON *.* TO monty@localhost
           IDENTIFIED BY 'some_pass' WITH GRANT OPTION;
mysql> GRANT ALL PRIVILEGES ON *.* TO monty@"%"
           IDENTIFIED BY 'some_pass' WITH GRANT OPTION;
mysql> GRANT RELOAD,PROCESS ON *.* TO admin@localhost;
mysql> GRANT USAGE ON *.* TO dummy@localhost;

These GRANT statements set up three new users:

monty
A full superuser who can connect to the server from anywhere, but who must use a password 'some_pass' to do so. Note that we must issue GRANT statements for both monty@localhost and monty@"%". If we don't add the entry with localhost, the anonymous user entry for localhost that is created by mysql_install_db will take precedence when we connect from the local host, because it has a more specific Host field value and thus comes earlier in the user table sort order.
admin
A user who can connect from localhost without a password and who is granted the reload and process administrative privileges. This allows the user to execute the mysqladmin reload, mysqladmin refresh, and mysqladmin flush-* commands, as well as mysqladmin processlist . No database-related privileges are granted. (They can be granted later by issuing additional GRANT statements.)
dummy
A user who can connect without a password, but only from the local host. The global privileges are all set to 'N' -- the USAGE privilege type allows you to create a user with no privileges. It is assumed that you will grant database-specific privileges later.

You can also add the same user access information directly by issuing INSERT statements and then telling the server to reload the grant tables: 

shell> mysql --user=root mysql
mysql> INSERT INTO user VALUES('localhost','monty',PASSWORD('some_pass'),
                'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO user VALUES('%','monty',PASSWORD('some_pass'),
                'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO user SET Host='localhost',User='admin',
                 Reload_priv='Y', Process_priv='Y';
mysql> INSERT INTO user (Host,User,Password)
                        VALUES('localhost','dummy','');
mysql> FLUSH PRIVILEGES;

Depending on your MySQL version, you may have to use a different number of 'Y' values above (versions prior to Version 3.22.11 had fewer privilege columns). For the admin user, the more readable extended INSERT syntax that is available starting with Version 3.22.11 is used.

Note that to set up a superuser, you need only create a user table entry with the privilege fields set to 'Y'. No db or host table entries are necessary.

The privilege columns in the user table were not set explicitly in the last INSERT statement (for the dummy user), so those columns are assigned the default value of 'N'. This is the same thing that GRANT USAGE does.

The following example adds a user custom who can connect from hosts localhost, server.domain, and whitehouse.gov. He wants to access the bankaccount database only from localhost, the expenses database only from whitehouse.gov, and the customer database from all three hosts. He wants to use the password stupid from all three hosts.

To set up this user's privileges using GRANT statements, run these commands: 

shell> mysql --user=root mysql
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
           ON bankaccount.*
           TO custom@localhost
           IDENTIFIED BY 'stupid';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
           ON expenses.*
           TO custom@whitehouse.gov
           IDENTIFIED BY 'stupid';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
           ON customer.*
           TO custom@'%'
           IDENTIFIED BY 'stupid';

The reason that we do to grant statements for the user 'custom' is that we want the give the user access to MySQL both from the local machine with Unix sockets and from the remote machine 'whitehouse.gov' over TCP/IP.

To set up the user's privileges by modifying the grant tables directly, run these commands (note the FLUSH PRIVILEGES at the end): 

shell> mysql --user=root mysql
mysql> INSERT INTO user (Host,User,Password)
       VALUES('localhost','custom',PASSWORD('stupid'));
mysql> INSERT INTO user (Host,User,Password)
       VALUES('server.domain','custom',PASSWORD('stupid'));
mysql> INSERT INTO user (Host,User,Password)
       VALUES('whitehouse.gov','custom',PASSWORD('stupid'));
mysql> INSERT INTO db
       (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv,
        Create_priv,Drop_priv)
       VALUES
       ('localhost','bankaccount','custom','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
       (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv,
        Create_priv,Drop_priv)
       VALUES
       ('whitehouse.gov','expenses','custom','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
       (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv,
        Create_priv,Drop_priv)
       VALUES('%','customer','custom','Y','Y','Y','Y','Y','Y');
mysql> FLUSH PRIVILEGES;

The first three INSERT statements add user table entries that allow user custom to connect from the various hosts with the given password, but grant no permissions to him (all privileges are set to the default value of 'N'). The next three INSERT statements add db table entries that grant privileges to custom for the bankaccount, expenses, and customer databases, but only when accessed from the proper hosts. As usual, when the grant tables are modified directly, the server must be told to reload them (with FLUSH PRIVILEGES) so that the privilege changes take effect.

If you want to give a specific user access from any machine in a given domain, you can issue a GRANT statement like the following: 

mysql> GRANT ...
           ON *.*
           TO myusername@"%.mydomainname.com"
           IDENTIFIED BY 'mypassword';

To do the same thing by modifying the grant tables directly, do this: 

mysql> INSERT INTO user VALUES ('%.mydomainname.com', 'myusername',
           PASSWORD('mypassword'),...);
mysql> FLUSH PRIVILEGES;

You can also use xmysqladmin, mysql_webadmin, and even xmysql to insert, change, and update values in the grant tables. You can find these utilities in the Contrib directory of the MySQL Website.

4.3.6 Setting Up Passwords

In most cases you should use GRANT to set up your users/passwords, so the following only applies for advanced users. See section 4.3.1 GRANT and REVOKE Syntax.

The examples in the preceding sections illustrate an important principle: when you store a non-empty password using INSERT or UPDATE statements, you must use the PASSWORD() function to encrypt it. This is because the user table stores passwords in encrypted form, not as plaintext. If you forget that fact, you are likely to attempt to set passwords like this: 

shell> mysql -u root mysql
mysql> INSERT INTO user (Host,User,Password)
       VALUES('%','jeffrey','biscuit');
mysql> FLUSH PRIVILEGES;

The result is that the plaintext value 'biscuit' is stored as the password in the user table. When the user jeffrey attempts to connect to the server using this password, the mysql client encrypts it with PASSWORD(), generates an authentification vector based on encrypted password and a random number, obtained from server, and sends the result to the server. The server uses the password value in the user table (that is not encrypted value 'biscuit') to perform the same calculations, and compares results. The comparison fails and the server rejects the connection: 

shell> mysql -u jeffrey -pbiscuit test
Access denied

Passwords must be encrypted when they are inserted in the user table, so the INSERT statement should have been specified like this instead: 

mysql> INSERT INTO user (Host,User,Password)
       VALUES('%','jeffrey',PASSWORD('biscuit'));

You must also use the PASSWORD() function when you use SET PASSWORD statements: 

mysql> SET PASSWORD FOR jeffrey@"%" = PASSWORD('biscuit');

If you set passwords using the GRANT ... IDENTIFIED BY statement or the mysqladmin password command, the PASSWORD() function is unnecessary. They both take care of encrypting the password for you, so you would specify a password of 'biscuit' like this: 

mysql> GRANT USAGE ON *.* TO jeffrey@"%" IDENTIFIED BY 'biscuit';

or 

shell> mysqladmin -u jeffrey password biscuit

NOTE: PASSWORD() does not perform password encryption in the same way that Unix passwords are encrypted. You should not assume that if your Unix password and your MySQL password are the same, that PASSWORD() will result in the same encrypted value as is stored in the Unix password file. See section 4.3.2 MySQL User Names and Passwords.

4.3.7 Keeping Your Password Secure

It is inadvisable to specify your password in a way that exposes it to discovery by other users. The methods you can use to specify your password when you run client programs are listed below, along with an assessment of the risks of each method:

All in all, the safest methods are to have the client program prompt for the password or to specify the password in a properly protected `.my.cnf' file.

4.4 Disaster Prevention and Recovery

4.4.1 Database Backups

Because MySQL tables are stored as files, it is easy to do a backup. To get a consistent backup, do a LOCK TABLES on the relevant tables followed by FLUSH TABLES for the tables. See section 6.7.2 LOCK TABLES/UNLOCK TABLES Syntax. See section 4.5.3 FLUSH Syntax. You only need a read lock; this allows other threads to continue to query the tables while you are making a copy of the files in the database directory. The FLUSH TABLE is needed to ensure that the all active index pages is written to disk before you start the backup.

If you want to make a SQL level backup of a table, you can use SELECT INTO OUTFILE or BACKUP TABLE. See section 6.4.1 SELECT Syntax. See section 4.4.2 BACKUP TABLE Syntax.

Another way to back up a database is to use the mysqldump program or the mysqlhotcopy script. See section 4.8.5 mysqldump, Dumping Table Structure and Data. See section 4.8.6 mysqlhotcopy, Copying MySQL Databases and Tables.

  1. Do a full backup of your databases: 
    shell> mysqldump --tab=/path/to/some/dir --opt --full

or

shell> mysqlhotcopy database /path/to/some/dir
    You can also simply copy all table files (`*.frm', `*.MYD', and `*.MYI' files) as long as the server isn't updating anything. The script mysqlhotcopy does use this method.
  2. Stop mysqld if it's running, then start it with the --log-update[=file_name] option. See section 4.9.3 The Update Log. The update log file(s) provide you with the information you need to replicate changes to the database that are made subsequent to the point at which you executed mysqldump.

If you have to restore something, try to recover your tables using REPAIR TABLE or myisamchk -r first. That should work in 99.9% of all cases. If myisamchk fails, try the following procedure: (This will only work if you have started MySQL with --log-update. See section 4.9.3 The Update Log.):

  1. Restore the original mysqldump backup.
  2. Execute the following command to re-run the updates in the binary log: 
    shell> mysqlbinlog hostname-bin.[0-9]* | mysql
    If you are using the update log you can use: 
    shell> ls -1 -t -r hostname.[0-9]* | xargs cat | mysql

ls is used to get all the update log files in the right order.

You can also do selective backups with SELECT * INTO OUTFILE 'file_name' FROM tbl_name and restore with LOAD DATA INFILE 'file_name' REPLACE ... To avoid duplicate records, you need a PRIMARY KEY or a UNIQUE key in the table. The REPLACE keyword causes old records to be replaced with new ones when a new record duplicates an old record on a unique key value.

If you get performance problems in making backups on your system, you can solve this by setting up replication and do the backups on the slave instead of on the master. See section 4.10.1 Introduction.

If you are using a Veritas file system, you can do:

  1. Execute in a client (perl ?) FLUSH TABLES WITH READ LOCK
  2. Fork a shell or execute in another client mount vxfs snapshot.
  3. Execute in the first client UNLOCK TABLES
  4. Copy files from snapshot
  5. Unmount snapshot

4.4.2 BACKUP TABLE Syntax

BACKUP TABLE tbl_name[,tbl_name...] TO '/path/to/backup/directory'

Make a copy of all the table files to the backup directory that are the minimum needed to restore it. Currenlty only works for MyISAM tables. For MyISAM table, copies .frm (definition) and .MYD (data) files. The index file can be rebuilt from those two.

Before using this command, please see See section 4.4.1 Database Backups.

During the backup, read lock will be held for each table, one at time, as they are being backed up. If you want to backup several tables as a snapshot, you must first issue LOCK TABLES obtaining a read lock for each table in the group.

The command returns a table with the following columns:

Column Value
Table Table name
Op Always ``backup''
Msg_type One of status, error, info or warning.
Msg_text The message.

Note that BACKUP TABLE is only available in MySQL version 3.23.25 and later.

4.4.3 RESTORE TABLE Syntax

RESTORE TABLE tbl_name[,tbl_name...] FROM '/path/to/backup/directory'

Restores the table(s) from the backup that was made with BACKUP TABLE. Existing tables will not be overwritten - if you try to restore over an existing table, you will get an error. Restore will take longer than BACKUP due to the need to rebuilt the index. The more keys you have, the longer it is going to take. Just as BACKUP TABLE, currently only works of MyISAM tables.

The command returns a table with the following columns:

Column Value
Table Table name
Op Always ``restore''
Msg_type One of status, error, info or warning.
Msg_text The message.

4.4.4 CHECK TABLE Syntax

CHECK TABLE tbl_name[,tbl_name...] [option [option...]]

option = QUICK | FAST | MEDIUM | EXTENDED | CHANGED

CHECK TABLE only works on MyISAM tables. On MyISAM tables it's the same thing as running myisamchk -m table_name on the table.

If you don't specify any option MEDIUM is used.

Checks the table(s) for errors. For MyISAM tables the key statistics is updated. The command returns a table with the following columns:

Column Value
Table Table name.
Op Always ``check''.
Msg_type One of status, error, info, or warning.
Msg_text The message.

Note that you can get many rows of information for each checked table. The last row will be of Msg_type status and should normally be OK. If you don't get OK, or Not checked you should normally run a repair of the table. See section 4.4.6 Using myisamchk for Table Maintenance and Crash Recovery. Not checked means that the table the given TYPE told MySQL that there wasn't any need to check the table.

The different check types stand for the following:

Type Meaning
QUICK Don't scan the rows to check for wrong links.
FAST Only check tables which haven't been closed properly.
CHANGED Only check tables which have been changed since last check or haven't been closed properly.
MEDIUM Scan rows to verify that deleted links are ok. This also calculates a key checksum for the rows and verifies this with a calcualted checksum for the keys.
EXTENDED Do a full key lookup for all keys for each row. This ensures that the table is 100 % consistent, but will take a long time!

For dynamic sized MyISAM tables a started check will always do a MEDIUM check. For static size rows we skip the row scan for QUICK and FAST as the rows are very seldom corrupted.

You can combine check options as in: 

CHECK TABLE test_table FAST QUICK;

Which only would do a quick check on the table if it wasn't closed properly.

NOTE: that in some case CHECK TABLE will change the table! This happens if the table is marked as 'corrupted' or 'not closed properly' but CHECK TABLE didn't find any problems in the table. In this case CHECK TABLE will mark the table as ok.

If a table is corrupted, then it's most likely that the problem is in the indexes and not in the data part. All of the above check types checks the indexes throughly and should thus find most errors.

If you just want to check a table that you assume is ok, you should use no check options or the QUICK option. The later should be used when you are in a hurry and can take the very small risk that QUICK didn't find an error in the data file (In most cases MySQL should find, under normal usage, any error in the data file. If this happens then the table will be marked as 'corrupted', in which case the table can't be used until it's repaired).

FAST and CHANGED are mostly intended to be used from a script (for example to be executed from cron) if you want to check your table from time to time. In most cases you FAST is to be prefered over CHANGED. (The only case when it isn't is when you suspect a bug you have found a bug in the MyISAM code.).

EXTENDED is only to be used after you have run a normal check but still get strange errors from a table when MySQL tries to update a row or find a row by key (this is VERY unlikely to happen if a normal check has succeeded!).

Some things reported by check table, can't be corrected automatically:

4.4.5 REPAIR TABLE Syntax

REPAIR TABLE tbl_name[,tbl_name...] [QUICK] [EXTENDED]

REPAIR TABLE only works on MyISAM tables and is the same as running myisamchk -r table_name on the table.

Normally you should never have to run this command, but if disaster strikes you are very likely to get back all your data from a MyISAM table with REPAIR TABLE. If your tables get corrupted a lot you should try to find the reason for this! See section A.4.1 What To Do If MySQL Keeps Crashing. See section 7.1.3 MyISAM table problems..

REPAIR TABLE repairs a possible corrupted table. The command returns a table with the following columns:

Column Value
Table Table name
Op Always ``repair''
Msg_type One of status, error, info or warning.
Msg_text The message.

Note that you can get many rows of information for each repaired table. The last one row will be of Msg_type status and should normally be OK. If you don't get OK, you should try repairing the table with myisamchk -o, as REPAIR TABLE does not yet implement all the options of myisamchk. In the near future, we will make it more flexible.

If QUICK is given then MySQL will try to do a REPAIR of only the index tree.

If you use EXTENDED then MySQL will create the index row by row instead of creating one index at a time with sorting; This may be better than sorting on fixed-length keys if you have long char() keys that compress very good.

4.4.6 Using myisamchk for Table Maintenance and Crash Recovery

Starting with MySQL Version 3.23.13, you can check MyISAM tables with the CHECK TABLE command. See section 4.4.4 CHECK TABLE Syntax. You can repair tables with the REPAIR TABLE command. See section 4.4.5 REPAIR TABLE Syntax.

To check/repair MyISAM tables (.MYI and .MYD) you should use the myisamchk utility. To check/repair ISAM tables (.ISM and .ISD) you should use the isamchk utility. See section 7 MySQL Table Types.

In the following text we will talk about myisamchk, but everything also applies to the old isamchk.

You can use the myisamchk utility to get information about your database tables, check and repair them, or optimize them. The following sections describe how to invoke myisamchk (including a description of its options), how to set up a table maintenance schedule, and how to use myisamchk to perform its various functions.

You can, in most cases, also use the command OPTIMIZE TABLES to optimize and repair tables, but this is not as fast or reliable (in case of real fatal errors) as myisamchk. On the other hand, OPTIMIZE TABLE is easier to use and you don't have to worry about flushing tables. See section 4.5.1 OPTIMIZE TABLE Syntax.

Even that the repair in myisamchk is quite secure, it's always a good idea to make a backup BEFORE doing a repair (or anything that could make a lot of changes to a table)

4.4.6.1 myisamchk Invocation Syntax

myisamchk is invoked like this: 

shell> myisamchk [options] tbl_name

The options specify what you want myisamchk to do. They are described below. (You can also get a list of options by invoking myisamchk --help.) With no options, myisamchk simply checks your table. To get more information or to tell myisamchk to take corrective action, specify options as described below and in the following sections.

tbl_name is the database table you want to check/repair. If you run myisamchk somewhere other than in the database directory, you must specify the path to the file, because myisamchk has no idea where your database is located. Actually, myisamchk doesn't care whether or not the files you are working on are located in a database directory; you can copy the files that correspond to a database table into another location and perform recovery operations on them there.

You can name several tables on the myisamchk command line if you wish. You can also specify a name as an index file name (with the `.MYI' suffix), which allows you to specify all tables in a directory by using the pattern `*.MYI'. For example, if you are in a database directory, you can check all the tables in the directory like this: 

shell> myisamchk *.MYI

If you are not in the database directory, you can check all the tables there by specifying the path to the directory: 

shell> myisamchk /path/to/database_dir/*.MYI

You can even check all tables in all databases by specifying a wild card with the path to the MySQL data directory: 

shell> myisamchk /path/to/datadir/*/*.MYI

The recommended way to quickly check all tables is: 

myisamchk --silent --fast /path/to/datadir/*/*.MYI
isamchk --silent /path/to/datadir/*/*.ISM

If you want to check all tables and repair all tables that are corrupted, you can use the following line: 

myisamchk --silent --force --fast --update-state -O key_buffer=64M -O sort_buffer=64M -O read_buffer=1M -O write_buffer=1M /path/to/datadir/*/*.MYI
isamchk --silent --force -O key_buffer=64M -O sort_buffer=64M -O read_buffer=1M -O write_buffer=1M /path/to/datadir/*/*.ISM

The above assumes that you have more than 64 M free.

Note that if you get an error like: 

myisamchk: warning: 1 clients is using or hasn't closed the table properly

This means that you are trying to check a table that has been updated by the another program (like the mysqld server) that hasn't yet closed the file or that has died without closing the file properly.

If you mysqld is running, you must force a sync/close of all tables with FLUSH TABLES and ensure that no one is using the tables while you are running myisamchk. In MySQL Version 3.23 the easiest way to avoid this problem is to use CHECK TABLE instead of myisamchk to check tables.

4.4.6.2 General Options for myisamchk

myisamchk supports the following options.

-# or --debug=debug_options
Output debug log. The debug_options string often is 'd:t:o,filename'.
-? or --help
Display a help message and exit.
-O var=option, --set-variable var=option
Set the value of a variable. The possible variables and their default values for myisamchk can be examined with myisamchk --help:
key_buffer_size 523264
read_buffer_size 262136
write_buffer_size 262136
sort_buffer_size 2097144
sort_key_blocks 16
decode_bits 9
sort_buffer_size is used when the keys are repaired by sorting keys, which is the normal case when you use --recover. key_buffer_size is used when you are checking the table with --extended-check or when the keys are repaired by inserting key row by row in to the table (like when doing normal inserts). Repairing through the key buffer is used in the following cases: Reparing through the key buffer takes much less disk space than using sorting, but is also much slower. If you want a faster repair, set the above variables to about 1/4 of your available memory. You can set both variables to big values, as only one of the above buffers will be used at a time.
-s or --silent
Silent mode. Write output only when errors occur. You can use -s twice (-ss) to make myisamchk very silent.
-v or --verbose
Verbose mode. Print more information. This can be used with -d and -e. Use -v multiple times (-vv, -vvv) for more verbosity!
-V or --version
Print the myisamchk version and exit.
-w or, --wait
Instead of giving an error if the table is locked, wait until the table is unlocked before continuing. Note that if you are running mysqld on the table with --skip-locking, the table can only be locked by another myisamchk command.

4.4.6.3 Check Options for myisamchk

-c or --check
Check table for errors. This is the default operation if you are not giving myisamchk any options that override this.
-e or --extend-check
Check the table VERY thoroughly (which is quite slow if you have many indexes). This option should only be used in extreme cases. Normally, myisamchk or myisamchk --medium-check should, in most cases, be able to find out if there are any errors in the table. If you are using --extended-check and have much memory, you should increase the value of key_buffer_size a lot!
-F or --fast
Check only tables that haven't been closed properly.
-C or --check-only-changed
Check only tables that have changed since the last check.
-f or --force
Restart myisamchk with -r (repair) on the table, if myisamchk finds any errors in the table.
-i or --information
Print informational statistics about the table that is checked.
-m or --medium-check
Faster than extended-check, but only finds 99.99% of all errors. Should, however, be good enough for most cases.
-U or --update-state
Store in the `.MYI' file when the table was checked and if the table crashed. This should be used to get full benefit of the --check-only-changed option, but you shouldn't use this option if the mysqld server is using the table and you are running mysqld with --skip-locking.
-T or --read-only
Don't mark table as checked. This is useful if you use myisamchk to check a table that is in use by some other application that doesn't use locking (like mysqld --skip-locking).

4.4.6.4 Repair Options for myisamchk

The following options are used if you start myisamchk with -r or -o:

-D # or --data-file-length=#
Max length of data file (when re-creating data file when it's 'full').
-e or --extend-check
Try to recover every possible row from the data file. Normally this will also find a lot of garbage rows. Don't use this option if you are not totally desperate.
-f or --force
Overwrite old temporary files (table_name.TMD) instead of aborting.
-k # or keys-used=#
If you are using ISAM, tells the ISAM table handler to update only the first # indexes. If you are using MyISAM, tells which keys to use, where each binary bit stands for one key (first key is bit 0). This can be used to get faster inserts! Deactivated indexes can be reactivated by using myisamchk -r. keys.
-l or --no-symlinks
Do not follow symbolic links. Normally myisamchk repairs the table a symlink points at. This option doesn't exist in MySQL 4.0, as MySQL 4.0 will not remove symlinks during repair.
-r or --recover
Can fix almost anything except unique keys that aren't unique (which is an extremely unlikely error with ISAM/MyISAM tables). If you want to recover a table, this is the option to try first. Only if myisamchk reports that the table can't be recovered by -r, you should then try -o. (Note that in the unlikely case that -r fails, the data file is still intact.) If you have lots of memory, you should increase the size of sort_buffer_size!
-o or --safe-recover
Uses an old recovery method (reads through all rows in order and updates all index trees based on the found rows); this is a magnitude slower than -r, but can handle a couple of very unlikely cases that -r cannot handle. This recovery method also uses much less disk space than -r. Normally one should always first repair with -r, and only if this fails use -o. If you have lots of memory, you should increase the size of key_buffer_size!
-n or --sort-recover
Force myisamchk to use sorting to resolve the keys even if the temporary files should be very big. This will not have any effect if you have fulltext keys in the table.
--character-sets-dir=...
Directory where character sets are stored.
--set-character-set=name
Change the character set used by the index
.t or --tmpdir=path
Path for storing temporary files. If this is not set, myisamchk will use the environment variable TMPDIR for this.
-q or --quick
Faster repair by not modifying the data file. One can give a second -q to force myisamchk to modify the original datafile in case of duplicate keys
-u or --unpack
Unpack file packed with myisampack.

4.4.6.5 Other Options for myisamchk

Other actions that myisamchk can do, besides repair and check tables:

-a or --analyze
Analyze the distribution of keys. This improves join performance by enabling the join optimizer to better choose in which order it should join the tables and which keys it should use: myisamchk --describe --verbose table_name' or using SHOW KEYS in MySQL.
-d or --description
Prints some information about table.
-A or --set-auto-increment[=value]
Force auto_increment to start at this or higher value. If no value is given, then sets the next auto_increment value to the highest used value for the auto key + 1.
-S or --sort-index
Sort the index tree blocks in high-low order. This will optimize seeks and will make table scanning by key faster.
-R or --sort-records=#
Sorts records according to an index. This makes your data much more localized and may speed up ranged SELECT and ORDER BY operations on this index. (It may be VERY slow to do a sort the first time!) To find out a table's index numbers, use SHOW INDEX, which shows a table's indexes in the same order that myisamchk sees them. Indexes are numbered beginning with 1.

4.4.6.6 myisamchk Memory Usage

Memory allocation is important when you run myisamchk. myisamchk uses no more memory than you specify with the -O options. If you are going to use myisamchk on very large files, you should first decide how much memory you want it to use. The default is to use only about 3M to fix things. By using larger values, you can get myisamchk to operate faster. For example, if you have more than 32M RAM, you could use options such as these (in addition to any other options you might specify): 

shell> myisamchk -O sort=16M -O key=16M -O read=1M -O write=1M ...

Using -O sort=16M should probably be enough for most cases.

Be aware that myisamchk uses temporary files in TMPDIR. If TMPDIR points to a memory file system, you may easily get out of memory errors. If this happens, set TMPDIR to point at some directory with more space and restart myisamchk.

When repairing, myisamchk will also need a lot of disk space:

If you have a problem with disk space during repair, you can try to use --safe-recover instead of --recover.

4.4.6.7 Using myisamchk for Crash Recovery

If you run mysqld with --skip-locking (which is the default on some systems, like Linux), you can't reliably use myisamchk to check a table when mysqld is using the same table. If you can be sure that no one is accessing the tables through mysqld while you run myisamchk, you only have to do mysqladmin flush-tables before you start checking the tables. If you can't guarantee the above, then you must take down mysqld while you check the tables. If you run myisamchk while mysqld is updating the tables, you may get a warning that a table is corrupt even if it isn't.

If you are not using --skip-locking, you can use myisamchk to check tables at any time. While you do this, all clients that try to update the table will wait until myisamchk is ready before continuing.

If you use myisamchk to repair or optimize tables, you MUST always ensure that the mysqld server is not using the table (this also applies if you are using --skip-locking). If you don't take down mysqld you should at least do a mysqladmin flush-tables before you run myisamchk.

This chapter describes how to check for and deal with data corruption in MySQL databases. If your tables get corrupted a lot you should try to find the reason for this! See section A.4.1 What To Do If MySQL Keeps Crashing.

The MyISAM table section contains reason for why a table could be corrupted. See section 7.1.3 MyISAM table problems..

When performing crash recovery, it is important to understand that each table tbl_name in a database corresponds to three files in the database directory:

File Purpose
`tbl_name.frm' Table definition (form) file
`tbl_name.MYD' Data file
`tbl_name.MYI' Index file

Each of these three file types is subject to corruption in various ways, but problems occur most often in data files and index files.

myisamchk works by creating a copy of the `.MYD' (data) file row by row. It ends the repair stage by removing the old `.MYD' file and renaming the new file to the original file name. If you use --quick, myisamchk does not create a temporary `.MYD' file, but instead assumes that the `.MYD' file is correct and only generates a new index file without touching the `.MYD' file. This is safe, because myisamchk automatically detects if the `.MYD' file is corrupt and aborts the repair in this case. You can also give two --quick options to myisamchk. In this case, myisamchk does not abort on some errors (like duplicate key) but instead tries to resolve them by modifying the `.MYD' file. Normally the use of two --quick options is useful only if you have too little free disk space to perform a normal repair. In this case you should at least make a backup before running myisamchk.

4.4.6.8 How to Check Tables for Errors

To check a MyISAM table, use the following commands:

myisamchk tbl_name
This finds 99.99% of all errors. What it can't find is corruption that involves ONLY the data file (which is very unusual). If you want to check a table, you should normally run myisamchk without options or with either the -s or --silent option.
myisamchk -m tbl_name
This finds 99.999% of all errors. It checks first all index entries for errors and then it reads through all rows. It calculates a checksum for all keys in the rows and verifies that they checksum matches the checksum for the keys in the index tree.
myisamchk -e tbl_name
This does a complete and thorough check of all data (-e means ``extended check''). It does a check-read of every key for each row to verify that they indeed point to the correct row. This may take a LONG time on a big table with many keys. myisamchk will normally stop after the first error it finds. If you want to obtain more information, you can add the --verbose (-v) option. This causes myisamchk to keep going, up through a maximum of 20 errors. In normal usage, a simple myisamchk (with no arguments other than the table name) is sufficient.
myisamchk -e -i tbl_name
Like the previous command, but the -i option tells myisamchk to print some informational statistics, too.

4.4.6.9 How to Repair Tables

In the following section we only talk about using myisamchk on MyISAM tables (extensions .MYI and .MYD). If you are using ISAM tables (extensions .ISM and .ISD), you should use isamchk instead.

Starting with MySQL Version 3.23.14, you can repair MyISAM tables with the REPAIR TABLE command. See section 4.4.5 REPAIR TABLE Syntax.

The symptoms of a corrupted table include queries that abort unexpectedly and observable errors such as these:

In the other cases, you must repair your tables. myisamchk can usually detect and fix most things that go wrong.

The repair process involves up to four stages, described below. Before you begin, you should cd to the database directory and check the permissions of the table files. Make sure they are readable by the Unix user that mysqld runs as (and to you, because you need to access the files you are checking). If it turns out you need to modify files, they must also be writable by you.

If you are using MySQL Version 3.23.16 and above, you can (and should) use the CHECK and REPAIR commands to check and repair MyISAM tables. See section 4.4.4 CHECK TABLE Syntax. See section 4.4.5 REPAIR TABLE Syntax.

The manual section about table maintenance includes the options to isamchk/myisamchk. See section 4.4.6 Using myisamchk for Table Maintenance and Crash Recovery.

The following section is for the cases where the above command fails or if you want to use the extended features that isamchk/myisamchk provides.

If you are going to repair a table from the command line, you must first take down the mysqld server. Note that when you do mysqladmin shutdown on a remote server, the mysqld server will still be alive for a while after mysqladmin returns, until all queries are stopped and all keys have been flushed to disk.

Stage 1: Checking your tables

Run myisamchk *.MYI or myisamchk -e *.MYI if you have more time. Use the -s (silent) option to suppress unnecessary information.

If the mysqld server is done you should use the --update option to tell myisamchk to mark the table as 'checked'.

You have to repair only those tables for which myisamchk announces an error. For such tables, proceed to Stage 2.

If you get weird errors when checking (such as out of memory errors), or if myisamchk crashes, go to Stage 3.

Stage 2: Easy safe repair

NOTE: If you want repairing to go much faster, you should add: -O sort_buffer=# -O key_buffer=# (where # is about 1/4 of the available memory) to all isamchk/myisamchk commands.

First, try myisamchk -r -q tbl_name (-r -q means ``quick recovery mode''). This will attempt to repair the index file without touching the data file. If the data file contains everything that it should and the delete links point at the correct locations within the data file, this should work, and the table is fixed. Start repairing the next table. Otherwise, use the following procedure:

  1. Make a backup of the data file before continuing.
  2. Use myisamchk -r tbl_name (-r means ``recovery mode''). This will remove incorrect records and deleted records from the data file and reconstruct the index file.
  3. If the preceding step fails, use myisamchk --safe-recover tbl_name. Safe recovery mode uses an old recovery method that handles a few cases that regular recovery mode doesn't (but is slower).

If you get weird errors when repairing (such as out of memory errors), or if myisamchk crashes, go to Stage 3.

Stage 3: Difficult repair

You should only reach this stage if the first 16K block in the index file is destroyed or contains incorrect information, or if the index file is missing. In this case, it's necessary to create a new index file. Do so as follows:

  1. Move the data file to some safe place.
  2. Use the table description file to create new (empty) data and index files: 
    shell> mysql db_name
mysql> SET AUTOCOMMIT=1;
mysql> TRUNCATE TABLE table_name;
mysql> quit
    If your SQL version doesn't have TRUNCATE TABLE, use DELETE FROM table_name instead.
  3. Copy the old data file back onto the newly created data file. (Don't just move the old file back onto the new file; you want to retain a copy in case something goes wrong.)

Go back to Stage 2. myisamchk -r -q should work now. (This shouldn't be an endless loop.)

Stage 4: Very difficult repair

You should reach this stage only if the description file has also crashed. That should never happen, because the description file isn't changed after the table is created:

  1. Restore the description file from a backup and go back to Stage 3. You can also restore the index file and go back to Stage 2. In the latter case, you should start with myisamchk -r.
  2. If you don't have a backup but know exactly how the table was created, create a copy of the table in another database. Remove the new data file, then move the description and index files from the other database to your crashed database. This gives you new description and index files, but leaves the data file alone. Go back to Stage 2 and attempt to reconstruct the index file.

4.4.6.10 Table Optimization

To coalesce fragmented records and eliminate wasted space resulting from deleting or updating records, run myisamchk in recovery mode: 

shell> myisamchk -r tbl_name

You can optimize a table in the same way using the SQL OPTIMIZE TABLE statement. OPTIMIZE TABLE does a repair of the table, a key analyzes and also sorts the index tree to give faster key lookups. There is also no possibility of unwanted interaction between a utility and the server, because the server does all the work when you use OPTIMIZE TABLE. See section 4.5.1 OPTIMIZE TABLE Syntax.

myisamchk also has a number of other options you can use to improve the performance of a table:

-S, --sort-index
-R index_num, --sort-records=index_num
-a, --analyze

For a full description of the option. See section 4.4.6.1 myisamchk Invocation Syntax.

4.4.7 Setting Up a Table Maintenance Regimen

Starting with MySQL Version 3.23.13, you can check MyISAM tables with the CHECK TABLE command. See section 4.4.4 CHECK TABLE Syntax. You can repair tables with the REPAIR TABLE command. See section 4.4.5 REPAIR TABLE Syntax.

It is a good idea to perform table checks on a regular basis rather than waiting for problems to occur. For maintenance purposes, you can use myisamchk -s to check tables. The -s option (short for --silent) causes myisamchk to run in silent mode, printing messages only when errors occur.

It's also a good idea to check tables when the server starts up. For example, whenever the machine has done a reboot in the middle of an update, you usually need to check all the tables that could have been affected. (This is an ``expected crashed table''.) You could add a test to safe_mysqld that runs myisamchk to check all tables that have been modified during the last 24 hours if there is an old `.pid' (process ID) file left after a reboot. (The `.pid' file is created by mysqld when it starts up and removed when it terminates normally. The presence of a `.pid' file at system startup time indicates that mysqld terminated abnormally.)

An even better test would be to check any table whose last-modified time is more recent than that of the `.pid' file.

You should also check your tables regularly during normal system operation. At MySQL AB, we run a cron job to check all our important tables once a week, using a line like this in a `crontab' file: 

35 0 * * 0 /path/to/myisamchk --fast --silent /path/to/datadir/*/*.MYI

This prints out information about crashed tables so we can examine and repair them when needed.

As we haven't had any unexpectedly crashed tables (tables that become corrupted for reasons other than hardware trouble) for a couple of years now (this is really true), once a week is more than enough for us.

We recommend that to start with, you execute myisamchk -s each night on all tables that have been updated during the last 24 hours, until you come to trust MySQL as much as we do.

Normally you don't need to maintain MySQL tables that much. If you are changing tables with dynamic size rows (tables with VARCHAR, BLOB or TEXT columns) or have tables with many deleted rows you may want to from time to time (once a month?) defragment/reclaim space from the tables.

You can do this by using OPTIMIZE TABLE on the tables in question or if you can take the mysqld server down for a while do: 

isamchk -r --silent --sort-index -O sort_buffer_size=16M */*.ISM
myisamchk -r --silent --sort-index  -O sort_buffer_size=16M */*.MYI

4.4.8 Getting Information About a Table

To get a description of a table or statistics about it, use the commands shown below. We explain some of the information in more detail later:

myisamchk -d tbl_name
Runs myisamchk in ``describe mode'' to produce a description of your table. If you start the MySQL server using the --skip-locking option, myisamchk may report an error for a table that is updated while it runs. However, because myisamchk doesn't change the table in describe mode, there isn't any risk of destroying data.
myisamchk -d -v tbl_name
To produce more information about what myisamchk is doing, add -v to tell it to run in verbose mode.
myisamchk -eis tbl_name
Shows only the most important information from a table. It is slow because it must read the whole table.
myisamchk -eiv tbl_name
This is like -eis, but tells you what is being done.

Example of myisamchk -d output: 

MyISAM file:     company.MYI
Record format:   Fixed length
Data records:    1403698  Deleted blocks:         0
Recordlength:    226

table description:
Key Start Len Index   Type
1   2     8   unique  double
2   15    10  multip. text packed stripped
3   219   8   multip. double
4   63    10  multip. text packed stripped
5   167   2   multip. unsigned short
6   177   4   multip. unsigned long
7   155   4   multip. text
8   138   4   multip. unsigned long
9   177   4   multip. unsigned long
    193   1           text

Example of myisamchk -d -v output: 

MyISAM file:         company
Record format:       Fixed length
File-version:        1
Creation time:       1999-10-30 12:12:51
Recover time:        1999-10-31 19:13:01
Status:              checked
Data records:           1403698  Deleted blocks:              0
Datafile parts:         1403698  Deleted data:                0
Datafilepointer (bytes):      3  Keyfile pointer (bytes):     3
Max datafile length: 3791650815  Max keyfile length: 4294967294
Recordlength:               226

table description:
Key Start Len Index   Type                  Rec/key     Root Blocksize
1   2     8   unique  double                      1 15845376      1024
2   15    10  multip. text packed stripped        2 25062400      1024
3   219   8   multip. double                     73 40907776      1024
4   63    10  multip. text packed stripped        5 48097280      1024
5   167   2   multip. unsigned short           4840 55200768      1024
6   177   4   multip. unsigned long            1346 65145856      1024
7   155   4   multip. text                     4995 75090944      1024
8   138   4   multip. unsigned long              87 85036032      1024
9   177   4   multip. unsigned long             178 96481280      1024
    193   1           text

Example of myisamchk -eis output: 

Checking MyISAM file: company
Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
Total:    Keyblocks used:  98%  Packed:   17%

Records:          1403698    M.recordlength:     226   Packed:             0%
Recordspace used:     100%   Empty space:          0%  Blocks/Record:   1.00
Record blocks:    1403698    Delete blocks:        0
Recorddata:     317235748    Deleted data:         0
Lost space:             0    Linkdata:             0

User time 1626.51, System time 232.36
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 627, Swaps 0
Blocks in 0 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 639, Involuntary context switches 28966

Example of myisamchk -eiv output: 

Checking MyISAM file: company
Data records: 1403698   Deleted blocks:       0
- check file-size
- check delete-chain
block_size 1024:
index  1:
index  2:
index  3:
index  4:
index  5:
index  6:
index  7:
index  8:
index  9:
No recordlinks
- check index reference
- check data record references index: 1
Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
- check data record references index: 2
Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
- check data record references index: 3
Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
- check data record references index: 4
Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
- check data record references index: 5
Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 6
Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 7
Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 8
Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 9
Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
Total:    Keyblocks used:   9%  Packed:   17%

- check records and index references
[LOTS OF ROW NUMBERS DELETED]

Records:          1403698    M.recordlength:     226   Packed:             0%
Recordspace used:     100%   Empty space:          0%  Blocks/Record:   1.00
Record blocks:    1403698    Delete blocks:        0
Recorddata:     317235748    Deleted data:         0
Lost space:             0    Linkdata:             0

User time 1639.63, System time 251.61
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 10580, Swaps 0
Blocks in 4 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 10604, Involuntary context switches 122798

Here are the sizes of the data and index files for the table used in the preceding examples: 

-rw-rw-r--   1 monty    tcx     317235748 Jan 12 17:30 company.MYD
-rw-rw-r--   1 davida   tcx      96482304 Jan 12 18:35 company.MYM

Explanations for the types of information myisamchk produces are given below. The ``keyfile'' is the index file. ``Record'' and ``row'' are synonymous:

ISAM file
Name of the ISAM (index) file.
Isam-version
Version of ISAM format. Currently always 2.
Creation time
When the data file was created.
Recover time
When the index/data file was last reconstructed.
Data records
How many records are in the table.
Deleted blocks
How many deleted blocks still have reserved space. You can optimize your table to minimize this space. See section 4.4.6.10 Table Optimization.
Datafile: Parts
For dynamic record format, this indicates how many data blocks there are. For an optimized table without fragmented records, this is the same as Data records.
Deleted data
How many bytes of non-reclaimed deleted data there are. You can optimize your table to minimize this space. See section 4.4.6.10 Table Optimization.
Datafile pointer
The size of the data file pointer, in bytes. It is usually 2, 3, 4, or 5 bytes. Most tables manage with 2 bytes, but this cannot be controlled from MySQL yet. For fixed tables, this is a record address. For dynamic tables, this is a byte address.
Keyfile pointer
The size of the index file pointer, in bytes. It is usually 1, 2, or 3 bytes. Most tables manage with 2 bytes, but this is calculated automatically by MySQL. It is always a block address.
Max datafile length
How long the table's data file (.MYD file) can become, in bytes.
Max keyfile length
How long the table's key file (.MYI file) can become, in bytes.
Recordlength
How much space each record takes, in bytes.
Record format
The format used to store table rows. The examples shown above use Fixed length. Other possible values are Compressed and Packed.
table description
A list of all keys in the table. For each key, some low-level information is presented:
Key
This key's number.
Start
Where in the record this index part starts.
Len
How long this index part is. For packed numbers, this should always be the full length of the column. For strings, it may be shorter than the full length of the indexed column, because you can index a prefix of a string column.
Index
unique or multip. (multiple). Indicates whether or not one value can exist multiple times in this index.
Type
What data-type this index part has. This is an ISAM data-type with the options packed, stripped or empty.
Root
Address of the root index block.
Blocksize
The size of each index block. By default this is 1024, but the value may be changed at compile time.
Rec/key
This is a statistical value used by the optimizer. It tells how many records there are per value for this key. A unique key always has a value of 1. This may be updated after a table is loaded (or greatly changed) with myisamchk -a. If this is not updated at all, a default value of 30 is given.
In the first example above, the 9th key is a multi-part key with two parts.
Keyblocks used
What percentage of the keyblocks are used. Because the table used in the examples had just been reorganized with myisamchk, the values are very high (very near the theoretical maximum).
Packed
MySQL tries to pack keys with a common suffix. This can only be used for CHAR/VARCHAR/DECIMAL keys. For long strings like names, this can significantly reduce the space used. In the third example above, the 4th key is 10 characters long and a 60% reduction in space is achieved.
Max levels
How deep the B-tree for this key is. Large tables with long keys get high values.
Records
How many rows are in the table.
M.recordlength
The average record length. For tables with fixed-length records, this is the exact record length.
Packed
MySQL strips spaces from the end of strings. The Packed value indicates the percentage of savings achieved by doing this.
Recordspace used
What percentage of the data file is used.
Empty space
What percentage of the data file is unused.
Blocks/Record
Average number of blocks per record (that is, how many links a fragmented record is composed of). This is always 1 for fixed-format tables. This value should stay as close to 1.0 as possible. If it gets too big, you can reorganize the table with myisamchk. See section 4.4.6.10 Table Optimization.
Recordblocks
How many blocks (links) are used. For fixed format, this is the same as the number of records.
Deleteblocks
How many blocks (links) are deleted.
Recorddata
How many bytes in the data file are used.
Deleted data
How many bytes in the data file are deleted (unused).
Lost space
If a record is updated to a shorter length, some space is lost. This is the sum of all such losses, in bytes.
Linkdata
When the dynamic table format is used, record fragments are linked with pointers (4 to 7 bytes each). Linkdata is the sum of the amount of storage used by all such pointers.

If a table has been compressed with myisampack, myisamchk -d prints additional information about each table column. See section 4.7.4 myisampack, The MySQL Compressed Read-only Table Generator, for an example of this information and a description of what it means.

4.5 Database Administration Language Reference

4.5.1 OPTIMIZE TABLE Syntax

OPTIMIZE TABLE tbl_name[,tbl_name]...

OPTIMIZE TABLE should be used if you have deleted a large part of a table or if you have made many changes to a table with variable-length rows (tables that have VARCHAR, BLOB, or TEXT columns). Deleted records are maintained in a linked list and subsequent INSERT operations reuse old record positions. You can use OPTIMIZE TABLE to reclaim the unused space and to defragment the data file.

For the moment OPTIMIZE TABLE only works on MyISAM and BDB tables. For BDB tables, OPTIMIZE TABLE is currently mapped to ANALYZE TABLE. See section 4.5.2 ANALYZE TABLE Syntax.

You can get optimize table to work on other table types by starting mysqld with --skip-new or --safe-mode, but in this case OPTIMIZE TABLE is just mapped to ALTER TABLE.

OPTIMIZE TABLE works the following way:

OPTIMIZE TABLE for MyISAM tables is equvialent of running myisamchk --quick --check-changed-tables --sort-index --analyze on the table.

Note that the table is locked during the time OPTIMIZE TABLE is running!

4.5.2 ANALYZE TABLE Syntax

ANALYZE TABLE tbl_name[,tbl_name...]

Analyze and store the key distribution for the table. During the analyze the table is locked with a read lock. This works on MyISAM and BDB tables.

This is equivalent to running myisamchk -a on the table.

MySQL uses the stored key distribution to decide in which order tables should be joined when one does a join on something else than a constant.

The command returns a table with the following columns:

Column Value
Table Table name
Op Always ``analyze''
Msg_type One of status, error, info or warning.
Msg_text The message.

You can check the stored key distribution with the SHOW INDEX command. See section 4.5.5.1 Retrieving information about Database, Tables, Columns, and Indexes.

If the table hasn't changed since the last ANALYZE TABLE command, the table will not be analyzed again.

4.5.3 FLUSH Syntax

FLUSH flush_option [,flush_option]

You should use the FLUSH command if you want to clear some of the internal caches MySQL uses. To execute FLUSH, you must have the RELOAD privilege.

flush_option can be any of the following:

HOSTS Empties the host cache tables. You should flush the host tables if some of your hosts change IP number or if you get the error message Host ... is blocked. When more than max_connect_errors errors occur in a row for a given host while connection to the MySQL server, MySQL assumes something is wrong and blocks the host from further connection requests. Flushing the host tables allows the host to attempt to connect again. See section A.2.4 Host '...' is blocked Error.) You can start mysqld with -O max_connection_errors=999999999 to avoid this error message.
LOGS Closes and reopens all log files. If you have specified the update log file or a binary log file without an extension, the extension number of the log file will be incremented by one relative to the previous file. If you have used an extension in the file name, MySQL will close and reopen the update log file. See section 4.9.3 The Update Log. This is the same thing as sending the SIGHUP signal to the mysqld server.
PRIVILEGES Reloads the privileges from the grant tables in the mysql database.
TABLES Closes all open tables and force all tables in use to be closed.
[TABLE | TABLES] table_name [,table_name...] Flushes only the given tables.
TABLES WITH READ LOCK Closes all open tables and locks all tables for all databases with a read until one executes UNLOCK TABLES. This is very convenient way to get backups if you have a file system, like Veritas,that can take snapshots in time.
STATUS Resets most status variables to zero. This is something one should only use when debugging a query.

You can also access each of the commands shown above with the mysqladmin utility, using the flush-hosts, flush-logs, reload, or flush-tables commands.

Take also a look at the RESET command used with replication. See section 4.10.6 SQL Commands Related to Replication.

4.5.4 KILL Syntax

KILL thread_id

Each connection to mysqld runs in a separate thread. You can see which threads are running with the SHOW PROCESSLIST command and kill a thread with the KILL thread_id command.

If you have the process privilege, you can see and kill all threads. Otherwise, you can see and kill only your own threads.

You can also use the mysqladmin processlist and mysqladmin kill commands to examine and kill threads.

When you do a KILL, a thread specific kill flag is set for the thread.

In most cases it may take some time for the thread to die as the kill flag is only checked at specific intervals.

4.5.5 SHOW Syntax

   SHOW DATABASES [LIKE wild]
or SHOW [OPEN] TABLES [FROM db_name] [LIKE wild]
or SHOW [FULL] COLUMNS FROM tbl_name [FROM db_name] [LIKE wild]
or SHOW INDEX FROM tbl_name [FROM db_name]
or SHOW TABLE STATUS [FROM db_name] [LIKE wild]
or SHOW STATUS [LIKE wild]
or SHOW VARIABLES [LIKE wild]
or SHOW LOGS
or SHOW [FULL] PROCESSLIST
or SHOW GRANTS FOR user
or SHOW CREATE TABLE table_name
or SHOW MASTER STATUS
or SHOW MASTER LOGS
or SHOW SLAVE STATUS

SHOW provides information about databases, tables, columns, or status information about the server. If the LIKE wild part is used, the wild string can be a string that uses the SQL `%' and `_' wild-card characters.

4.5.5.1 Retrieving information about Database, Tables, Columns, and Indexes

You can use db_name.tbl_name as an alternative to the tbl_name FROM db_name syntax. These two statements are equivalent: 

mysql> SHOW INDEX FROM mytable FROM mydb;
mysql> SHOW INDEX FROM mydb.mytable;

SHOW DATABASES lists the databases on the MySQL server host. You can also get this list using the mysqlshow command.

SHOW TABLES lists the tables in a given database. You can also get this list using the mysqlshow db_name command.

NOTE: If a user doesn't have any privileges for a table, the table will not show up in the output from SHOW TABLES or mysqlshow db_name.

SHOW OPEN TABLES lists the tables that are currently open in the table cache. See section 5.4.6 How MySQL Opens and Closes Tables. The Comment field tells how many times the table is cached and in_use.

SHOW COLUMNS lists the columns in a given table. If you specify the FULL option, you will also get the privileges you have for each column. If the column types are different than you expect them to be based on a CREATE TABLE statement, note that MySQL sometimes changes column types. See section 6.5.3.1 Silent Column Specification Changes.

The DESCRIBE statement provides information similar to SHOW COLUMNS. See section 6.6.2 DESCRIBE Syntax (Get Information About Columns).

SHOW FIELDS is a synonym for SHOW COLUMNS, and SHOW KEYS is a synonym for SHOW INDEX. You can also list a table's columns or indexes with mysqlshow db_name tbl_name or mysqlshow -k db_name tbl_name.

SHOW INDEX returns the index information in a format that closely resembles the SQLStatistics call in ODBC. The following columns are returned:

Column Meaning
Table Name of the table.
Non_unique 0 if the index can't contain duplicates.
Key_name Name of the index.
Seq_in_index Column sequence number in index, starting with 1.
Column_name Column name.
Collation How the column is sorted in the index. In MySQL, this can have values `A' (Ascending) or NULL (Not sorted).
Cardinality Number of unique values in the index. This is updated by running isamchk -a.
Sub_part Number of indexed characters if the column is only partly indexed. NULL if the entire key is indexed.
Comment Various remarks. For now, it tells whether index is FULLTEXT or not.

Note that as the Cardinality is counted based on statistics stored as integers, it's not necessarily accurate for small tables.

4.5.5.2 SHOW TABLE STATUS

SHOW TABLE STATUS [FROM db_name] [LIKE wild]

SHOW TABLE STATUS (new in Version 3.23) works likes SHOW STATUS, but provides a lot of information about each table. You can also get this list using the mysqlshow --status db_name command. The following columns are returned:

Column Meaning
Name Name of the table.
Type Type of table. See section 7 MySQL Table Types.
Row_format The row storage format (Fixed, Dynamic, or Compressed).
Rows Number of rows.
Avg_row_length Average row length.
Data_length Length of the data file.
Max_data_length Max length of the data file.
Index_length Length of the index file.
Data_free Number of allocated but not used bytes.
Auto_increment Next autoincrement value.
Create_time When the table was created.
Update_time When the data file was last updated.
Check_time When the table was last checked.
Create_options Extra options used with CREATE TABLE.
Comment The comment used when creating the table (or some information why MySQL couldn't access the table information).

InnoDB tables will report the free space in the tablespace in the table comment.

4.5.5.3 SHOW STATUS

SHOW STATUS provides server status information (like mysqladmin extended-status). The output resembles that shown below, though the format and numbers probably differ: 

+--------------------------+------------+
| Variable_name            | Value      |
+--------------------------+------------+
| Aborted_clients          | 0          |
| Aborted_connects         | 0          |
| Bytes_received           | 155372598  |
| Bytes_sent               | 1176560426 |
| Connections              | 30023      |
| Created_tmp_disk_tables  | 0          |
| Created_tmp_tables       | 8340       |
| Created_tmp_files        | 60         |
| Delayed_insert_threads   | 0          |
| Delayed_writes           | 0          |
| Delayed_errors           | 0          |
| Flush_commands           | 1          |
| Handler_delete           | 462604     |
| Handler_read_first       | 105881     |
| Handler_read_key         | 27820558   |
| Handler_read_next        | 390681754  |
| Handler_read_prev        | 6022500    |
| Handler_read_rnd         | 30546748   |
| Handler_read_rnd_next    | 246216530  |
| Handler_update           | 16945404   |
| Handler_write            | 60356676   |
| Key_blocks_used          | 14955      |
| Key_read_requests        | 96854827   |
| Key_reads                | 162040     |
| Key_write_requests       | 7589728    |
| Key_writes               | 3813196    |
| Max_used_connections     | 0          |
| Not_flushed_key_blocks   | 0          |
| Not_flushed_delayed_rows | 0          |
| Open_tables              | 1          |
| Open_files               | 2          |
| Open_streams             | 0          |
| Opened_tables            | 44600      |
| Questions                | 2026873    |
| Select_full_join         | 0          |
| Select_full_range_join   | 0          |
| Select_range             | 99646      |
| Select_range_check       | 0          |
| Select_scan              | 30802      |
| Slave_running            | OFF        |
| Slave_open_temp_tables   | 0          |
| Slow_launch_threads      | 0          |
| Slow_queries             | 0          |
| Sort_merge_passes        | 30         |
| Sort_range               | 500        |
| Sort_rows                | 30296250   |
| Sort_scan                | 4650       |
| Table_locks_immediate    | 1920382    |
| Table_locks_waited       | 0          |
| Threads_cached           | 0          |
| Threads_created          | 30022      |
| Threads_connected        | 1          |
| Threads_running          | 1          |
| Uptime                   | 80380      |
+--------------------------+------------+

The status variables listed above have the following meaning:

Variable Meaning
Aborted_clients Number of connections aborted because the client died without closing the connection properly. See section A.2.9 Communication Errors / Aborted Connection.
Aborted_connects Number of tries to connect to the MySQL server that failed. See section A.2.9 Communication Errors / Aborted Connection.
Bytes_received Number of bytes received from all clients.
Bytes_sent Number of bytes sent to all clients.
Com_xxxx Number of times the xxx commands has been executed.
Connections Number of connection attempts to the MySQL server.
Created_tmp_disk_tables Number of implicit temporary tables on disk created while executing statements.
Created_tmp_tables Number of implicit temporary tables in memory created while executing statements.
Created_tmp_files How many temporary files mysqld have created.
Delayed_insert_threads Number of delayed insert handler threads in use.
Delayed_writes Number of rows written with INSERT DELAYED.
Delayed_errors Number of rows written with INSERT DELAYED for which some error occurred (probably duplicate key).
Flush_commands Number of executed FLUSH commands.
Handler_delete Number of times a row was deleted from a table.
Handler_read_first Number of times the first entry was read from an index. If this is high, it suggests that the server is doing a lot of full index scans, for example, SELECT col1 FROM foo, assuming that col1 is indexed.
Handler_read_key Number of requests to read a row based on a key. If this is high, it is a good indication that your queries and tables are properly indexed.
Handler_read_next Number of requests to read next row in key order. This will be incremented if you are querying an index column with a range constraint. This also will be incremented if you are doing an index scan.
Handler_read_rnd Number of requests to read a row based on a fixed position. This will be high if you are doing a lot of queries that require sorting of the result.
Handler_read_rnd_next Number of requests to read the next row in the datafile. This will be high if you are doing a lot of table scans. Generally this suggests that your tables are not properly indexed or that your queries are not written to take advantage of the indexes you have.
Handler_update Number of requests to update a row in a table.
Handler_write Number of requests to insert a row in a table.
Key_blocks_used The number of used blocks in the key cache.
Key_read_requests The number of requests to read a key block from the cache.
Key_reads The number of physical reads of a key block from disk.
Key_write_requests The number of requests to write a key block to the cache.
Key_writes The number of physical writes of a key block to disk.
Max_used_connections The maximum number of connections in use simultaneously.
Not_flushed_key_blocks Keys blocks in the key cache that has changed but hasn't yet been flushed to disk.
Not_flushed_delayed_rows Number of rows waiting to be written in INSERT DELAY queues.
Open_tables Number of tables that are open.
Open_files Number of files that are open.
Open_streams Number of streams that are open (used mainly for logging).
Opened_tables Number of tables that have been opened.
Select_full_join Number of joins without keys (Should be 0).
Select_full_range_join Number of joins where we used a range search on reference table.
Select_range Number of joins where we used ranges on the first table. (It's normally not critical even if this is big.)
Select_scan Number of joins where we scanned the first table.
Select_range_check Number of joins without keys where we check for key usage after each row (Should be 0).
Questions Number of queries sent to the server.
Slave_open_temp_tables Number of temporary tables currently open by the slave thread
Slow_launch_threads Number of threads that have taken more than slow_launch_time to connect.
Slow_queries Number of queries that have taken more than long_query_time. See section 4.9.5 The Slow Query Log.
Sort_merge_passes Number of merges the sort has to do. If this value is large you should consider increasing sort_buffer.
Sort_range Number of sorts that where done with ranges.
Sort_rows Number of sorted rows.
Sort_scan Number of sorts that where done by scanning the table.
Table_locks_immediate Number of times a table lock was acquired immediately. Available after 3.23.33.
Table_locks_waited Number of times a table lock could not be acquired immediately and a wait was needed. If this is high, and you have performance problems, you should first optimize your queries, and then either split your table(s) or use replication. Available after 3.23.33.
Threads_cached Number of threads in the thread cache.
Threads_connected Number of currently open connections.
Threads_created Number of threads created to handle connections.
Threads_running Number of threads that are not sleeping.
Uptime How many seconds the server has been up.

Some comments about the above:

4.5.5.4 SHOW VARIABLES

SHOW VARIABLES [LIKE wild]

SHOW VARIABLES shows the values of some MySQL system variables. You can also get this information using the mysqladmin variables command. If the default values are unsuitable, you can set most of these variables using command-line options when mysqld starts up. See section 4.1.1 mysqld Command-line Options.

The output resembles that shown below, though the format and numbers may differ somewhat: 

+-------------------------+---------------------------+
| Variable_name           | Value                     |
+-------------------------+---------------------------+
| ansi_mode               | OFF                       |
| back_log                | 50                        |
| basedir                 | /my/monty/                |
| bdb_cache_size          | 16777216                  |
| bdb_log_buffer_size     | 32768                     |
| bdb_home                | /my/monty/data/           |
| bdb_max_lock            | 10000                     |
| bdb_logdir              |                           |
| bdb_shared_data         | OFF                       |
| bdb_tmpdir              | /tmp/                     |
| binlog_cache_size       | 32768                     |
| concurrent_insert       | ON                        |
| connect_timeout         | 5                         |
| datadir                 | /my/monty/data/           |
| delay_key_write         | ON                        |
| delayed_insert_limit    | 100                       |
| delayed_insert_timeout  | 300                       |
| delayed_queue_size      | 1000                      |
| flush                   | OFF                       |
| flush_time              | 0                         |
| have_bdb                | YES                       |
| have_innodb             | YES                       |
| have_raid               | YES                       |
| have_ssl                | NO                        |
| init_file               |                           |
| interactive_timeout     | 28800                     |
| join_buffer_size        | 131072                    |
| key_buffer_size         | 16776192                  |
| language                | /my/monty/share/english/  |
| large_files_support     | ON                        |
| log                     | OFF                       |
| log_update              | OFF                       |
| log_bin                 | OFF                       |
| log_slave_updates       | OFF                       |
| long_query_time         | 10                        |
| low_priority_updates    | OFF                       |
| lower_case_table_names  | 0                         |
| max_allowed_packet      | 1048576                   |
| max_binlog_cache_size   | 4294967295                |
| max_connections         | 100                       |
| max_connect_errors      | 10                        |
| max_delayed_threads     | 20                        |
| max_heap_table_size     | 16777216                  |
| max_join_size           | 4294967295                |
| max_sort_length         | 1024                      |
| max_tmp_tables          | 32                        |
| max_write_lock_count    | 4294967295                |
| myisam_recover_options  | DEFAULT                   |
| myisam_sort_buffer_size | 8388608                   |
| net_buffer_length       | 16384                     |
| net_read_timeout        | 30                        |
| net_retry_count         | 10                        |
| net_write_timeout       | 60                        |
| open_files_limit        | 0                         |
| pid_file                | /my/monty/data/donna.pid  |
| port                    | 3306                      |
| protocol_version        | 10                        |
| record_buffer           | 131072                    |
| query_buffer_size       | 0                         |
| safe_show_database      | OFF                       |
| server_id               | 0                         |
| skip_locking            | ON                        |
| skip_networking         | OFF                       |
| skip_show_database      | OFF                       |
| slow_launch_time        | 2                         |
| socket                  | /tmp/mysql.sock           |
| sort_buffer             | 2097116                   |
| table_cache             | 64                        |
| table_type              | MYISAM                    |
| thread_cache_size       | 4                         |
| thread_stack            | 65536                     |
| tmp_table_size          | 1048576                   |
| tmpdir                  | /tmp/                     |
| version                 | 3.23.29a-gamma-debug      |
| wait_timeout            | 28800                     |
+-------------------------+---------------------------+

Each option is described below. Values for buffer sizes, lengths, and stack sizes are given in bytes. You can specify values with a suffix of `K' or `M' to indicate kilobytes or megabytes. For example, 16M indicates 16 megabytes. The case of suffix letters does not matter; 16M and 16m are equivalent:

ansi_mode.
Is ON if mysqld was started with --ansi. See section 1.4.3 Running MySQL in ANSI Mode.
back_log
The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets VERY many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The back_log value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time. In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix listen(2) system call should have more details. Check your OS documentation for the maximum value for this variable. Attempting to set back_log higher than your operating system limit will be ineffective.
basedir
The value of the --basedir option.
bdb_cache_size
The buffer that is allocated to cache index and rows for BDB tables. If you don't use BDB tables, you should start mysqld with --skip-bdb to not waste memory for this cache.
bdb_log_buffer_size
The buffer that is allocated to cache index and rows for BDB tables. If you don't use BDB tables, you should set this to 0 or start mysqld with --skip-bdb to not waste memory for this cache.
bdb_home
The value of the --bdb-home option.
bdb_max_lock
The maximum number of locks (1000 by default) you can have active on a BDB table. You should increase this if you get errors of type bdb: Lock table is out of available locks or Got error 12 from ... when you have do long transactions or when mysqld has to examine a lot of rows to calculate the query.
bdb_logdir
The value of the --bdb-logdir option.
bdb_shared_data
Is ON if you are using --bdb-shared-data.
bdb_tmpdir
The value of the --bdb-tmpdir option.
binlog_cache_size. The size of the cache to hold the SQL
statements for the binary log during a transaction. If you often use big, multi-statement transactions you can increase this to get more performance. See section 6.7.1 BEGIN/COMMIT/ROLLBACK Syntax.
character_set
The default character set.
character_sets
The supported character sets.
concurrent_inserts
If ON (the default), MySQL will allow you to use INSERT on MyISAM tables at the same time as you run SELECT queries on them. You can turn this option off by starting mysqld with --safe or --skip-new.
connect_timeout
The number of seconds the mysqld server is waiting for a connect packet before responding with Bad handshake.
datadir
The value of the --datadir option.
delay_key_write
If enabled (is on by default), MySQL will honor the delay_key_write option CREATE TABLE. This means that the key buffer for tables with this option will not get flushed on every index update, but only when a table is closed. This will speed up writes on keys a lot, but you should add automatic checking of all tables with myisamchk --fast --force if you use this. Note that if you start mysqld with the --delay-key-write-for-all-tables option this means that all tables will be treated as if they were created with the delay_key_write option. You can clear this flag by starting mysqld with --skip-new or --safe-mode.
delayed_insert_limit
After inserting delayed_insert_limit rows, the INSERT DELAYED handler will check if there are any SELECT statements pending. If so, it allows these to execute before continuing.
delayed_insert_timeout
How long a INSERT DELAYED thread should wait for INSERT statements before terminating.
delayed_queue_size
What size queue (in rows) should be allocated for handling INSERT DELAYED. If the queue becomes full, any client that does INSERT DELAYED will wait until there is room in the queue again.
flush
This is ON if you have started MySQL with the --flush option.
flush_time
If this is set to a non-zero value, then every flush_time seconds all tables will be closed (to free up resources and sync things to disk). We only recommend this option on Win95, Win98, or on systems where you have very little resources.
have_bdb
YES if mysqld supports Berkeley DB tables. DISABLED if --skip-bdb is used.
have_innodb
YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used.
have_raid
YES if mysqld supports the RAID option.
have_ssl
YES if mysqld supports SSL (encryption) on the client/server protocol.
init_file
The name of the file specified with the --init-file option when you start the server. This is a file of SQL statements you want the server to execute when it starts.
interactive_timeout
The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). See also wait_timeout.
join_buffer_size
The size of the buffer that is used for full joins (joins that do not use indexes). The buffer is allocated one time for each full join between two tables. Increase this value to get a faster full join when adding indexes is not possible. (Normally the best way to get fast joins is to add indexes.)
key_buffer_size
Index blocks are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. Increase this to get better index handling (for all reads and multiple writes) to as much as you can afford; 64M on a 256M machine that mainly runs MySQL is quite common. If you, however, make this too big (more than 50% of your total memory?) your system may start to page and become REALLY slow. Remember that because MySQL does not cache data read, that you will have to leave some room for the OS filesystem cache. You can check the performance of the key buffer by doing show status and examine the variables Key_read_requests, Key_reads, Key_write_requests, and Key_writes. The Key_reads/Key_read_request ratio should normally be < 0.01. The Key_write/Key_write_requests is usually near 1 if you are using mostly updates/deletes but may be much smaller if you tend to do updates that affect many at the same time or if you are using delay_key_write. See section 4.5.5 SHOW Syntax. To get even more speed when writing many rows at the same time, use LOCK TABLES. See section 6.7.2 LOCK TABLES/UNLOCK TABLES Syntax.
language
The language used for error messages.
large_file_support
If mysqld was compiled with options for big file support.
locked_in_memory
If mysqld was locked in memory with --memlock
log
If logging of all queries is enabled.
log_update
If the update log is enabled.
log_bin
If the binary log is enabled.
log_slave_updates
If the updates from the slave should be logged.
long_query_time
If a query takes longer than this (in seconds), the Slow_queries counter will be incremented. If you are using --log-slow-queries, the query will be logged to the slow query logfile. See section 4.9.5 The Slow Query Log.
lower_case_table_names
If set to 1 table names are stored in lowercase on disk and table names will be case-insensitive. See section 6.1.3 Case Sensitivity in Names.
max_allowed_packet
The maximum size of one packet. The message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet bytes when needed. This value by default is small, to catch big (possibly wrong) packets. You must increase this value if you are using big BLOB columns. It should be as big as the biggest BLOB you want to use. The current protocol limits max_allowed_packet to 16M.
max_binlog_cache_size
If a multi-statement transaction requires more than this amount of memory, one will get the error "Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage".
max_binlog_size
Available after 3.23.33. If a write to the binary (replication) log exceeds the given value, rotate the logs. You cannot set it to less than 1024 bytes, or more than 1 GB. Default is 1 GB.
max_connections
The number of simultaneous clients allowed. Increasing this value increases the number of file descriptors that mysqld requires. See below for comments on file descriptor limits. See section A.2.5 Too many connections Error.
max_connect_errors
If there is more than this number of interrupted connections from a host this host will be blocked from further connections. You can unblock a host with the command FLUSH HOSTS.
max_delayed_threads
Don't start more than this number of threads to handle INSERT DELAYED statements. If you try to insert data into a new table after all INSERT DELAYED threads are in use, the row will be inserted as if the DELAYED attribute wasn't specified.
max_heap_table_size
Don't allow creation of heap tables bigger than this.
max_join_size
Joins that are probably going to read more than max_join_size records return an error. Set this value if your users tend to perform joins that lack a WHERE clause, that take a long time, and that return millions of rows.
max_sort_length
The number of bytes to use when sorting BLOB or TEXT values (only the first max_sort_length bytes of each value are used; the rest are ignored).
max_user_connections
The maximum number of active connections for a single user (0 = no limit).
max_tmp_tables
(This option doesn't yet do anything.) Maximum number of temporary tables a client can keep open at the same time.
max_write_lock_count
After this many write locks, allow some read locks to run in between.
myisam_recover_options
The value of the --myisam-recover option.
myisam_sort_buffer_size
The buffer that is allocated when sorting the index when doing a REPAIR or when creating indexes with CREATE INDEX or ALTER TABLE.
myisam_max_extra_sort_file_size.
If the creating of the temporary file for fast index creation would be this much bigger than using the key cache, then prefer the key cache method. This is mainly used to force long character keys in large tables to use the slower key cache method to create the index. NOTE that this parameter is given in megabytes!
myisam_max_sort_file_size
The maximum size of the temporary file MySQL is allowed to use while recreating the index (during REPAIR, ALTER TABLE or LOAD DATA INFILE. If the file size would be bigger than this, the index will be created through the key cache (which is slower). NOTE that this parameter is given in megabytes!
net_buffer_length
The communication buffer is reset to this size between queries. This should not normally be changed, but if you have very little memory, you can set it to the expected size of a query. (That is, the expected length of SQL statements sent by clients. If statements exceed this length, the buffer is automatically enlarged, up to max_allowed_packet bytes.)
net_read_timeout
Number of seconds to wait for more data from a connection before aborting the read. Note that when we don't expect data from a connection, the timeout is defined by write_timeout. See also slave_read_timeout.
net_retry_count
If a read on a communication port is interrupted, retry this many times before giving up. This value should be quite high on FreeBSD as internal interrupts are sent to all threads.
net_write_timeout
Number of seconds to wait for a block to be written to a connection before aborting the write.
open_files_limit
If this is not 0, then mysqld will use this value to reserve file descriptors to use with setrlimit(). If this value is 0 then mysqld will reserve max_connections*5 or max_connections + table_cache*2 (whichever is larger) number of files. You should try increasing this if mysqld gives you the error 'Too many open files'.
pid_file
The value of the --pid-file option.
port
The value of the --port option.
protocol_version
The protocol version used by the MySQL server.
record_buffer
Each thread that does a sequential scan allocates a buffer of this size for each table it scans. If you do many sequential scans, you may want to increase this value.
record_rnd_buffer
When reading rows in sorted order after a sort, the rows are read through this buffer to avoid a disk seeks. If not set, then it's set to the value of record_buffer.
query_buffer_size
The initial allocation of the query buffer. If most of your queries are long (like when inserting blobs), you should increase this!
safe_show_databases
Don't show databases for which the user doesn't have any database or table privileges. This can improve security if you're concerned about people being able to see what databases other users have. See also skip_show_databases.
server_id
The value of the --server-id option.
skip_locking
Is OFF if mysqld uses external locking.
skip_networking
Is ON if we only allow local (socket) connections.
skip_show_databases
This prevents people from doing SHOW DATABASES if they don't have the PROCESS_PRIV privilege. This can improve security if you're concerned about people being able to see what databases other users have. See also safe_show_databases.
slave_read_timeout
Number of seconds to wait for more data from a master/slave connection before aborting the read.
slow_launch_time
If creating the thread takes longer than this value (in seconds), the Slow_launch_threads counter will be incremented.
socket
The Unix socket used by the server.
sort_buffer
Each thread that needs to do a sort allocates a buffer of this size. Increase this value for faster ORDER BY or GROUP BY operations. See section A.4.4 Where MySQL Stores Temporary Files.
table_cache
The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. MySQL needs two file descriptors for each unique open table. See below for comments on file descriptor limits. You can check if you need to increase the table cache by checking the Opened_tables variable. See section 4.5.5 SHOW Syntax. If this variable is big and you don't do FLUSH TABLES a lot (which just forces all tables to be closed and reopenend), then you should increase the value of this variable. Make sure that your operating system can handle the number of open file descriptors implied by the table_cache setting. If table_cache is set too high, MySQL may run out of file descriptors and refuse connections, fail to perform queries, and be very unreliable. For information about how the table cache works, see section 5.4.6 How MySQL Opens and Closes Tables.
table_type
The default table type
thread_cache_size
How many threads we should keep in a cache for reuse. When a client disconnects, the client's threads are put in the cache if there aren't more than thread_cache_size threads from before. All new threads are first taken from the cache, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. (Normally this doesn't give a notable performance improvement if you have a good thread implementation.) By examing the difference between the Connections and Threads_created you can see how efficient the current thread cache is for you.
thread_concurrency
On Solaris, mysqld will call thr_setconcurrency() with this value. thr_setconcurrency() permits the application to give the threads system a hint for the desired number of threads that should be run at the same time.
thread_stack
The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. The default is large enough for normal operation. See section 5.1.4 The MySQL Benchmark Suite.
timezone
The timezone for the server.
tmp_table_size
If an in-memory temporary table exceeds this size, MySQL will automatically convert it to an on-disk MyISAM table. Increase the value of tmp_table_size if you do many advanced GROUP BY queries and you have lots of memory.
tmpdir
The directory used for temporary files and temporary tables.
version
The version number for the server.
wait_timeout
The number of seconds the server waits for activity on a connection before closing it. See also interactive_timeout.

The manual section that describes tuning MySQL contains some information of how to tune the above variables. See section 5.5.2 Tuning Server Parameters.

4.5.5.5 SHOW LOGS

SHOW LOGS shows you status information about existing log files. It currently only displays information about Berkeley DB log files.

4.5.5.6 SHOW PROCESSLIST

SHOW PROCESSLIST shows you which threads are running. You can also get this information using the mysqladmin processlist command. If you have the process privilege, you can see all threads. Otherwise, you can see only your own threads. See section 4.5.4 KILL Syntax. If you don't use the FULL option, then only the first 100 characters of each query will be shown.

This command is very useful if you get the 'too many connections' error message and want to find out what's going on. MySQL reserves one extra connection for a client with the Process_priv privilege to ensure that you should always be able to login and check the system (assuming you are not giving this privilege to all your users).

4.5.5.7 SHOW GRANTS

SHOW GRANTS FOR user lists the grant commands that must be issued to duplicate the grants for a user. 

mysql> SHOW GRANTS FOR root@localhost;
+---------------------------------------------------------------------+
| Grants for root@localhost                                           |
+---------------------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' WITH GRANT OPTION |
+---------------------------------------------------------------------+

4.5.5.8 SHOW CREATE TABLE

Shows a CREATE TABLE statement that will create the given table: 

mysql> show create table t\G
*************************** 1. row ***************************
       Table: t
Create Table: CREATE TABLE t (
  id int(11) default NULL auto_increment,
  s char(60) default NULL,
  PRIMARY KEY (id)
) TYPE=MyISAM

SHOW CREATE TABLE will quote table and column names according to SQL_QUOTE_SHOW_CREATE option. section 5.5.6 SET Syntax.

4.6 MySQL Localization and International Usage

4.6.1 The Character Set Used for Data and Sorting

By default, MySQL uses the ISO-8859-1 (Latin1) character set with sorting according to Swedish/Finnish. This is the character set suitable in the USA and western Europe.

All standard MySQL binaries are compiled with --with-extra-charsets=complex. This will add code to all standard programs to be able to handle latin1 and all multi-byte character sets within the binary. Other character sets will be loaded from a character-set definition file when needed.

The character set determines what characters are allowed in names and how things are sorted by the ORDER BY and GROUP BY clauses of the SELECT statement.

You can change the character set with the --default-character-set option when you start the server. The character sets available depend on the --with-charset=charset and --with-extra-charset= list-of-charset | complex | all options to configure, and the character set configuration files listed in `SHAREDIR/charsets/Index'. See section 2.3.3 Typical configure Options.

If you change the character set when running MySQL (which may also change the sort order), you must run myisamchk -r -q on all tables. Otherwise your indexes may not be ordered correctly.

When a client connects to a MySQL server, the server sends the default character set in use to the client. The client will switch to use this character set for this connection.

One should use mysql_real_escape_string() when escaping strings for a SQL query. mysql_real_escape_string() is identical to the old mysql_escape_string() function, except that it takes the MYSQL connection handle as the first parameter.

If the client is compiled with different paths than where the server is installed and the user who configured MySQL didn't included all character sets in the MySQL binary, one must specify for the client where it can find the additional character sets it will need if the server runs with a different character set than the client.

One can specify this by putting in a MySQL option file: 

[client]
character-sets-dir=/usr/local/mysql/share/mysql/charsets

where the path points to where the dynamic MySQL character sets are stored.

One can force the client to use specific character set by specifying: 

[client]
default-character-set=character-set-name

but normally this is never needed.

4.6.2 Non-English Error Messages

mysqld can issue error messages in the following languages: Czech, Danish, Dutch, English (the default), Estonian, French, German, Greek, Hungarian, Italian, Japanese, Korean, Norwegian, Norwegian-ny, Polish, Portuguese, Romanian, Russian, Slovak, Spanish, and Swedish.

To start mysqld with a particular language, use either the --language=lang or -L lang options. For example: 

shell> mysqld --language=swedish

or: 

shell> mysqld --language=/usr/local/share/swedish

Note that all language names are specified in lowercase.

The language files are located (by default) in `mysql_base_dir/share/LANGUAGE/'.

To update the error message file, you should edit the `errmsg.txt' file and execute the following command to generate the `errmsg.sys' file: 

shell> comp_err errmsg.txt errmsg.sys

If you upgrade to a newer version of MySQL, remember to repeat your changes with the new `errmsg.txt' file.

4.6.3 Adding a New Character Set

To add another character set to MySQL, use the following procedure.

Decide if the set is simple or complex. If the character set does not need to use special string collating routines for sorting and does not need multi-byte character support, it is simple. If it needs either of those features, it is complex.

For example, latin1 and danish are simple charactersets while big5 or czech are complex character sets.

In the following section, we have assumed that you name your character set MYSET.

For a simple character set do the following:

  1. Add MYSET to the end of the `sql/share/charsets/Index' file Assign an unique number to it.
  2. Create the file `sql/share/charsets/MYSET.conf'. (You can use `sql/share/charsets/latin1.conf' as a base for this). The syntax for the file very simple: See section 4.6.4 The character definition arrays.
  3. Add the character set name to the CHARSETS_AVAILABLE and COMPILED_CHARSETS lists in configure.in.
  4. Reconfigure, recompile, and test.

For a complex character set do the following:

  1. Create the file `strings/ctype-MYSET.c' in the MySQL source distribution.
  2. Add MYSET to the end of the `sql/share/charsets/Index' file. Assign an unique number to it.
  3. Look at one of the existing `ctype-*.c' files to see what needs to be defined, for example `strings/ctype-big5.c'. Note that the arrays in your file must have names like ctype_MYSET, to_lower_MYSET, and so on. This corresponds to the arrays in the simple character set. See section 4.6.4 The character definition arrays. For a complex character set
  4. Near the top of the file, place a special comment like this: 
    /*
 * This comment is parsed by configure to create ctype.c,
 * so don't change it unless you know what you are doing.
 *
 * .configure. number_MYSET=MYNUMBER
 * .configure. strxfrm_multiply_MYSET=N
 * .configure. mbmaxlen_MYSET=N
 */
    The configure program uses this comment to include the character set into the MySQL library automatically. The strxfrm_multiply and mbmaxlen lines will be explained in the following sections. Only include them if you the string collating functions or the multi-byte character set functions, respectively.
  5. You should then create some of the following functions: See section 4.6.5 String Collating Support.
  6. Add the character set name to the CHARSETS_AVAILABLE and COMPILED_CHARSETS lists in configure.in.
  7. Reconfigure, recompile, and test.

The file `sql/share/charsets/README' includes some more instructions.

If you want to have the character set included in the MySQL distribution, mail a patch to internals@lists.mysql.com.

4.6.4 The character definition arrays

to_lower[] and to_upper[] are simple arrays that hold the lowercase and uppercase characters corresponding to each member of the character set. For example: 

to_lower['A'] should contain 'a'
to_upper['a'] should contain 'A'

sort_order[] is a map indicating how characters should be ordered for comparison and sorting purposes. For many character sets, this is the same as to_upper[] (which means sorting will be case insensitive). MySQL will sort characters based on the value of sort_order[character]. For more complicated sorting rules, see the discussion of string collating below. See section 4.6.5 String Collating Support.

ctype[] is an array of bit values, with one element for one character. (Note that to_lower[], to_upper[], and sort_order[] are indexed by character value, but ctype[] is indexed by character value + 1. This is an old legacy to be able to handle EOF.)

You can find the following bitmask definitions in `m_ctype.h': 

#define _U      01      /* Uppercase */
#define _L      02      /* Lowercase */
#define _N      04      /* Numeral (digit) */
#define _S      010     /* Spacing character */
#define _P      020     /* Punctuation */
#define _C      040     /* Control character */
#define _B      0100    /* Blank */
#define _X      0200    /* heXadecimal digit */

The ctype[] entry for each character should be the union of the applicable bitmask values that describe the character. For example, 'A' is an uppercase character (_U) as well as a hexadecimal digit (_X), so ctype['A'+1] should contain the value: 

_U + _X = 01 + 0200 = 0201

4.6.5 String Collating Support

If the sorting rules for your language are too complex to be handled with the simple sort_order[] table, you need to use the string collating functions.

Right now the best documentation on this is the character sets that are already implemented. Look at the big5, czech, gbk, sjis, and tis160 character sets for examples.

You must specify the strxfrm_multiply_MYSET=N value in the special comment at the top of the file. N should be set to the maximum ratio the strings may grow during my_strxfrm_MYSET (it must be a positive integer).

4.6.6 Multi-byte Character Support

If your want to add support for a new character set that includes multi-byte characters, you need to use the multi-byte character functions.

Right now the best documentation on this is the character sets that are already implemented. Look at the euc_kr, gb2312, gbk, sjis and ujis character sets for examples. These are implemented in the ctype-'charset'.c files in the `strings' directory.

You must specify the mbmaxlen_MYSET=N value in the special comment at the top of the source file. N should be set to the size in bytes of the largest character in the set.

4.7 MySQL Server-Side Scripts and Utilities

4.7.1 Overview of the Server-Side Scripts and Utilities

All MySQL clients that communicate with the server using the mysqlclient library use the following environment variables:

Name Description
MYSQL_UNIX_PORT The default socket; used for connections to localhost
MYSQL_TCP_PORT The default TCP/IP port
MYSQL_PWD The default password
MYSQL_DEBUG Debug-trace options when debugging
TMPDIR The directory where temporary tables/files are created

Use of MYSQL_PWD is insecure. See section 4.2.7 Connecting to the MySQL Server.

The `mysql' client uses the file named in the MYSQL_HISTFILE environment variable to save the command-line history. The default value for the history file is `$HOME/.mysql_history', where $HOME is the value of the HOME environment variable. See section H Environment Variables.

All MySQL programs take many different options. However, every MySQL program provides a --help option that you can use to get a full description of the program's different options. For example, try mysql --help.

You can override default options for all standard client programs with an option file. section 4.1.2 my.cnf Option Files.

The list below briefly describes the MySQL programs:

myisamchk
Utility to describe, check, optimize, and repair MySQL tables. Because myisamchk has many functions, it is described in its own chapter. See section 4 MySQL Database Administration.
make_binary_distribution
Makes a binary release of a compiled MySQL. This could be sent by FTP to `/pub/mysql/Incoming' on support.mysql.com for the convenience of other MySQL users.
msql2mysql
A shell script that converts mSQL programs to MySQL. It doesn't handle all cases, but it gives a good start when converting.
mysqlaccess
A script that checks the access privileges for a host, user, and database combination.
mysqladmin
Utility for performing administrative operations, such as creating or dropping databases, reloading the grant tables, flushing tables to disk, and reopening log files. mysqladmin can also be used to retrieve version, process, and status information from the server. See section 4.8.3 mysqladmin, Administrating a MySQL Server.
mysqlbug
The MySQL bug report script. This script should always be used when filing a bug report to the MySQL list.
mysqld
The SQL daemon. This should always be running.
mysqldump
Dumps a MySQL database into a file as SQL statements or as tab-separated text files. Enhanced freeware originally by Igor Romanenko. See section 4.8.5 mysqldump, Dumping Table Structure and Data.
mysqlimport
Imports text files into their respective tables using LOAD DATA INFILE. See section 4.8.7 mysqlimport, Importing Data from Text Files.
mysqlshow
Displays information about databases, tables, columns, and indexes.
mysql_install_db
Creates the MySQL grant tables with default privileges. This is usually executed only once, when first installing MySQL on a system.
replace
A utility program that is used by msql2mysql, but that has more general applicability as well. replace changes strings in place in files or on the standard input. Uses a finite state machine to match longer strings first. Can be used to swap strings. For example, this command swaps a and b in the given files: 
shell> replace a b b a -- file1 file2 ...

4.7.2 safe_mysqld, the wrapper around mysqld

safe_mysqld is the recommended way to start a mysqld daemon on Unix. safe_mysqld adds some safety features such as restarting the server when an error occurs and logging run-time information to a log file.

If you don't use --mysqld=# or --mysqld-version=# safe_mysqld will use an executable named mysqld-max if it exists. If not, safe_mysqld will start mysqld. This makes it very easy to test to use mysqld-max instead of mysqld; Just copy mysqld-max to where you have mysqld and it will be used.

Normally one should never edit the safe_mysqld script, but instead put the options to safe_mysqld in the [safe_mysqld] section in the my.cnf file. safe_mysqld will read all options from the [mysqld], [server] and [safe_mysqld] sections from the option files. See section 4.1.2 my.cnf Option Files.

Note that all options on the command line to safe_mysqld are passed to mysqld. If you wants to use any options in safe_mysqld that mysqld doesn't support, you must specify these in the option file.

Most of the options to safe_mysqld are the same as the options to mysqld. See section 4.1.1 mysqld Command-line Options.

safe_mysqld supports the following options:

--basedir=path
--core-file-size=#
Size of the core file mysqld should be able to create. Passed to ulimit -c.
--datadir=path
--defaults-extra-file=path
--defaults-file=path
--err-log=path
--ledir=path
Path to mysqld
--log=path
--mysqld=mysqld-version
Name of the mysqld version in the ledir directory you want to start.
--mysqld-version=version
Similar to --mysqld= but here you only give the suffix for mysqld. For example if you use --mysqld-version=max, safe_mysqld will start the ledir/mysqld-max version. If the argument to --mysqld-version is empty, ledir/mysqld will be used.
--no-defaults
--open-files-limit=#
Number of files mysqld should be able to open. Passed to ulimit -n. Note that you need to start safe_mysqld as root for this to work properly!
--pid-file=path
--port=#
--socket=path
--timezone=#
Set the timezone (the TZ) variable to the value of this parameter.
--user=#

The safe_mysqld script is written so that it normally is able to start a server that was installed from either a source or a binary version of MySQL, even if these install the server in slightly different locations. safe_mysqld expects one of these conditions to be true:

Because safe_mysqld will try to find the server and databases relative to its own working directory, you can install a binary distribution of MySQL anywhere, as long as you start safe_mysqld from the MySQL installation directory: 

shell> cd mysql_installation_directory
shell> bin/safe_mysqld &

If safe_mysqld fails, even when invoked from the MySQL installation directory, you can modify it to use the path to mysqld and the pathname options that are correct for your system. Note that if you upgrade MySQL in the future, your modified version of safe_mysqld will be overwritten, so you should make a copy of your edited version that you can reinstall.

4.7.3 mysqld_multi, program for managing multiple MySQL servers

mysqld_multi is meant for managing several mysqld processes running in different UNIX sockets and TCP/IP ports.

The program will search for group(s) named [mysqld#] from my.cnf (or the given --config-file=...), where # can be any positive number starting from 1. These groups should be the same as the usual [mysqld] group (e.g. options to mysqld, see MySQL manual for detailed information about this group), but with those port, socket etc. options that are wanted for each separate mysqld processes. The number in the group name has another function; it can be used for starting, stopping, or reporting some specific mysqld servers with this program. See the usage and options below for more information. 

Usage: mysqld_multi [OPTIONS] {start|stop|report} [GNR,GNR,GNR...]
or     mysqld_multi [OPTIONS] {start|stop|report} [GNR-GNR,GNR,GNR-GNR,...]

The GNR above means the group number. You can start, stop or report any GNR, or several of them at the same time. (See --example) The GNRs list can be comma separated, or a dash combined, of which the latter means that all the GNRs between GNR1-GNR2 will be affected. Without GNR argument all the found groups will be either started, stopped, or reported. Note that you must not have any white spaces in the GNR list. Anything after a white space is ignored.

mysqld_multi supports the following options:

--config-file=...
Alternative config file. NOTE: This will not affect this program's own options (group [mysqld_multi]), but only groups [mysqld#]. Without this option everything will be searched from the ordinary my.cnf file.
--example
Give an example of a config file.
--help
Print this help and exit.
--log=...
Log file. Full path to and the name for the log file. NOTE: If the file exists, everything will be appended.
--mysqladmin=...
mysqladmin binary to be used for a server shutdown.
--mysqld=...
mysqld binary to be used. Note that you can give safe_mysqld to this option also. The options are passed to mysqld. Just make sure you have mysqld in your environment variable PATH or fix safe_mysqld.
--no-log
Print to stdout instead of the log file. By default the log file is turned on.
--password=...
Password for user for mysqladmin.
--tcp-ip
Connect to the MySQL server(s) via the TCP/IP port instead of the UNIX socket. This affects stopping and reporting. If a socket file is missing, the server may still be running, but can be accessed only via the TCP/IP port. By default connecting is done via the UNIX socket.
--user=...
MySQL user for mysqladmin.
--version
Print the version number and exit.

Some notes about mysqld_multi:

See section 4.1.4 Running Multiple MySQL Servers on the Same Machine.

This is an example of the config file on behalf of mysqld_multi. 

# This file should probably be in your home dir (~/.my.cnf) or /etc/my.cnf
# Version 2.1 by Jani Tolonen

[mysqld_multi]
mysqld     = /usr/local/bin/safe_mysqld
mysqladmin = /usr/local/bin/mysqladmin
user       = multi_admin
password   = multipass

[mysqld2]
socket     = /tmp/mysql.sock2
port       = 3307
pid-file   = /usr/local/mysql/var2/hostname.pid2
datadir    = /usr/local/mysql/var2
language   = /usr/local/share/mysql/english
user       = john

[mysqld3]
socket     = /tmp/mysql.sock3
port       = 3308
pid-file   = /usr/local/mysql/var3/hostname.pid3
datadir    = /usr/local/mysql/var3
language   = /usr/local/share/mysql/swedish
user       = monty

[mysqld4]
socket     = /tmp/mysql.sock4
port       = 3309
pid-file   = /usr/local/mysql/var4/hostname.pid4
datadir    = /usr/local/mysql/var4
language   = /usr/local/share/mysql/estonia
user       = tonu

[mysqld6]
socket     = /tmp/mysql.sock6
port       = 3311
pid-file   = /usr/local/mysql/var6/hostname.pid6
datadir    = /usr/local/mysql/var6
language   = /usr/local/share/mysql/japanese
user       = jani

See section 4.1.2 my.cnf Option Files.

4.7.4 myisampack, The MySQL Compressed Read-only Table Generator

myisampack is used to compress MyISAM tables, and pack_isam is used to compress ISAM tables. Because ISAM tables are deprecated, we will only discuss myisampack here, but everything said about myisampack should also be true for pack_isam.

myisampack works by compressing each column in the table separately. The information needed to decompress columns is read into memory when the table is opened. This results in much better performance when accessing individual records, because you only have to uncompress exactly one record, not a much larger disk block as when using Stacker on MS-DOS. Usually, myisampack packs the data file 40%-70%.

MySQL uses memory mapping (mmap()) on compressed tables and falls back to normal read/write file usage if mmap() doesn't work.

There are currently two limitations with myisampack:

Fixing these limitations is on our TODO list but with low priority.

myisampack is invoked like this: 

shell> myisampack [options] filename ...

Each filename should be the name of an index (`.MYI') file. If you are not in the database directory, you should specify the pathname to the file. It is permissible to omit the `.MYI' extension.

myisampack supports the following options:

-b, --backup
Make a backup of the table as tbl_name.OLD.
-#, --debug=debug_options
Output debug log. The debug_options string often is 'd:t:o,filename'.
-f, --force
Force packing of the table even if it becomes bigger or if the temporary file exists. myisampack creates a temporary file named `tbl_name.TMD' while it compresses the table. If you kill myisampack, the `.TMD' file may not be deleted. Normally, myisampack exits with an error if it finds that `tbl_name.TMD' exists. With --force, myisampack packs the table anyway.
-?, --help
Display a help message and exit.
-j big_tbl_name, --join=big_tbl_name
Join all tables named on the command line into a single table big_tbl_name. All tables that are to be combined MUST be identical (same column names and types, same indexes, etc.).
-p #, --packlength=#
Specify the record length storage size, in bytes. The value should be 1, 2, or 3. (myisampack stores all rows with length pointers of 1, 2, or 3 bytes. In most normal cases, myisampack can determine the right length value before it begins packing the file, but it may notice during the packing process that it could have used a shorter length. In this case, myisampack will print a note that the next time you pack the same file, you could use a shorter record length.)
-s, --silent
Silent mode. Write output only when errors occur.
-t, --test
Don't actually pack table, just test packing it.
-T dir_name, --tmp_dir=dir_name
Use the named directory as the location in which to write the temporary table.
-v, --verbose
Verbose mode. Write information about progress and packing result.
-V, --version
Display version information and exit.
-w, --wait
Wait and retry if table is in use. If the mysqld server was invoked with the --skip-locking option, it is not a good idea to invoke myisampack if the table might be updated during the packing process.

The sequence of commands shown below illustrates a typical table compression session: 

shell> ls -l station.*
-rw-rw-r--   1 monty    my         994128 Apr 17 19:00 station.MYD
-rw-rw-r--   1 monty    my          53248 Apr 17 19:00 station.MYI
-rw-rw-r--   1 monty    my           5767 Apr 17 19:00 station.frm

shell> myisamchk -dvv station

MyISAM file:     station
Isam-version:  2
Creation time: 1996-03-13 10:08:58
Recover time:  1997-02-02  3:06:43
Data records:              1192  Deleted blocks:              0
Datafile: Parts:           1192  Deleted data:                0
Datafile pointer (bytes):     2  Keyfile pointer (bytes):     2
Max datafile length:   54657023  Max keyfile length:   33554431
Recordlength:               834
Record format: Fixed length

table description:
Key Start Len Index   Type                       Root  Blocksize    Rec/key
1   2     4   unique  unsigned long              1024       1024          1
2   32    30  multip. text                      10240       1024          1

Field Start Length Type
1     1     1
2     2     4
3     6     4
4     10    1
5     11    20
6     31    1
7     32    30
8     62    35
9     97    35
10    132   35
11    167   4
12    171   16
13    187   35
14    222   4
15    226   16
16    242   20
17    262   20
18    282   20
19    302   30
20    332   4
21    336   4
22    340   1
23    341   8
24    349   8
25    357   8
26    365   2
27    367   2
28    369   4
29    373   4
30    377   1
31    378   2
32    380   8
33    388   4
34    392   4
35    396   4
36    400   4
37    404   1
38    405   4
39    409   4
40    413   4
41    417   4
42    421   4
43    425   4
44    429   20
45    449   30
46    479   1
47    480   1
48    481   79
49    560   79
50    639   79
51    718   79
52    797   8
53    805   1
54    806   1
55    807   20
56    827   4
57    831   4

shell> myisampack station.MYI
Compressing station.MYI: (1192 records)
- Calculating statistics

normal:     20  empty-space:      16  empty-zero:        12  empty-fill:  11
pre-space:   0  end-space:        12  table-lookups:      5  zero:         7
Original trees:  57  After join: 17
- Compressing file
87.14%

shell> ls -l station.*
-rw-rw-r--   1 monty    my         127874 Apr 17 19:00 station.MYD
-rw-rw-r--   1 monty    my          55296 Apr 17 19:04 station.MYI
-rw-rw-r--   1 monty    my           5767 Apr 17 19:00 station.frm

shell> myisamchk -dvv station

MyISAM file:     station
Isam-version:  2
Creation time: 1996-03-13 10:08:58
Recover time:  1997-04-17 19:04:26
Data records:              1192  Deleted blocks:              0
Datafile: Parts:           1192  Deleted data:                0
Datafilepointer (bytes):      3  Keyfile pointer (bytes):     1
Max datafile length:   16777215  Max keyfile length:     131071
Recordlength:               834
Record format: Compressed

table description:
Key Start Len Index   Type                       Root  Blocksize    Rec/key
1   2     4   unique  unsigned long             10240       1024          1
2   32    30  multip. text                      54272       1024          1

Field Start Length Type                         Huff tree  Bits
1     1     1      constant                             1     0
2     2     4      zerofill(1)                          2     9
3     6     4      no zeros, zerofill(1)                2     9
4     10    1                                           3     9
5     11    20     table-lookup                         4     0
6     31    1                                           3     9
7     32    30     no endspace, not_always              5     9
8     62    35     no endspace, not_always, no empty    6     9
9     97    35     no empty                             7     9
10    132   35     no endspace, not_always, no empty    6     9
11    167   4      zerofill(1)                          2     9
12    171   16     no endspace, not_always, no empty    5     9
13    187   35     no endspace, not_always, no empty    6     9
14    222   4      zerofill(1)                          2     9
15    226   16     no endspace, not_always, no empty    5     9
16    242   20     no endspace, not_always              8     9
17    262   20     no endspace, no empty                8     9
18    282   20     no endspace, no empty                5     9
19    302   30     no endspace, no empty                6     9
20    332   4      always zero                          2     9
21    336   4      always zero                          2     9
22    340   1                                           3     9
23    341   8      table-lookup                         9     0
24    349   8      table-lookup                        10     0
25    357   8      always zero                          2     9
26    365   2                                           2     9
27    367   2      no zeros, zerofill(1)                2     9
28    369   4      no zeros, zerofill(1)                2     9
29    373   4      table-lookup                        11     0
30    377   1                                           3     9
31    378   2      no zeros, zerofill(1)                2     9
32    380   8      no zeros                             2     9
33    388   4      always zero                          2     9
34    392   4      table-lookup                        12     0
35    396   4      no zeros, zerofill(1)               13     9
36    400   4      no zeros, zerofill(1)                2     9
37    404   1                                           2     9
38    405   4      no zeros                             2     9
39    409   4      always zero                          2     9
40    413   4      no zeros                             2     9
41    417   4      always zero                          2     9
42    421   4      no zeros                             2     9
43    425   4      always zero                          2     9
44    429   20     no empty                             3     9
45    449   30     no empty                             3     9
46    479   1                                          14     4
47    480   1                                          14     4
48    481   79     no endspace, no empty               15     9
49    560   79     no empty                             2     9
50    639   79     no empty                             2     9
51    718   79     no endspace                         16     9
52    797   8      no empty                             2     9
53    805   1                                          17     1
54    806   1                                           3     9
55    807   20     no empty                             3     9
56    827   4      no zeros, zerofill(2)                2     9
57    831   4      no zeros, zerofill(1)                2     9

The information printed by myisampack is described below:

normal
The number of columns for which no extra packing is used.
empty-space
The number of columns containing values that are only spaces; these will occupy 1 bit.
empty-zero
The number of columns containing values that are only binary 0's; these will occupy 1 bit.
empty-fill
The number of integer columns that don't occupy the full byte range of their type; these are changed to a smaller type (for example, an INTEGER column may be changed to MEDIUMINT).
pre-space
The number of decimal columns that are stored with leading spaces. In this case, each value will contain a count for the number of leading spaces.
end-space
The number of columns that have a lot of trailing spaces. In this case, each value will contain a count for the number of trailing spaces.
table-lookup
The column had only a small number of different values, which were converted to an ENUM before Huffman compression.
zero
The number of columns for which all values are zero.
Original trees
The initial number of Huffman trees.
After join
The number of distinct Huffman trees left after joining trees to save some header space.

After a table has been compressed, myisamchk -dvv prints additional information about each field:

Type
The field type may contain the following descriptors:
constant
All rows have the same value.
no endspace
Don't store endspace.
no endspace, not_always
Don't store endspace and don't do end space compression for all values.
no endspace, no empty
Don't store endspace. Don't store empty values.
table-lookup
The column was converted to an ENUM.
zerofill(n)
The most significant n bytes in the value are always 0 and are not stored.
no zeros
Don't store zeros.
always zero
0 values are stored in 1 bit.
Huff tree
The Huffman tree associated with the field.
Bits
The number of bits used in the Huffman tree.

After you have run pack_isam/myisampack you must run isamchk/myisamchk to re-create the index. At this time you can also sort the index blocks and create statistics needed for the MySQL optimizer to work more efficiently: 

myisamchk -rq --analyze --sort-index table_name.MYI
isamchk   -rq --analyze --sort-index table_name.ISM

After you have installed the packed table into the MySQL database directory you should do mysqladmin flush-tables to force mysqld to start using the new table.

If you want to unpack a packed table, you can do this with the --unpack option to isamchk or myisamchk.

4.7.5 mysqld-max, An extended mysqld server

mysqld-max is the MySQL server (mysqld) configured with the following configure options:

Option Comment
--with-server-suffix=-max Add a suffix to the mysqld version string.
--with-bdb Support for Berkeley DB (BDB) tables
--with-innodb Support for InnoDB tables.
CFLAGS=-DUSE_SYMDIR Symbolic links support for Windows.

You can find the MySQL-max binaries at http://www.mysql.com/downloads/mysql-max-3.23.html.

The Windows MySQL 3.23 binary distribution includes both the standard mysqld.exe binary and the mysqld-max.exe binary. http://www.mysql.com/downloads/mysql-3.23.html. See section 2.1.2 Installing MySQL on Windows.

Note that as Berkeley DB and InnoDB are not available for all platforms, some of the Max binaries may not have support for both of these. You can check which table types are supported by doing the following query: 

mysql> show variables like "have_%";
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| have_bdb      | YES   |
| have_innodb   | NO    |
| have_isam     | YES   |
| have_raid     | NO    |
| have_ssl      | NO    |
+---------------+-------+

The meaning of the values are:

Value Meaning.
YES The option is activated and usable.
NO MySQL is not compiled with support for this option.
DISABLED The xxxx option is disabled because one started mysqld with --skip-xxxx or because one didn't start mysqld with all needed options to enable the option. In this case the hostname.err file should contain a reason for why the option is disabled.

NOTE: To be able to create InnoDB tables you MUST edit your startup options to include at least the innodb_data_file_path option. See section 7.6.2 InnoDB startup options.

To get better performance for BDB tables, you should add some configuration options for these too. See section 7.5.3 BDB startup options.

safe_mysqld will automatically try to start any mysqld binary with the -max prefix. This makes it very easy to test out a another mysqld binary in an existing installation. Just run configure with the options you want and then install the new mysqld binary as mysqld-max in the same directory where your old mysqld binary is. See section 4.7.2 safe_mysqld, the wrapper around mysqld.

The mysqld-max RPM uses the above mentioned safe_mysqld feature. It just installs the mysqld-max executable and safe_mysqld will automatically use this executable when safe_mysqld is restarted.

The following table shows which table types our standard MySQL-Max binaries includes:

System BDB InnoDB
AIX 4.3 N Y
HP-UX 11.0 N Y
Linux-Alpha N Y
Linux-Intel Y Y
Linux-Ia64 N Y
Solaris-intel N Y
Solaris-sparc Y Y
SCO OSR5 Y Y
UnixWare Y Y
Windows/NT Y Y

4.8 MySQL Client-Side Scripts and Utilities

4.8.1 Overview of the Client-Side Scripts and Utilities

All MySQL clients that communicate with the server using the mysqlclient library use the following environment variables:

Name Description
MYSQL_UNIX_PORT The default socket; used for connections to localhost
MYSQL_TCP_PORT The default TCP/IP port
MYSQL_PWD The default password
MYSQL_DEBUG Debug-trace options when debugging
TMPDIR The directory where temporary tables/files are created

Use of MYSQL_PWD is insecure. See section 4.2.7 Connecting to the MySQL Server.

The `mysql' client uses the file named in the MYSQL_HISTFILE environment variable to save the command-line history. The default value for the history file is `$HOME/.mysql_history', where $HOME is the value of the HOME environment variable. See section H Environment Variables.

All MySQL programs take many different options. However, every MySQL program provides a --help option that you can use to get a full description of the program's different options. For example, try mysql --help.

You can override default options for all standard client programs with an option file. section 4.1.2 my.cnf Option Files.

The list below briefly describes the MySQL programs:

myisamchk
Utility to describe, check, optimize, and repair MySQL tables. Because myisamchk has many functions, it is described in its own chapter. See section 4 MySQL Database Administration.
make_binary_distribution
Makes a binary release of a compiled MySQL. This could be sent by FTP to `/pub/mysql/Incoming' on support.mysql.com for the convenience of other MySQL users.
msql2mysql
A shell script that converts mSQL programs to MySQL. It doesn't handle all cases, but it gives a good start when converting.
mysqlaccess
A script that checks the access privileges for a host, user, and database combination.
mysqladmin
Utility for performing administrative operations, such as creating or dropping databases, reloading the grant tables, flushing tables to disk, and reopening log files. mysqladmin can also be used to retrieve version, process, and status information from the server. See section 4.8.3 mysqladmin, Administrating a MySQL Server.
mysqlbug
The MySQL bug report script. This script should always be used when filing a bug report to the MySQL list.
mysqld
The SQL daemon. This should always be running.
mysqldump
Dumps a MySQL database into a file as SQL statements or as tab-separated text files. Enhanced freeware originally by Igor Romanenko. See section 4.8.5 mysqldump, Dumping Table Structure and Data.
mysqlimport
Imports text files into their respective tables using LOAD DATA INFILE. See section 4.8.7 mysqlimport, Importing Data from Text Files.
mysqlshow
Displays information about databases, tables, columns, and indexes.
mysql_install_db
Creates the MySQL grant tables with default privileges. This is usually executed only once, when first installing MySQL on a system.
replace
A utility program that is used by msql2mysql, but that has more general applicability as well. replace changes strings in place in files or on the standard input. Uses a finite state machine to match longer strings first. Can be used to swap strings. For example, this command swaps a and b in the given files: 
shell> replace a b b a -- file1 file2 ...

4.8.2 The Command-line Tool

mysql is a simple SQL shell (with GNU readline capabilities). It supports interactive and non-interactive use. When used interactively, query results are presented in an ASCII-table format. When used non-interactively (for example, as a filter), the result is presented in tab-separated format. (The output format can be changed using command-line options.) You can run scripts simply like this: 

shell> mysql database < script.sql > output.tab

If you have problems due to insufficient memory in the client, use the --quick option! This forces mysql to use mysql_use_result() rather than mysql_store_result() to retrieve the result set.

Using mysql is very easy. Just start it as follows: mysql database or mysql --user=user_name --password=your_password database. Type a SQL statement, end it with `;', `\g', or `\G' and press RETURN/ENTER.

mysql supports the following options:

-?, --help
Display this help and exit.
-A, --no-auto-rehash
No automatic rehashing. One has to use 'rehash' to get table and field completion. This gives a quicker start of mysql.
-B, --batch
Print results with a tab as separator, each row on a new line. Doesn't use history file.
--character-sets-dir=...
Directory where character sets are located.
-C, --compress
Use compression in server/client protocol.
-#, --debug[=...]
Debug log. Default is 'd:t:o,/tmp/mysql.trace'.
-D, --database=...
Database to use. This is mainly useful in the my.cnf file.
--default-character-set=...
Set the default character set.
-e, --execute=...
Execute command and quit. (Output like with --batch)
-E, --vertical
Print the output of a query (rows) vertically. Without this option you can also force this output by ending your statements with \G.
-f, --force
Continue even if we get a SQL error.
-g, --no-named-commands
Named commands are disabled. Use \* form only, or use named commands only in the beginning of a line ending with a semicolon (;). Since Version 10.9, the client now starts with this option ENABLED by default! With the -g option, long format commands will still work from the first line, however.
-G, --enable-named-commands
Named commands are enabled. Long format commands are allowed as well as shortened \* commands.
-i, --ignore-space
Ignore space after function names.
-h, --host=...
Connect to the given host.
-H, --html
Produce HTML output.
-L, --skip-line-numbers
Don't write line number for errors. Useful when one wants to compare result files that includes error messages
--no-pager
Disable pager and print to stdout. See interactive help (\h) also.
--no-tee
Disable outfile. See interactive help (\h) also.
-n, --unbuffered
Flush buffer after each query.
-N, --skip-column-names
Don't write column names in results.
-O, --set-variable var=option
Give a variable a value. --help lists variables.
-o, --one-database
Only update the default database. This is useful for skipping updates to other database in the update log.
--pager[=...]
Output type. Default is your ENV variable PAGER. Valid pagers are less, more, cat [> filename], etc. See interactive help (\h) also. This option does not work in batch mode. Pager works only in UNIX.
-p[password], --password[=...]
Password to use when connecting to server. If a password is not given on the command line, you will be prompted for it. Note that if you use the short form -p you can't have a space between the option and the password.
-P --port=...
TCP/IP port number to use for connection.
-q, --quick
Don't cache result, print it row-by-row. This may slow down the server if the output is suspended. Doesn't use history file.
-r, --raw
Write column values without escape conversion. Used with --batch
-s, --silent
Be more silent.
-S --socket=...
Socket file to use for connection.
-t --table
Output in table format. This is default in non-batch mode.
-T, --debug-info
Print some debug information at exit.
--tee=...
Append everything into outfile. See interactive help (\h) also. Does not work in batch mode.
-u, --user=#
User for login if not current user.
-U, --safe-updates[=#], --i-am-a-dummy[=#]
Only allow UPDATE and DELETE that uses keys. See below for more information about this option. You can reset this option if you have it in your my.cnf file by using --safe-updates=0.
-v, --verbose
More verbose output (-v -v -v gives the table output format).
-V, --version
Output version information and exit.
-w, --wait
Wait and retry if connection is down instead of aborting.

You can also set the following variables with -O or --set-variable:

Variable Name Default Description
connect_timeout 0 Number of seconds before timeout connection.
max_allowed_packet 16777216 Max packetlength to send/receive from to server
net_buffer_length 16384 Buffer for TCP/IP and socket communication
select_limit 1000 Automatic limit for SELECT when using --i-am-a-dummy
max_join_size 1000000 Automatic limit for rows in a join when using --i-am-a-dummy.

If you type 'help' on the command line, mysql will print out the commands that it supports: 

mysql> help

MySQL commands:
help    (\h)    Display this text.
?       (\h)    Synonym for `help'.
clear   (\c)    Clear command.
connect (\r)    Reconnect to the server. Optional arguments are db and host.
edit    (\e)    Edit command with $EDITOR.
ego     (\G)    Send command to mysql server, display result vertically.
exit    (\q)    Exit mysql. Same as quit.
go      (\g)    Send command to mysql server.
nopager (\n)    Disable pager, print to stdout.
notee   (\t)    Don't write into outfile.
pager   (\P)    Set PAGER [to_pager]. Print the query results via PAGER.
print   (\p)    Print current command.
quit    (\q)    Quit mysql.
rehash  (\#)    Rebuild completion hash.
source  (\.)    Execute a SQL script file. Takes a file name as an argument.
status  (\s)    Get status information from the server.
tee     (\T)    Set outfile [to_outfile]. Append everything into given outfile.
use     (\u)    Use another database. Takes database name as argument.

From the above, pager only works in UNIX.

The status command gives you some information about the connection and the server you are using. If you are running in the --safe-updates mode, status will also print the values for the mysql variables that affect your queries.

A useful startup option for beginners (introduced in MySQL Version 3.23.11) is --safe-updates (or --i-am-a-dummy for users that has at some time done a DELETE FROM table_name but forgot the WHERE clause). When using this option, mysql sends the following command to the MySQL server when opening the connection: 

SET SQL_SAFE_UPDATES=1,SQL_SELECT_LIMIT=#select_limit#,
    SQL_MAX_JOIN_SIZE=#max_join_size#"

where #select_limit# and #max_join_size# are variables that can be set from the mysql command line. See section 5.5.6 SET Syntax.

The effect of the above is:

Some useful hints about the mysql client:

Some data is much more readable when displayed vertically, instead of the usual horizontal box type output. For example longer text, which includes new lines, is often much easier to be read with vertical output. 

mysql> select * from mails where length(txt) < 300 limit 300,1\G
*************************** 1. row ***************************
  msg_nro: 3068
     date: 2000-03-01 23:29:50
time_zone: +0200
mail_from: Monty
    reply: monty@no.spam.com
  mail_to: "Thimble Smith" <tim@no.spam.com>
      sbj: UTF-8
      txt: >>>>> "Thimble" == Thimble Smith writes:

Thimble> Hi.  I think this is a good idea.  Is anyone familiar with UTF-8
Thimble> or Unicode?  Otherwise I'll put this on my TODO list and see what
Thimble> happens.

Yes, please do that.

Regards,
Monty
     file: inbox-jani-1
     hash: 190402944
1 row in set (0.09 sec)

4.8.3 mysqladmin, Administrating a MySQL Server

A utility for performing administrative operations. The syntax is: 

shell> mysqladmin [OPTIONS] command [command-option] command ...

You can get a list of the options your version of mysqladmin supports by executing mysqladmin --help.

The current mysqladmin supports the following commands:

create databasename
Create a new database.
drop databasename
Delete a database and all its tables.
extended-status
Gives an extended status message from the server.
flush-hosts
Flush all cached hosts.
flush-logs
Flush all logs.
flush-tables
Flush all tables.
flush-privileges
Reload grant tables (same as reload).
kill id,id,...
Kill mysql threads.
password
Set a new password. Change old password to new-password.
ping
Check if mysqld is alive.
processlist
Show list of active threads in server.
reload
Reload grant tables.
refresh
Flush all tables and close and open logfiles.
shutdown
Take server down.
slave-start
Start slave replication thread.
slave-stop
Stop slave replication thread.
status
Gives a short status message from the server.
variables
Prints variables available.
version
Get version info from server.

All commands can be shortened to their unique prefix. For example: 

shell> mysqladmin proc stat
+----+-------+-----------+----+-------------+------+-------+------+
| Id | User  | Host      | db | Command     | Time | State | Info |
+----+-------+-----------+----+-------------+------+-------+------+
| 6  | monty | localhost |    | Processlist | 0    |       |      |
+----+-------+-----------+----+-------------+------+-------+------+
Uptime: 10077  Threads: 1  Questions: 9  Slow queries: 0  Opens: 6  Flush tables: 1  Open tables: 2  Memory in use: 1092K  Max memory used: 1116K

The mysqladmin status command result has the following columns:

Uptime Number of seconds the MySQL server has been up.
Threads Number of active threads (clients).
Questions Number of questions from clients since mysqld was started.
Slow queries Queries that have taken more than long_query_time seconds. See section 4.9.5 The Slow Query Log.
Opens How many tables mysqld has opened.
Flush tables Number of flush ..., refresh, and reload commands.
Open tables Number of tables that are open now.
Memory in use Memory allocated directly by the mysqld code (only available when MySQL is compiled with --with-debug=full).
Max memory used Maximum memory allocated directly by the mysqld code (only available when MySQL is compiled with --with-debug=full).

If you do myslqadmin shutdown on a socket (in other words, on a the computer where mysqld is running), mysqladmin will wait until the MySQL pid-file is removed to ensure that the mysqld server has stopped properly.

4.8.4 Using mysqlcheck for Table Maintenance and Crash Recovery

Since MySQL version 3.23.38 you will be able to use a new checking and repairing tool for MyISAM tables. The difference to myisamchk is that mysqlcheck should be used when the mysqld server is running, where as myisamchk should be used when it is not. The benefit is that you no longer have to take the server down for checking or repairing your tables.

mysqlcheck uses MySQL server commands CHECK, REPAIR, ANALYZE and OPTIMIZE in a convenient way for the user.

There are three alternative ways to invoke mysqlcheck: 

shell> mysqlcheck [OPTIONS] database [tables]
shell> mysqlcheck [OPTIONS] --databases DB1 [DB2 DB3...]
shell> mysqlcheck [OPTIONS] --all-databases

So it can be used in a similar way as mysqldump when it comes to what databases and tables you want to choose.

mysqlcheck does have a special feature compared to the other clients; the default behavior, checking tables (-c), can be changed by renaming the binary. So if you want to have a tool that repairs tables by default, you should just copy mysqlcheck to your harddrive with a new name, mysqlrepair, or alternatively make a symbolic link to mysqlrepair and name the symbolic link as mysqlrepair. If you invoke mysqlrepair now, it will repair tables by default.

The names that you can use to change mysqlcheck default behavior are here: 

mysqlrepair:   The default option will be -r
mysqlanalyze:  The default option will be -a
mysqloptimize: The default option will be -o

The options available for mysqlcheck are listed here, please check what your version supports with mysqlcheck --help.

-A, --all-databases
Check all the databases. This will be same as --databases with all databases selected
-1, --all-in-1
Instead of making one query for each table, execute all queries in 1 query separately for each database. Table names will be in a comma separated list.
-a, --analyze
Analyze given tables.
--auto-repair
If a checked table is corrupted, automatically fix it. Repairing will be done after all tables have been checked, if corrupted ones were found.
-#, --debug=...
Output debug log. Often this is 'd:t:o,filename'
--character-sets-dir=...
Directory where character sets are
-c, --check
Check table for errors
-C, --check-only-changed
Check only tables that have changed since last check or haven't been closed properly.
--compress
Use compression in server/client protocol.
-?, --help
Display this help message and exit.
-B, --databases
To check several databases. Note the difference in usage; In this case no tables are given. All name arguments are regarded as database names.
--default-character-set=...
Set the default character set
-F, --fast
Check only tables that hasn't been closed properly
-f, --force
Continue even if we get an sql-error.
-e, --extended
If you are using this option with CHECK TABLE, it will ensure that the table is 100 percent consistent, but will take a long time. If you are using this option with REPAIR TABLE, it will run an extended repair on the table, which may not only take a long time to execute, but may produce a lot of garbage rows also!
-h, --host=...
Connect to host.
-m, --medium-check
Faster than extended-check, but only finds 99.99 percent of all errors. Should be good enough for most cases.
-o, --optimize
Optimize table
-p, --password[=...]
Password to use when connecting to server. If password is not given it's solicited on the tty.
-P, --port=...
Port number to use for connection.
-q, --quick
If you are using this option with CHECK TABLE, it prevents the check from scanning the rows to check for wrong links. This is the fastest check. If you are using this option with REPAIR TABLE, it will try to repair only the index tree. This is the fastest repair method for a table.
-r, --repair
Can fix almost anything except unique keys that aren't unique.
-s, --silent
Print only error messages.
-S, --socket=...
Socket file to use for connection.
--tables
Overrides option --databases (-B).
-u, --user=#
User for login if not current user.
-v, --verbose
Print info about the various stages.
-V, --version
Output version information and exit.

4.8.5 mysqldump, Dumping Table Structure and Data

Utility to dump a database or a collection of database for backup or for transferring the data to another SQL server (not necessarily a MySQL server). The dump will contain SQL statements to create the table and/or populate the table.

If you are doing a backup on the server, you should consider using the mysqlhotcopy instead. See section 4.8.6 mysqlhotcopy, Copying MySQL Databases and Tables. 

shell> mysqldump [OPTIONS] database [tables]
OR     mysqldump [OPTIONS] --databases [OPTIONS] DB1 [DB2 DB3...]
OR     mysqldump [OPTIONS] --all-databases [OPTIONS]

If you don't give any tables or use the --databases or --all-databases, the whole database(s) will be dumped.

You can get a list of the options your version of mysqldump supports by executing mysqldump --help.

Note that if you run mysqldump without --quick or --opt, mysqldump will load the whole result set into memory before dumping the result. This will probably be a problem if you are dumping a big database.

Note that if you are using a new copy of the mysqldump program and you are going to do a dump that will be read into a very old MySQL server, you should not use the --opt or -e options.

mysqldump supports the following options:

--add-locks
Add LOCK TABLES before and UNLOCK TABLE after each table dump. (To get faster inserts into MySQL.)
--add-drop-table
Add a drop table before each create statement.
-A, --all-databases
Dump all the databases. This will be same as --databases with all databases selected.
-a, --all
Include all MySQL-specific create options.
--allow-keywords
Allow creation of column names that are keywords. This works by prefixing each column name with the table name.
-c, --complete-insert
Use complete insert statements (with column names).
-C, --compress
Compress all information between the client and the server if both support compression.
-B, --databases
To dump several databases. Note the difference in usage. In this case no tables are given. All name arguments are regarded as database names. USE db_name; will be included in the output before each new database.
--delayed
Insert rows with the INSERT DELAYED command.
-e, --extended-insert
Use the new multiline INSERT syntax. (Gives more compact and faster inserts statements.)
-#, --debug[=option_string]
Trace usage of the program (for debugging).
--help
Display a help message and exit.
--fields-terminated-by=...
--fields-enclosed-by=...
--fields-optionally-enclosed-by=...
--fields-escaped-by=...
--lines-terminated-by=...
These options are used with the -T option and have the same meaning as the corresponding clauses for LOAD DATA INFILE. See section 6.4.8 LOAD DATA INFILE Syntax.
-F, --flush-logs
Flush log file in the MySQL server before starting the dump.
-f, --force,
Continue even if we get a SQL error during a table dump.
-h, --host=..
Dump data from the MySQL server on the named host. The default host is localhost.
-l, --lock-tables.
Lock all tables before starting the dump. The tables are locked with READ LOCAL to allow concurrent inserts in the case of MyISAM tables.
-n, --no-create-db
'CREATE DATABASE /*!32312 IF NOT EXISTS*/ db_name;' will not be put in the output. The above line will be added otherwise, if --databases or --all-databases option was given.
-t, --no-create-info
Don't write table creation information (The CREATE TABLE statement.)
-d, --no-data
Don't write any row information for the table. This is very useful if you just want to get a dump of the structure for a table!
--opt
Same as --quick --add-drop-table --add-locks --extended-insert --lock-tables. Should give you the fastest possible dump for reading into a MySQL server.
-pyour_pass, --password[=your_pass]
The password to use when connecting to the server. If you specify no `=your_pass' part, mysqldump you will be prompted for a password.
-P port_num, --port=port_num
The TCP/IP port number to use for connecting to a host. (This is used for connections to hosts other than localhost, for which Unix sockets are used.)
-q, --quick
Don't buffer query, dump directly to stdout. Uses mysql_use_result() to do this.
-r, --result-file=...
Direct output to a given file. This option should be used in MSDOS, because it prevents new line '\n' from being converted to '\n\r' (new line + carriage return).
-S /path/to/socket, --socket=/path/to/socket
The socket file to use when connecting to localhost (which is the default host).
--tables
Overrides option --databases (-B).
-T, --tab=path-to-some-directory
Creates a table_name.sql file, that contains the SQL CREATE commands, and a table_name.txt file, that contains the data, for each give table. NOTE: This only works if mysqldump is run on the same machine as the mysqld daemon. The format of the .txt file is made according to the --fields-xxx and --lines--xxx options.
-u user_name, --user=user_name
The MySQL user name to use when connecting to the server. The default value is your Unix login name.
-O var=option, --set-variable var=option
Set the value of a variable. The possible variables are listed below.
-v, --verbose
Verbose mode. Print out more information on what the program does.
-V, --version
Print version information and exit.
-w, --where='where-condition'
Dump only selected records. Note that QUOTES are mandatory: 
"--where=user='jimf'" "-wuserid>1" "-wuserid<1"
-O net_buffer_length=#, where # < 16M
When creating multi-row-insert statements (as with option --extended-insert or --opt), mysqldump will create rows up to net_buffer_length length. If you increase this variable, you should also ensure that the max_allowed_packet variable in the MySQL server is bigger than the net_buffer_length.

The most normal use of mysqldump is probably for making a backup of whole databases. See section 4.4.1 Database Backups. 

mysqldump --opt database > backup-file.sql

You can read this back into MySQL with: 

mysql database < backup-file.sql

or 

mysql -e "source /patch-to-backup/backup-file.sql" database

However, it's also very useful to populate another MySQL server with information from a database: 

mysqldump --opt database | mysql --host=remote-host -C database

It is possible to dump several databases with one command: 

mysqldump --databases database1 [database2 database3...] > my_databases.sql

If all the databases are wanted, one can use: 

mysqldump --all-databases > all_databases.sql

4.8.6 mysqlhotcopy, Copying MySQL Databases and Tables

mysqlhotcopy is a perl script that uses LOCK TABLES, FLUSH TABLES and cp or scp to quickly make a backup of a database. It's the fastest way to make a backup of the database, of single tables but it can only be run on the same machine where the database directories are. 

mysqlhotcopy db_name [/path/to/new_directory]

mysqlhotcopy db_name_1 ... db_name_n /path/to/new_directory

mysqlhotcopy db_name./regex/

mysqlhotcopy supports the following options:

-?, --help
Display a help screen and exit
-u, --user=#
User for database login
-p, --password=#
Password to use when connecting to server
-P, --port=#
Port to use when connecting to local server
-S, --socket=#
Socket to use when connecting to local server
--allowold
Don't abort if target already exists (rename it _old)
--keepold
Don't delete previous (now renamed) target when done
--noindices
Don't include full index files in copy to make the backup smaller and faster The indexes can later be reconstructed with myisamchk -rq..
--method=#
Method for copy (cp or scp).
-q, --quiet
Be silent except for errors
--debug
Enable debug
-n, --dryrun
Report actions without doing them
--regexp=#
Copy all databases with names matching regexp
--suffix=#
Suffix for names of copied databases
--checkpoint=#
Insert checkpoint entry into specified db.table
--flushlog
Flush logs once all tables are locked.
--tmpdir=#
Temporary directory (instead of /tmp).

You can use perldoc mysqlhotcopy to get a more complete documentation for mysqlhotcopy.

mysqlhotcopy reads the groups [client] and [mysqlhotcopy] from the option files.

To be able to execute mysqlhotcopy you need write access to the backup directory, SELECT privilege to the tables you are about to copy and the MySQL Reload privilege (to be able to execute FLUSH TABLES).

4.8.7 mysqlimport, Importing Data from Text Files

mysqlimport provides a command-line interface to the LOAD DATA INFILE SQL statement. Most options to mysqlimport correspond directly to the same options to LOAD DATA INFILE. See section 6.4.8 LOAD DATA INFILE Syntax.

mysqlimport is invoked like this: 

shell> mysqlimport [options] database textfile1 [textfile2....]

For each text file named on the command line, mysqlimport strips any extension from the filename and uses the result to determine which table to import the file's contents into. For example, files named `patient.txt', `patient.text', and `patient' would all be imported into a table named patient.

mysqlimport supports the following options:

-c, --columns=...
This option takes a comma-separated list of field names as an argument. The field list is used to create a proper LOAD DATA INFILE command, which is then passed to MySQL. See section 6.4.8 LOAD DATA INFILE Syntax.
-C, --compress
Compress all information between the client and the server if both support compression.
-#, --debug[=option_string]
Trace usage of the program (for debugging).
-d, --delete
Empty the table before importing the text file.
--fields-terminated-by=...
--fields-enclosed-by=...
--fields-optionally-enclosed-by=...
--fields-escaped-by=...
--lines-terminated-by=...
These options have the same meaning as the corresponding clauses for LOAD DATA INFILE. See section 6.4.8 LOAD DATA INFILE Syntax.
-f, --force
Ignore errors. For example, if a table for a text file doesn't exist, continue processing any remaining files. Without --force, mysqlimport exits if a table doesn't exist.
--help
Display a help message and exit.
-h host_name, --host=host_name
Import data to the MySQL server on the named host. The default host is localhost.
-i, --ignore
See the description for the --replace option.
-l, --lock-tables
Lock ALL tables for writing before processing any text files. This ensures that all tables are synchronized on the server.
-L, --local
Read input files from the client. By default, text files are assumed to be on the server if you connect to localhost (which is the default host).
-pyour_pass, --password[=your_pass]
The password to use when connecting to the server. If you specify no `=your_pass' part, mysqlimport you will be prompted for a password.
-P port_num, --port=port_num
The TCP/IP port number to use for connecting to a host. (This is used for connections to hosts other than localhost, for which Unix sockets are used.)
-r, --replace
The --replace and --ignore options control handling of input records that duplicate existing records on unique key values. If you specify --replace, new rows replace existing rows that have the same unique key value. If you specify --ignore, input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, an error occurs when a duplicate key value is found, and the rest of the text file is ignored.
-s, --silent
Silent mode. Write output only when errors occur.
-S /path/to/socket, --socket=/path/to/socket
The socket file to use when connecting to localhost (which is the default host).
-u user_name, --user=user_name
The MySQL user name to use when connecting to the server. The default value is your Unix login name.
-v, --verbose
Verbose mode. Print out more information what the program does.
-V, --version
Print version information and exit.

Here is a sample run using mysqlimport: 

$ mysql --version
mysql  Ver 9.33 Distrib 3.22.25, for pc-linux-gnu (i686)
$ uname -a
Linux xxx.com 2.2.5-15 #1 Mon Apr 19 22:21:09 EDT 1999 i586 unknown
$ mysql -e 'CREATE TABLE imptest(id INT, n VARCHAR(30))' test
$ ed
a
100     Max Sydow
101     Count Dracula
.
w imptest.txt
32
q
$ od -c imptest.txt
0000000   1   0   0  \t   M   a   x       S   y   d   o   w  \n   1   0
0000020   1  \t   C   o   u   n   t       D   r   a   c   u   l   a  \n
0000040
$ mysqlimport --local test imptest.txt
test.imptest: Records: 2  Deleted: 0  Skipped: 0  Warnings: 0
$ mysql -e 'SELECT * FROM imptest' test
+------+---------------+
| id   | n             |
+------+---------------+
|  100 | Max Sydow     |
|  101 | Count Dracula |
+------+---------------+

4.8.8 Showing Databases, Tables, and Columns

mysqlshow can be used to quickly look at which databases exist, their tables, and the table's columns.

With the mysql program you can get the same information with the SHOW commands. See section 4.5.5 SHOW Syntax.

mysqlshow is invoked like this: 

shell> mysqlshow [OPTIONS] [database [table [column]]]

Note that in newer MySQL versions, you only see those database/tables/columns for which you have some privileges.

If the last argument contains a shell or SQL wild-card (*, ?, % or _) then only what's matched by the wild card is shown. This may cause some confusion when you try to display the columns for a table with a _ as in this case mysqlshow only shows you the table names that match the pattern. This is easily fixed by adding an extra % last on the command line (as a separate argument).

4.8.9 perror, Explaining Error Codes

perror can be used to print error message(s). perror can be invoked like this: 

shell> perror [OPTIONS] [ERRORCODE [ERRORCODE...]]

For example:

shell> perror 64 79
Error code  64:  Machine is not on the network
Error code  79:  Can not access a needed shared library

perror can be used to display a description for a system error code, or an MyISAM/ISAM table handler error code. The error messages are mostly system dependent.

4.8.10 How to Run SQL Commands from a Text File

The mysql client typically is used interactively, like this: 

shell> mysql database

However, it's also possible to put your SQL commands in a file and tell mysql to read its input from that file. To do so, create a text file `text_file' that contains the commands you wish to execute. Then invoke mysql as shown below: 

shell> mysql database < text_file

You can also start your text file with a USE db_name statement. In this case, it is unnecessary to specify the database name on the command line: 

shell> mysql < text_file

See section 4.8 MySQL Client-Side Scripts and Utilities.

4.9 The MySQL Log Files

MySQL has several different log files that can help you find out what's going on inside mysqld:

The error log Problems encountering starting, running or stopping mysqld.
The isam log Logs all changes to the ISAM tables. Used only for debugging the isam code.
The query log Established connections and executed queries.
The update log Deprecated: Stores all statements that changes data
The binary log Stores all statements that changes something. Used also for replication
The slow log Stores all queries that took more than long_query_time to execute or didn't use indexes.

All logs can be found in the mysqld data directory. You can force mysqld to reopen the log files (or in some cases switch to a new log) by executing FLUSH LOGS. See section 4.5.3 FLUSH Syntax.

4.9.1 The Error Log

mysqld writes all errors to the stderr, which the safe_mysqld script redirects to a file called 'hostname'.err. (On Windows, mysqld writes this directly to `\mysql\data\mysql.err').

This contains information indicating when mysqld was started and stopped and also any critical errors found when running. If mysqld dies unexpectedly and safe_mysqld needs to restart mysqld, safe_mysqld will write a restarted mysqld row in this file. This log also holds a warning if mysqld notices a table that needs to be automatically checked or repaired.

On some operating systems, the error log will contain a stack trace for where mysqld died. This can be used to find out where mysqld died. See section G.1.4 Using a stack trace.

4.9.2 The General Query Log

If you want to know what happens within mysqld, you should start it with --log[=file]. This will log all connections and queries to the log file (by default named `'hostname'.log'). This log can be very useful when you suspect an error in a client and want to know exactly what mysqld thought the client sent to it.

By default, the mysql.server script starts the MySQL server with the -l option. If you need better performance when you start using MySQL in a production environment, you can remove the -l option from mysql.server or change it to --log-bin.

The entries in this log are written as mysqld receives the questions. This may be different than the order in which the statements are executed. This is in contrast to the update log and the binary log which are written after the query is executed, but before any locks are released.

4.9.3 The Update Log

NOTE: The update log is replaced by the binary log. See section 4.9.4 The Binary Update Log. With this you can do anything that you can do with the update log.

When started with the --log-update[=file_name] option, mysqld writes a log file containing all SQL commands that update data. If no filename is given, it defaults to the name of the host machine. If a filename is given, but it doesn't contain a path, the file is written in the data directory. If `file_name' doesn't have an extension, mysqld will create log file names like so: `file_name.###', where ### is a number that is incremented each time you execute mysqladmin refresh, execute mysqladmin flush-logs, execute the FLUSH LOGS statement, or restart the server.

NOTE: For the above scheme to work, you should NOT create your own files with the same filename as the update log + some extensions that may be regarded as a number, in the directory used by the update log!

If you use the --log or -l options, mysqld writes a general log with a filename of `hostname.log', and restarts and refreshes do not cause a new log file to be generated (although it is closed and reopened). In this case you can copy it (on Unix) by doing: 

mv hostname.log hostname-old.log
mysqladmin flush-logs
cp hostname-old.log to-backup-directory
rm hostname-old.log

Update logging is smart because it logs only statements that really update data. So an UPDATE or a DELETE with a WHERE that finds no rows is not written to the log. It even skips UPDATE statements that set a column to the value it already has.

The update logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that the log will be logged in the execution order.

If you want to update a database from update log files, you could do the following (assuming your update logs have names of the form `file_name.###'): 

shell> ls -1 -t -r file_name.[0-9]* | xargs cat | mysql

ls is used to get all the log files in the right order.

This can be useful if you have to revert to backup files after a crash and you want to redo the updates that occurred between the time of the backup and the crash.

4.9.4 The Binary Update Log

The intention is that the binary log should replace the update log, so we recommend you to switch to this log format as soon as possible!

The binary log contains all information that is available in the update log in a more efficient format. It also contains information about how long every query that updated the database took.

The binary log is also used when you are replicating a slave from a master. See section 4.10 Replication in MySQL.

When started with the --log-bin[=file_name] option, mysqld writes a log file containing all SQL commands that update data. If no file name is given, it defaults to the name of the host machine followed by -bin. If file name is given, but it doesn't contain a path, the file is written in the data directory.

If you supply an extension to --log-bin=filename.extension, the extension will be silenty removed.

To the binary log filename mysqld will append an extension that is a number that is incremented each time you execute mysqladmin refresh, execute mysqladmin flush-logs, execute the FLUSH LOGS statement or restart the server.

You can use the following options to mysqld to affect what is logged to the binary log:

binlog-do-db=database_name Tells the master it should log updates for the specified database, and exclude all others not explicitly mentioned. (Example: binlog-do-db=some_database)
binlog-ignore-db=database_name Tells the master that updates to the given database should not be logged to the binary log (Example: binlog-ignore-db=some_database)

To be able to know which different binary log files have been used, mysqld will also create a binary log index file that contains the name of all used binary log files. By default this has the same name as the binary log file, with the extension '.index'. You can change the name of the binary log index file with the --log-bin-index=[filename] option.

If you are using replication, you should not delete old binary log files until you are sure that no slave will ever need to use them. One way to do this is to do mysqladmin flush-logs once a day and then remove any logs that are more than 3 days old.

You can examine the binary log file with the mysqlbinlog command. For example, you can update a MySQL server from the binary log as follows: 

mysqlbinlog log-file | mysql -h server_name

You can also use the mysqlbinlog program to read the binary log directly from a remote MySQL server!

mysqlbinlog --help will give you more information of how to use this program!

If you are using BEGIN [WORK] or SET AUTOCOMMIT=0, you must use the MySQL binary log for backups instead of the old update log.

The binary logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that the log will be logged in the execution order.

All updates (UPDATE, DELETE or INSERT) that change a transactional table (like BDB tables) are cached until a COMMIT. Any updates to a non-transactional table are stored in the binary log at once. Every thread will, on start, allocate a buffer of binlog_cache_size to buffer queries. If a query is bigger than this, the thread will open a temporary file to handle the bigger cache. The temporary file will be deleted when the thread ends.

The max_binlog_cache_size can be used to restrict the total size used to cache a multi-transaction query.

If you are using the update or binary log, concurrent inserts will not work together with CREATE ... INSERT and INSERT ... SELECT. This is to ensure that you can recreate an exact copy of your tables by applying the log on a backup.

4.9.5 The Slow Query Log

When started with the --log-slow-queries[=file_name] option, mysqld writes a log file containing all SQL commands that took more than long_query_time to execute. The time to get the initial table locks are not counted as execution time.

The slow query log is logged after the query is executed and after all locks has been released. This may be different than the order in which the statements are executed.

If no file name is given, it defaults to the name of the host machine suffixed with -slow.log. If a filename is given, but doesn't contain a path, the file is written in the data directory.

The slow query log can be used to find queries that take a long time to execute and are thus candidates for optimization. With a large log, that can become a difficult task. You can pipe the slow query log through the mysqldumpslow command to get a summary of the queries which appear in the log.

You are using --log-long-format then also queries that are not using indexes are printed. See section 4.1.1 mysqld Command-line Options.

4.9.6 Log File Maintenance

MySQL has a lot of log files which make it easy to see what is going. See section 4.9 The MySQL Log Files. One must however from time to time clean up after MysQL to ensure that the logs don't take up too much disk space.

When using MySQL with log files, you will, from time to time, want to remove/backup old log files and tell MySQL to start logging on new files. See section 4.4.1 Database Backups.

On a Linux (Redhat) installation, you can use the mysql-log-rotate script for this. If you installed MySQL from an RPM distribution, the script should have been installed automatically. Note that you should be careful with this if you are using the log for replication!

On other systems you must install a short script yourself that you start from cron to handle log files.

You can force MySQL to start using new log files by using mysqladmin flush-logs or by using the SQL command FLUSH LOGS. If you are using MySQL Version 3.21 you must use mysqladmin refresh.

The above command does the following:

If you are using only an update log, you only have to flush the logs and then move away the old update log files to a backup. If you are using the normal logging, you can do something like: 

shell> cd mysql-data-directory
shell> mv mysql.log mysql.old
shell> mysqladmin flush-logs

and then take a backup and remove `mysql.old'.

4.10 Replication in MySQL

This chapter describes the various replication features in MySQL. It serves as a reference to the options available with replication. You will be introduced to replication and learn how to implement it. Towards the end, there are some frequently asked questions and descriptions of problems and how to solve them.

4.10.1 Introduction

One way replication can be used is to increase both robustness and speed. For robustness you can have two systems and can switch to the backup if you have problems with the master. The extra speed is achieved by sending a part of the non-updating queries to the replica server. Of course this only works if non-updating queries dominate, but that is the normal case.

Starting in Version 3.23.15, MySQL supports one-way replication internally. One server acts as the master, while the other acts as the slave. Note that one server could play the roles of master in one pair and slave in the other. The master server keeps a binary log of updates (See section 4.9.4 The Binary Update Log.) and an index file to binary logs to keep track of log rotation. The slave, upon connecting, informs the master where it left off since the last successfully propagated update, catches up on the updates, and then blocks and waits for the master to notify it of the new updates.

Note that if you are replicating a database, all updates to this database should be done through the master!

Another benefit of using replication is that one can get live backups of the system by doing a backup on a slave instead of doing it on the master. See section 4.4.1 Database Backups.

4.10.2 Replication Implementation Overview

MySQL replication is based on the server keeping track of all changes to your database (updates, deletes, etc) in the binary log. (See section 4.9.4 The Binary Update Log.) and the slave server(s) reading the saved queries from the master server's binary log so that the slave can execute the same queries on its copy of the data.

It is very important to realize that the binary log is simply a record starting from a fixed point in time (the moment you enable binary logging). Any slaves which you set up will need copies of all the data from your master as it existed the moment that you enabled binary logging on the master. If you start your slaves with data that doesn't agree with what was on the master when the binary log was started, your slaves may fail.

A future version (4.0) of MySQL will remove the need to keep a (possibly large) snapshot of data for new slaves that you might wish to set up through the live backup functionality with no locking required. However, at this time, it is necessary to block all writes either with a global read lock or by shutting down the master while taking a snapshot.

Once a slave is properly configured and running, it will simply connect to the master and wait for updates to process. If the master goes away or the slave loses connectivity with your master, it will keep trying to connect every master-connect-retry seconds until it is able to reconnect and resume listening for updates.

Each slave keeps track of where it left off. The master server has no knowledge of how many slaves there are or which ones are up-to-date at any given time.

The next section explains the master/slave setup process in more detail.

4.10.3 How To Set Up Replication

Below is a quick description of how to set up complete replication on your current MySQL server. It assumes you want to replicate all your databases and have not configured replication before. You will need to shutdown your master server briefly to complete the steps outlined below.

  1. Make sure you have a recent version of MySQL installed on the master and slave(s). Use Version 3.23.29 or higher. Previous releases used a different binary log format and had bugs which have been fixed in newer releases. Please, do not report bugs until you have verified that the problem is present in the latest release.
  2. Set up special a replication user on the master with the FILE privilege and permission to connect from all the slaves. If the user is only doing replication (which is recommended), you don't need to grant any additional privileges. For example, to create a user named repl which can access your master from any host, you might use this command: 
    GRANT FILE ON *.* TO repl@"%" IDENTIFIED BY '<password>';
  3. Shut down MySQL on the master. 
    mysqladmin -u root -p<password> shutdown
  4. Snapshot all the data on your master server. The easiest way to do this (on Unix) is to simply use tar to produce an archive of your entire data directory. The exact data directory location depends on your installation. 
    tar -cvf /tmp/mysql-snapshot.tar /path/to/data-dir
    Windows users can use WinZip or similar software to create an archive of the data directory.
  5. In my.cnf on the master add log-bin and server-id=unique number to the [mysqld] section and restart it. It is very important that the id of the slave is different from the id of the master. Think of server-id as something similar to the IP address - it uniquely identifies the server instance in the community of replication partners. 
    [mysqld]
log-bin
server-id=1
  6. Restart MySQL on the master.
  7. Add the following to my.cnf on the slave(s): 
    master-host=<hostname of the master>
master-user=<replication user name>
master-password=<replication user password>
master-port=<TCP/IP port for master>
server-id=<some unique number between 2 and 2^32-1>
    replacing the values in <> with what is relevant to your system. server-id must be different for each server participating in replication. If you don't specify a server-id, it will be set to 1 if you have not defined master-host, else it will be set to 2. Note that in the case of server-id omission the master will refuse connections from all slaves, and the slave will refuse to connect to a master. Thus, omitting server-id is only good for backup with a binary log.
  8. Copy the snapshot data into your data directory on your slave(s). Make sure that the privileges on the files and directories are correct. The user which MySQL runs as needs to be able to read and write to them, just as on the master.
  9. Restart the slave(s).

After you have done the above, the slave(s) should connect to the master and catch up on any updates which happened since the snapshot was taken.

If you have forgotten to set server-id for the slave you will get the following error in the error log file: 

Warning: one should set server_id to a non-0 value if master_host is set.
The server will not act as a slave.

If you have forgot to do this for the master, the slaves will not be able to connect to the master.

If a slave is not able to replicate for any reason, you will find error messages in the error log on the slave.

Once a slave is replicating, you will find a file called master.info in the same directory as your error log. The master.info file is used by the slave to keep track of how much of the master's binary log is has processed. Do not remove or edit the file, unless you really know what you are doing. Even in that case, it is preferred that you use CHANGE MASTER TO command.

4.10.4 Replication Features and Known Problems

Below is an explanation of what is supported and what is not:

4.10.5 Replication Options in my.cnf

If you are using replication, we recommend you to use MySQL Version 3.23.30 or later. Older versions work, but they do have some bugs and are missing some features.

On both master and slave you need to use the server-id option. This sets an unique replication id. You should pick a unique value in the range between 1 to 2^32-1 for each master and slave. Example: server-id=3

The following table has the options you can use for the MASTER:

Option Description
log-bin=filename Write to a binary update log to the specified location. Note that if you give it a parameter with an extension (for example, log-bin=/mysql/logs/replication.log ) versions up to 3.23.24 will not work right during replication if you do FLUSH LOGS . The problem is fixed in Version 3.23.25. If you are using this kind of log name, FLUSH LOGS will be ignored on binlog. To clear the log, run FLUSH MASTER, and do not forget to run FLUSH SLAVE on all slaves. In Version 3.23.26 and in later versions you should use RESET MASTER and RESET SLAVE
log-bin-index=filename Because the user could issue the FLUSH LOGS command, we need to know which log is currently active and which ones have been rotated out and in what sequence. This information is stored in the binary log index file. The default is `hostname`.index. You can use this option if you want to be a rebel. Example: log-bin-index=db.index.
sql-bin-update-same If set, setting SQL_LOG_BIN to a value will automatically set SQL_LOG_UPDATE to the same value and vice versa.
binlog-do-db=database_name Tells the master that it should log updates to the binary log if the current database is 'database_name'. All others database are ignored. Note that if you use this you should ensure that you only do updates in the current database. Example: binlog-do-db=some_database.
binlog-ignore-db=database_name Tells the master that updates where the current database is 'database_name' should not be stored in the binary log. Note that if you use this you should ensure that you only do updates in the current database. Example: binlog-ignore-db=some_database

The following table has the options you can use for the SLAVE:

Option Description
master-host=host Master hostname or IP address for replication. If not set, the slave thread will not be started. Example: master-host=db-master.mycompany.com.
master-user=username The user the slave thread will us for authentication when connecting to the master. The user must have FILE privilege. If the master user is not set, user test is assumed. Example: master-user=scott.
master-password=password The password the slave thread will authenticate with when connecting to the master. If not set, an empty password is assumed. Example: master-password=tiger.
master-port=portnumber The port the master is listening on. If not set, the compiled setting of MYSQL_PORT is assumed. If you have not tinkered with configure options, this should be 3306. Example: master-port=3306.
master-connect-retry=seconds The number of seconds the slave thread will sleep before retrying to connect to the master in case the master goes down or the connection is lost. Default is 60. Example: master-connect-retry=60.
master-info-file=filename The location of the file that remembers where we left off on the master during the replication process. The default is master.info in the data directory. Sasha: The only reason I see for ever changing the default is the desire to be rebelious. Example: master-info-file=master.info.
replicate-do-table=db_name.table_name Tells the slave thread to restrict replication to the specified table. To specify more than one table, use the directive multiple times, once for each table. This will work for cross-database updates, in contrast to replicate-do-db. Example: replicate-do-table=some_db.some_table.
replicate-ignore-table=db_name.table_name Tells the slave thread to not replicate to the specified table. To specify more than one table to ignore, use the directive multiple times, once for each table. This will work for cross-datbase updates, in contrast to replicate-ignore-db. Example: replicate-ignore-table=db_name.some_table.
replicate-wild-do-table=db_name.table_name Tells the slave thread to restrict replication to the tables that match the specified wildcard pattern. To specify more than one table, use the directive multiple times, once for each table. This will work for cross-database updates. Example: replicate-wild-do-table=foo%.bar% will replicate only updates to tables in all databases that start with foo and whose table names start with bar.
replicate-wild-ignore-table=db_name.table_name Tells the slave thread to not replicate to the tables that match the given wild card pattern. To specify more than one table to ignore, use the directive multiple times, once for each table. This will work for cross-database updates. Example: replicate-wild-ignore-table=foo%.bar% will not do updates to tables in databases that start with foo and whose table names start with bar.
replicate-ignore-db=database_name Tells the slave thread to not replicate to the specified database. To specify more than one database to ignore, use the directive multiple times, once for each database. This option will not work if you use cross database updates. If you need cross database updates to work, make sure you have 3.23.28 or later, and use replicate-wild-ignore-table=db_name.% Example: replicate-ignore-db=some_db.
replicate-do-db=database_name Tells the slave thread to restrict replication to the specified database. To specify more than one database, use the directive multiple times, once for each database. Note that this will only work if you do not use cross-database queries such as UPDATE some_db.some_table SET foo='bar' while having selected a different or no database. If you need cross database updates to work, make sure you have 3.23.28 or later, and use replicate-wild-do-table=db_name.% Example: replicate-do-db=some_db.
log-slave-updates Tells the slave to log the updates from the slave thread to the binary log. Off by default. You will need to turn it on if you plan to daisy-chain the slaves.
replicate-rewrite-db=from_name->to_name Updates to a database with a different name than the original Example: replicate-rewrite-db=master_db_name->slave_db_name.
slave-skip-errors=err_code1,err_code2,.. Available only in 3.23.47 and later. Tells the slave thread to continue replication when a query returns an error from the provided list. Normally, replication will discontinue when an error is encountered giving the user a chance to resolve the inconsistency in the data manually. Do not use this option unless you fully understand why you are getting the errors. If there are no bugs in your replication setup and client programs, and no bugs in MySQL itself, you should never get an abort with error.Indiscriminate use of this option will result in slaves being hopelessly out of sync with the master and you having no idea how the problem happened. For error codes, you should use the numbers provided by the error message in your slave error log and in the output of SHOW SLAVE STATUS. Full list of error messages can be found in the source distribution in Docs/mysqld_error.txt. You can ( but should not) also use a very non-recommended value of all which will ignore all error messages and keep barging along regardless. Needless to say, if you use it, we make no promises regarding your data integrity. Please do not complain if your data on the slave is not anywhere close to what it is on the master in this case - you have been warned. Example: slave-skip-errors=1062,1053 or slave-skip-errors=all
skip-slave-start Tells the slave server not to start the slave on the startup. The user can start it later with SLAVE START.
slave_read_timeout=# Number of seconds to wait for more data from the master before aborting the read.

4.10.6 SQL Commands Related to Replication

Replication can be controlled through the SQL interface. Below is the summary of commands:

Command Description
SLAVE START Starts the slave thread. (Slave)
SLAVE STOP Stops the slave thread. (Slave)
SET SQL_LOG_BIN=0 Disables update logging if the user has process privilege. Ignored otherwise. (Master)
SET SQL_LOG_BIN=1 Re-enables update logging if the user has process privilege. Ignored otherwise. (Master)
SET SQL_SLAVE_SKIP_COUNTER=n Skip the next n events from the master. Only valid when the slave thread is not running, otherwise, gives an error. Useful for recovering from replication glitches.
RESET MASTER Deletes all binary logs listed in the index file, resetting the binlog index file to be empty. In pre-3.23.26 versions, FLUSH MASTER (Master)
RESET SLAVE Makes the slave forget its replication position in the master logs. In pre 3.23.26 versions the command was called FLUSH SLAVE(Slave)
LOAD TABLE tblname FROM MASTER Downloads a copy of the table from master to the slave. (Slave)
CHANGE MASTER TO master_def_list Changes the master parameters to the values specified in master_def_list and restarts the slave thread. master_def_list is a comma-separated list of master_def where master_def is one of the following: MASTER_HOST, MASTER_USER, MASTER_PASSWORD, MASTER_PORT, MASTER_CONNECT_RETRY, MASTER_LOG_FILE, MASTER_LOG_POS. Example: 

CHANGE MASTER TO
  MASTER_HOST='master2.mycompany.com',
  MASTER_USER='replication',
  MASTER_PASSWORD='bigs3cret',
  MASTER_PORT=3306,
  MASTER_LOG_FILE='master2-bin.001',
  MASTER_LOG_POS=4;
You only need to specify the values that need to be changed. The values that you omit will stay the same with the exception of when you change the host or the port. In that case, the slave will assume that since you are connecting to a different host or a different port, the master is different. Therefore, the old values of log and position are not applicable anymore, and will automatically be reset to an empty string and 0, respectively (the start values). Note that if you restart the slave, it will remember its last master. If this is not desirable, you should delete the `master.info' file before restarting, and the slave will read its master from my.cnf or the command line. (Slave)
SHOW MASTER STATUS Provides status information on the binlog of the master. (Master)
SHOW SLAVE STATUS Provides status information on essential parameters of the slave thread. (Slave)
SHOW MASTER LOGS Only available starting in Version 3.23.28. Lists the binary logs on the master. You should use this command prior to PURGE MASTER LOGS TO to find out how far you should go.
PURGE MASTER LOGS TO 'logname' Available starting in Version 3.23.28. Deletes all the replication logs that are listed in the log index as being prior to the specified log, and removed them from the log index, so that the given log now becomes first. Example: 
PURGE MASTER LOGS TO 'mysql-bin.010'
This command will do nothing and fail with an error if you have an active slave that is currently reading one of the logs you are trying to delete. However, if you have a dormant slave, and happen to purge one of the logs it wants to read, the slave will be unable to replicate once it comes up. The command is safe to run while slaves are replicating - you do not need to stop them. You must first check all the slaves with SHOW SLAVE STATUS to see which log they are on, then do a listing of the logs on the master with SHOW MASTER LOGS, find the earliest log among all the slaves (if all the slaves are up to date, this will be the last log on the list), backup all the logs you are about to delete (optional) and purge up to the target log.

4.10.7 Replication FAQ

Q: Why do I sometimes see more than one Binlog_Dump thread on the master after I have restarted the slave?

A: Binlog_Dump is a continuous process that is handled by the server in the following way:

So if the slave thread stops on the slave, the corresponding Binlog_Dump thread on the master will not notice it until after at least one update to the master (or a kill), which is needed to wake it up from pthread_cond_wait(). In the meantime, the slave could have opened another connection, which resulted in another Binlog_Dump thread.

The above problem should not be present in Version 3.23.26 and later versions. In Version 3.23.26 we added server-id to each replication server, and now all the old zombie threads are killed on the master when a new replication thread connects from the same slave

Q: How do I rotate replication logs?

A: In Version 3.23.28 you should use PURGE MASTER LOGS TO command after determining which logs can be deleted, and optionally backing them up first. In earlier versions the process is much more painful, and cannot be safely done without stopping all the slaves in the case that you plan to re-use log names. You will need to stop the slave threads, edit the binary log index file, delete all the old logs, restart the master, start slave threads, and then remove the old log files.

Q: How do I upgrade on a hot replication setup?

A: If you are upgrading pre-3.23.26 versions, you should just lock the master tables, let the slave catch up, then run FLUSH MASTER on the master, and FLUSH SLAVE on the slave to reset the logs, then restart new versions of the master and the slave. Note that the slave can stay down for some time - since the master is logging all the updates, the slave will be able to catch up once it is up and can connect.

After 3.23.26, we have locked the replication protocol for modifications, so you can upgrade masters and slave on the fly to a newer 3.23 version and you can have different versions of MySQL running on the slave and the master, as long as they are both newer than 3.23.26.

Q: What issues should I be aware of when setting up two-way replication?

A: MySQL replication currently does not support any locking protocol between master and slave to guarantee the atomicity of a distributed (cross-server) update. In in other words, it is possible for client A to make an update to co-master 1, and in the meantime, before it propagates to co-master 2, client B could make an update to co-master 2 that will make the update of client A work differently than it did on co-master 1. Thus when the update of client A will make it to co-master 2, it will produce tables that will be different than what you have on co-master 1, even after all the updates from co-master 2 have also propagated. So you should not co-chain two servers in a two-way replication relationship, unless you are sure that you updates can safely happen in any order, or unless you take care of mis-ordered updates somehow in the client code.

You must also realize that two-way replication actually does not improve performance very much, if at all, as far as updates are concerned. Both servers need to do the same amount of updates each, as you would have one server do. The only difference is that there will be a little less lock contention, because the updates originating on another server will be serialized in one slave thread. This benefit, though, might be offset by network delays.

Q: How can I use replication to improve performance of my system?

A: You should set up one server as the master, and direct all writes to it, and configure as many slaves as you have the money and rackspace for, distributing the reads among the master and the slaves. You can also start the slaves with --skip-bdb, --low-priority-updates and --delay-key-write-for-all-tables to get speed improvements for the slave. In this case the slave will use non-transactional MyISAM tables instead of BDB tables to get more speed.

Q: What should I do to prepare my client code to use performance-enhancing replication?

A: If the part of your code that is responsible for database access has been properly abstracted/modularized, converting it to run with the replicated setup should be very smooth and easy - just change the implementation of your database access to read from some slave or the master, and to always write to the master. If your code does not have this level of abstraction, setting up a replicated system will give you an opportunity/motivation to it clean up. You should start by creating a wrapper library /module with the following functions:

safe_ means that the function will take care of handling all the error conditions.

You should then convert your client code to use the wrapper library. It may be a painful and scary process at first, but it will pay off in the long run. All applications that follow the above pattern will be able to take advantage of one-master/many slaves solution. The code will be a lot easier to maintain, and adding troubleshooting options will be trivial. You will just need to modify one or two functions, for example, to log how long each query took, or which query, among your many thousands, gave you an error. If you have written a lot of code already, you may want to automate the conversion task by using Monty's replace utility, which comes with the standard distribution of MySQL, or just write your own Perl script. Hopefully, your code follows some recognizable pattern. If not, then you are probably better off re-writing it anyway, or at least going through and manually beating it into a pattern.

Note that, of course, you can use different names for the functions. What is important is having unified interface for connecting for reads, connecting for writes, doing a read, and doing a write.

Q: When and how much can MySQL replication improve the performance of my system?

A: MySQL replication is most beneficial for a system with frequent reads and not so frequent writes. In theory, by using a one master/many slaves setup you can scale by adding more slaves until you either run out of network bandwidth, or your update load grows to the point that the master cannot handle it.

In order to determine how many slaves you can get before the added benefits begin to level out, and how much you can improve performance of your site, you need to know your query patterns, and empirically (by benchmarking) determine the relationship between the throughput on reads (reads per second, or max_reads) and on writes max_writes) on a typical master and a typical slave. The example below will show you a rather simplified calculation of what you can get with replication for our imagined system.

Let's say our system load consists of 10% writes and 90% reads, and we have determined that max_reads = 1200 - 2 * max_writes, or in other words, our system can do 1200 reads per second with no writes, our average write is twice as slow as average read, and the relationship is linear. Let us suppose that our master and slave are of the same capacity, and we have N slaves and 1 master. Then we have for each server (master or slave):

reads = 1200 - 2 * writes (from bencmarks)

reads = 9* writes / (N + 1) (reads split, but writes go to all servers)

9*writes/(N+1) + 2 * writes = 1200

writes = 1200/(2 + 9/(N+1)

So if N = 0, which means we have no replication, our system can handle 1200/11, about 109 writes per second (which means we will have 9 times as many reads due to the nature of our application).

If N = 1, we can get up to 184 writes per second.

If N = 8, we get up to 400.

If N = 17, 480 writes.

Eventually as N approaches infinity (and our budget negative infinity), we can get very close to 600 writes per second, increasing system throughput about 5.5 times. However, with only 8 servers, we increased it almost 4 times already.

Note that our computations assumed infinite network bandwidth, and neglected several other factors that could turn out to be significant on your system. In many cases, you may not be able to make a computation similar to the one above that will accurately predict what will happen on your system if you add N replication slaves. However, answering the following questions should help you decided whether and how much, if at all, the replication will improve the performance of your system:

Q: How can I use replication to provide redundancy/high availability?

A: With the currently available features, you would have to set up a master and a slave (or several slaves), and write a script that will monitor the master to see if it is up, and instruct your applications and the slaves of the master change in case of failure. Some suggestions:

We are currently working on integrating an automatic master election system into MySQL, but until it is ready, you will have to create your own monitoring tools.

4.10.8 Troubleshooting Replication

If you have followed the instructions, and your replication setup is not working, first eliminate the user error factor by checking the following:

When you have determined that there is no user error involved, and replication still either does not work at all or is unstable, it is time to start working on a bug report. We need to get as much info as possible from you to be able to track down the bug. Please do spend some time and effort preparing a good bug report. Ideally, we would like to have a test case in the format found in mysql-test/t/rpl* directory of the source tree. If you submit a test case like that, you can expect a patch within a day or two in most cases, although, of course, you mileage may vary depending on a number of factors.

Second best option is a just program with easily configurable connection arguments for the master and the slave that will demonstrate the problem on our systems. You can write one in Perl or in C, depending on which language you know better.

If you have one of the above ways to demonstrate the bug, use mysqlbug to prepare a bug report and send it to bugs@lists.mysql.com. If you have a phantom - a problem that does occur but you cannot duplicate "at will":

Once you have collected the evidence on the phantom problem, try hard to isolate it into a separate test case first. Then report the problem to bugs@lists.mysql.com with as much info as possible.

5 MySQL Optimization

Optimization is a complicated task because it ultimately requires understanding of the whole system. While it may be possible to do some local optimizations with small knowledge of your system or application, the more optimal you want your system to become the more you will have to know about it.

This chapter will try to explain and give some examples of different ways to optimize MySQL. Remember, however, that there are always some (increasingly harder) additional ways to make the system even faster.

5.1 Optimization Overview

The most important part for getting a system fast is of course the basic design. You also need to know what kinds of things your system will be doing, and what your bottlenecks are.

The most common bottlenecks are:

5.1.1 MySQL Design Limitations/Tradeoffs

Because MySQL uses extremely fast table locking (multiple readers / single writers) the biggest remaining problem is a mix of a steady stream of inserts and slow selects on the same table.

We believe that for a huge number of systems the extremely fast performance in other cases make this choice a win. This case is usually also possible to solve by having multiple copies of the table, but it takes more effort and hardware.

We are also working on some extensions to solve this problem for some common application niches.

5.1.2 Portability

Because all SQL servers implement different parts of SQL, it takes work to write portable SQL applications. For very simple selects/inserts it is very easy, but the more you need the harder it gets. If you want an application that is fast with many databases it becomes even harder!

To make a complex application portable you need to choose a number of SQL servers that it should work with.

You can use the MySQL crash-me program/web-page http://www.mysql.com/information/crash-me.php to find functions, types, and limits you can use with a selection of database servers. Crash-me now tests far from everything possible, but it is still comprehensive with about 450 things tested.

For example, you shouldn't have column names longer than 18 characters if you want to be able to use Informix or DB2.

Both the MySQL benchmarks and crash-me programs are very database-independent. By taking a look at how we have handled this, you can get a feeling for what you have to do to write your application database-independent. The benchmarks themselves can be found in the `sql-bench' directory in the MySQL source distribution. They are written in Perl with DBI database interface (which solves the access part of the problem).

See http://www.mysql.com/information/benchmarks.html for the results from this benchmark.

As you can see in these results, all databases have some weak points. That is, they have different design compromises that lead to different behavior.

If you strive for database independence, you need to get a good feeling for each SQL server's bottlenecks. MySQL is VERY fast in retrieving and updating things, but will have a problem in mixing slow readers/writers on the same table. Oracle, on the other hand, has a big problem when you try to access rows that you have recently updated (until they are flushed to disk). Transaction databases in general are not very good at generating summary tables from log tables, as in this case row locking is almost useless.

To get your application really database-independent, you need to define an easy extendable interface through which you manipulate your data. As C++ is available on most systems, it makes sense to use a C++ classes interface to the databases.

If you use some specific feature for some database (like the REPLACE command in MySQL), you should code a method for the other SQL servers to implement the same feature (but slower). With MySQL you can use the /*! */ syntax to add MySQL-specific keywords to a query. The code inside /**/ will be treated as a comment (ignored) by most other SQL servers.

If REAL high performance is more important than exactness, as in some Web applications, a possibility is to create an application layer that caches all results to give you even higher performance. By letting old results 'expire' after a while, you can keep the cache reasonably fresh. This is quite nice in case of extremely high load, in which case you can dynamically increase the cache and set the expire timeout higher until things get back to normal.

In this case the table creation information should contain information of the initial size of the cache and how often the table should normally be refreshed.

5.1.3 What Have We Used MySQL For?

During MySQL initial development, the features of MySQL were made to fit our largest customer. They handle data warehousing for a couple of the biggest retailers in Sweden.

From all stores, we get weekly summaries of all bonus card transactions, and we are expected to provide useful information for the store owners to help them find how their advertisement campaigns are affecting their customers.

The data is quite huge (about 7 million summary transactions per month), and we have data for 4-10 years that we need to present to the users. We got weekly requests from the customers that they want to get 'instant' access to new reports from this data.

We solved this by storing all information per month in compressed 'transaction' tables. We have a set of simple macros (script) that generates summary tables grouped by different criteria (product group, customer id, store ...) from the transaction tables. The reports are Web pages that are dynamically generated by a small Perl script that parses a Web page, executes the SQL statements in it, and inserts the results. We would have used PHP or mod_perl instead but they were not available at that time.

For graphical data we wrote a simple tool in C that can produce GIFs based on the result of a SQL query (with some processing of the result). This is also dynamically executed from the Perl script that parses the HTML files.

In most cases a new report can simply be done by copying an existing script and modifying the SQL query in it. In some cases, we will need to add more fields to an existing summary table or generate a new one, but this is also quite simple, as we keep all transactions tables on disk. (Currently we have at least 50G of transactions tables and 200G of other customer data.)

We also let our customers access the summary tables directly with ODBC so that the advanced users can themselves experiment with the data.

We haven't had any problems handling this with quite modest Sun Ultra SPARCstation (2x200 Mhz). We recently upgraded one of our servers to a 2 CPU 400 Mhz UltraSPARC, and we are now planning to start handling transactions on the product level, which would mean a ten-fold increase of data. We think we can keep up with this by just adding more disk to our systems.

We are also experimenting with Intel-Linux to be able to get more CPU power cheaper. Now that we have the binary portable database format (new in Version 3.23), we will start to use this for some parts of the application.

Our initial feelings are that Linux will perform much better on low-to-medium load and Solaris will perform better when you start to get a high load because of extreme disk IO, but we don't yet have anything conclusive about this. After some discussion with a Linux Kernel developer, this might be a side effect of Linux giving so much resources to the batch job that the interactive performance gets very low. This makes the machine feel very slow and unresponsive while big batches are going. Hopefully this will be better handled in future Linux Kernels.

5.1.4 The MySQL Benchmark Suite

This should contain a technical description of the MySQL benchmark suite (and crash-me), but that description is not written yet. Currently, you can get a good idea of the benchmark by looking at the code and results in the `sql-bench' directory in any MySQL source distributions.

This benchmark suite is meant to be a benchmark that will tell any user what things a given SQL implementation performs well or poorly at.

Note that this benchmark is single threaded, so it measures the minimum time for the operations. We plan to in the future add a lot of multi-threaded tests to the benchmark suite.

For example, (run on the same NT 4.0 machine):

Reading 2000000 rows by index
Seconds Seconds
mysql 367 249
mysql_odbc 464
db2_odbc 1206
informix_odbc 121126
ms-sql_odbc 1634
oracle_odbc 20800
solid_odbc 877
sybase_odbc 17614
Inserting (350768) rows
Seconds Seconds
mysql 381 206
mysql_odbc 619
db2_odbc 3460
informix_odbc 2692
ms-sql_odbc 4012
oracle_odbc 11291
solid_odbc 1801
sybase_odbc 4802

In the above test MySQL was run with a 8M index cache.

We have gather some more benchmark results at http://www.mysql.com/information/benchmarks.html.

Note that Oracle is not included because they asked to be removed. All Oracle benchmarks have to be passed by Oracle! We believe that makes Oracle benchmarks VERY biased because the above benchmarks are supposed to show what a standard installation can do for a single client.

To run the benchmark suite, you have to download a MySQL source distribution, install the perl DBI driver, the perl DBD driver for the database you want to test and then do: 

cd sql-bench
perl run-all-tests --server=#

where # is one of supported servers. You can get a list of all options and supported servers by doing run-all-tests --help.

crash-me tries to determine what features a database supports and what its capabilities and limitations are by actually running queries. For example, it determines:

We can find the result from crash-me on a lot of different databases at http://www.mysql.com/information/crash-me.php.

5.1.5 Using Your Own Benchmarks

You should definitely benchmark your application and database to find out where the bottlenecks are. By fixing it (or by replacing the bottleneck with a 'dummy module') you can then easily identify the next bottleneck (and so on). Even if the overall performance for your application is sufficient, you should at least make a plan for each bottleneck, and decide how to solve it if someday you really need the extra performance.

For an example of portable benchmark programs, look at the MySQL benchmark suite. See section 5.1.4 The MySQL Benchmark Suite. You can take any program from this suite and modify it for your needs. By doing this, you can try different solutions to your problem and test which is really the fastest solution for you.

It is very common that some problems only occur when the system is very heavily loaded. We have had many customers who contact us when they have a (tested) system in production and have encountered load problems. In every one of these cases so far, it has been problems with basic design (table scans are NOT good at high load) or OS/Library issues. Most of this would be a LOT easier to fix if the systems were not already in production.

To avoid problems like this, you should put some effort into benchmarking your whole application under the worst possible load! You can use Super Smack for this, and it is available at: http://www.mysql.com/Downloads/super-smack/super-smack-1.0.tar.gz. As the name suggests, it can bring your system down to its knees if you ask it, so make sure to use it only on your development systems.

5.2 Optimizing SELECTs and Other Queries

First, one thing that affects all queries: The more complex permission system setup you have, the more overhead you get.

If you do not have any GRANT statements done, MySQL will optimize the permission checking somewhat. So if you have a very high volume it may be worth the time to avoid grants. Otherwise more permission check results in a larger overhead.

If your problem is with some explicit MySQL function, you can always time this in the MySQL client: 

mysql> select benchmark(1000000,1+1);
+------------------------+
| benchmark(1000000,1+1) |
+------------------------+
|                      0 |
+------------------------+
1 row in set (0.32 sec)

The above shows that MySQL can execute 1,000,000 + expressions in 0.32 seconds on a PentiumII 400MHz.

All MySQL functions should be very optimized, but there may be some exceptions, and the benchmark(loop_count,expression) is a great tool to find out if this is a problem with your query.

5.2.1 EXPLAIN Syntax (Get Information About a SELECT)

    EXPLAIN tbl_name
or  EXPLAIN SELECT select_options

EXPLAIN tbl_name is a synonym for DESCRIBE tbl_name or SHOW COLUMNS FROM tbl_name.

When you precede a SELECT statement with the keyword EXPLAIN, MySQL explains how it would process the SELECT, providing information about how tables are joined and in which order.

With the help of EXPLAIN, you can see when you must add indexes to tables to get a faster SELECT that uses indexes to find the records. You can also see if the optimizer joins the tables in an optimal order. To force the optimizer to use a specific join order for a SELECT statement, add a STRAIGHT_JOIN clause.

For non-simple joins, EXPLAIN returns a row of information for each table used in the SELECT statement. The tables are listed in the order they would be read. MySQL resolves all joins using a single-sweep multi-join method. This means that MySQL reads a row from the first table, then finds a matching row in the second table, then in the third table and so on. When all tables are processed, it outputs the selected columns and backtracks through the table list until a table is found for which there are more matching rows. The next row is read from this table and the process continues with the next table.

Output from EXPLAIN includes the following columns:

table
The table to which the row of output refers.
type
The join type. Information about the various types is given below.
possible_keys
The possible_keys column indicates which indexes MySQL could use to find the rows in this table. Note that this column is totally independent of the order of the tables. That means that some of the keys in possible_keys may not be usable in practice with the generated table order. If this column is empty, there are no relevant indexes. In this case, you may be able to improve the performance of your query by examining the WHERE clause to see if it refers to some column or columns that would be suitable for indexing. If so, create an appropriate index and check the query with EXPLAIN again. See section 6.5.4 ALTER TABLE Syntax. To see what indexes a table has, use SHOW INDEX FROM tbl_name.
key
The key column indicates the key that MySQL actually decided to use. The key is NULL if no index was chosen. If MySQL chooses the wrong index, you can probably force MySQL to use another index by using myisamchk --analyze, See section 4.4.6.1 myisamchk Invocation Syntax, or by using USE INDEX/IGNORE INDEX. See section 6.4.1.1 JOIN Syntax.
key_len
The key_len column indicates the length of the key that MySQL decided to use. The length is NULL if the key is NULL. Note that this tells us how many parts of a multi-part key MySQL will actually use.
ref
The ref column shows which columns or constants are used with the key to select rows from the table.
rows
The rows column indicates the number of rows MySQL believes it must examine to execute the query.
Extra
This column contains additional information of how MySQL will resolve the query. Here is an explanation of the different text strings that can be found in this column:
Distinct
MySQL will not continue searching for more rows for the current row combination after it has found the first matching row.
Not exists
MySQL was able to do a LEFT JOIN optimization on the query and will not examine more rows in this table for the previous row combination after it finds one row that matches the LEFT JOIN criteria. Here is an example for this: 
SELECT * FROM t1 LEFT JOIN t2 ON t1.id=t2.id WHERE t2.id IS NULL;
Assume that t2.id is defined with NOT NULL. In this case MySQL will scan t1 and look up the rows in t2 through t1.id. If MySQL finds a matching row in t2, it knows that t2.id can never be NULL, and will not scan through the rest of the rows in t2 that has the same id. In other words, for each row in t1, MySQL only needs to do a single lookup in t2, independent of how many matching rows there are in t2.
range checked for each record (index map: #)
MySQL didn't find a real good index to use. It will, instead, for each row combination in the preceding tables, do a check on which index to use (if any), and use this index to retrieve the rows from the table. This isn't very fast but is faster than having to do a join without an index.
Using filesort
MySQL will need to do an extra pass to find out how to retrieve the rows in sorted order. The sort is done by going through all rows according to the join type and storing the sort key + pointer to the row for all rows that match the WHERE. Then the keys are sorted. Finally the rows are retrieved in sorted order.
Using index
The column information is retrieved from the table using only information in the index tree without having to do an additional seek to read the actual row. This can be done when all the used columns for the table are part of the same index.
Using temporary
To resolve the query MySQL will need to create a temporary table to hold the result. This typically happens if you do an ORDER BY on a different column set than you did a GROUP BY on.
Where used
A WHERE clause will be used to restrict which rows will be matched against the next table or sent to the client. If you don't have this information and the table is of type ALL or index, you may have something wrong in your query (if you don't intend to fetch/examine all rows from the table).
If you want to get your queries as fast as possible, you should look out for Using filesort and Using temporary.

The different join types are listed below, ordered from best to worst type:

system
The table has only one row (= system table). This is a special case of the const join type.
const
The table has at most one matching row, which will be read at the start of the query. Because there is only one row, values from the column in this row can be regarded as constants by the rest of the optimizer. const tables are very fast as they are read only once!
eq_ref
One row will be read from this table for each combination of rows from the previous tables. This is the best possible join type, other than the const types. It is used when all parts of an index are used by the join and the index is UNIQUE or a PRIMARY KEY.
ref
All rows with matching index values will be read from this table for each combination of rows from the previous tables. ref is used if the join uses only a leftmost prefix of the key, or if the key is not UNIQUE or a PRIMARY KEY (in other words, if the join cannot select a single row based on the key value). If the key that is used matches only a few rows, this join type is good.
range
Only rows that are in a given range will be retrieved, using an index to select the rows. The key column indicates which index is used. The key_len contains the longest key part that was used. The ref column will be NULL for this type.
index
This is the same as ALL, except that only the index tree is scanned. This is usually faster than ALL, as the index file is usually smaller than the data file.
ALL
A full table scan will be done for each combination of rows from the previous tables. This is normally not good if the table is the first table not marked const, and usually very bad in all other cases. You normally can avoid ALL by adding more indexes, so that the row can be retrieved based on constant values or column values from earlier tables.

You can get a good indication of how good a join is by multiplying all values in the rows column of the EXPLAIN output. This should tell you roughly how many rows MySQL must examine to execute the query. This number is also used when you restrict queries with the max_join_size variable. See section 5.5.2 Tuning Server Parameters.

The following example shows how a JOIN can be optimized progressively using the information provided by EXPLAIN.

Suppose you have the SELECT statement shown below, that you examine using EXPLAIN: 

EXPLAIN SELECT tt.TicketNumber, tt.TimeIn,
            tt.ProjectReference, tt.EstimatedShipDate,
            tt.ActualShipDate, tt.ClientID,
            tt.ServiceCodes, tt.RepetitiveID,
            tt.CurrentProcess, tt.CurrentDPPerson,
            tt.RecordVolume, tt.DPPrinted, et.COUNTRY,
            et_1.COUNTRY, do.CUSTNAME
        FROM tt, et, et AS et_1, do
        WHERE tt.SubmitTime IS NULL
            AND tt.ActualPC = et.EMPLOYID
            AND tt.AssignedPC = et_1.EMPLOYID
            AND tt.ClientID = do.CUSTNMBR;

For this example, assume that:

Initially, before any optimizations have been performed, the EXPLAIN statement produces the following information: 

table type possible_keys                key  key_len ref  rows  Extra
et    ALL  PRIMARY                      NULL NULL    NULL 74
do    ALL  PRIMARY                      NULL NULL    NULL 2135
et_1  ALL  PRIMARY                      NULL NULL    NULL 74
tt    ALL  AssignedPC,ClientID,ActualPC NULL NULL    NULL 3872
      range checked for each record (key map: 35)

Because type is ALL for each table, this output indicates that MySQL is doing a full join for all tables! This will take quite a long time, as the product of the number of rows in each table must be examined! For the case at hand, this is 74 * 2135 * 74 * 3872 = 45,268,558,720 rows. If the tables were bigger, you can only imagine how long it would take.

One problem here is that MySQL can't (yet) use indexes on columns efficiently if they are declared differently. In this context, VARCHAR and CHAR are the same unless they are declared as different lengths. Because tt.ActualPC is declared as CHAR(10) and et.EMPLOYID is declared as CHAR(15), there is a length mismatch.

To fix this disparity between column lengths, use ALTER TABLE to lengthen ActualPC from 10 characters to 15 characters: 

mysql> ALTER TABLE tt MODIFY ActualPC VARCHAR(15);

Now tt.ActualPC and et.EMPLOYID are both VARCHAR(15). Executing the EXPLAIN statement again produces this result: 

table type   possible_keys   key     key_len ref         rows    Extra
tt    ALL    AssignedPC,ClientID,ActualPC NULL NULL NULL 3872    where used
do    ALL    PRIMARY         NULL    NULL    NULL        2135
      range checked for each record (key map: 1)
et_1  ALL    PRIMARY         NULL    NULL    NULL        74
      range checked for each record (key map: 1)
et    eq_ref PRIMARY         PRIMARY 15      tt.ActualPC 1

This is not perfect, but is much better (the product of the rows values is now less by a factor of 74). This version is executed in a couple of seconds.

A second alteration can be made to eliminate the column length mismatches for the tt.AssignedPC = et_1.EMPLOYID and tt.ClientID = do.CUSTNMBR comparisons: 

mysql> ALTER TABLE tt MODIFY AssignedPC VARCHAR(15),
                      MODIFY ClientID   VARCHAR(15);

Now EXPLAIN produces the output shown below: 

table type   possible_keys   key     key_len ref            rows     Extra
et    ALL    PRIMARY         NULL    NULL    NULL           74
tt    ref    AssignedPC,ClientID,ActualPC ActualPC 15 et.EMPLOYID 52 where used
et_1  eq_ref PRIMARY         PRIMARY 15      tt.AssignedPC  1
do    eq_ref PRIMARY         PRIMARY 15      tt.ClientID    1

This is almost as good as it can get.

The remaining problem is that, by default, MySQL assumes that values in the tt.ActualPC column are evenly distributed, and that isn't the case for the tt table. Fortunately, it is easy to tell MySQL about this: 

shell> myisamchk --analyze PATH_TO_MYSQL_DATABASE/tt
shell> mysqladmin refresh

Now the join is perfect, and EXPLAIN produces this result: 

table type   possible_keys   key     key_len ref            rows    Extra
tt    ALL    AssignedPC,ClientID,ActualPC NULL NULL NULL    3872    where used
et    eq_ref PRIMARY         PRIMARY 15      tt.ActualPC    1
et_1  eq_ref PRIMARY         PRIMARY 15      tt.AssignedPC  1
do    eq_ref PRIMARY         PRIMARY 15      tt.ClientID    1

Note that the rows column in the output from EXPLAIN is an educated guess from the MySQL join optimizer. To optimize a query, you should check if the numbers are even close to the truth. If not, you may get better performance by using STRAIGHT_JOIN in your SELECT statement and trying to list the tables in a different order in the FROM clause.

5.2.2 Estimating Query Performance

In most cases you can estimate the performance by counting disk seeks. For small tables, you can usually find the row in 1 disk seek (as the index is probably cached). For bigger tables, you can estimate that (using B++ tree indexes) you will need: log(row_count) / log(index_block_length / 3 * 2 / (index_length + data_pointer_length)) + 1 seeks to find a row.

In MySQL an index block is usually 1024 bytes and the data pointer is usually 4 bytes. A 500,000 row table with an index length of 3 (medium integer) gives you: log(500,000)/log(1024/3*2/(3+4)) + 1 = 4 seeks.

As the above index would require about 500,000 * 7 * 3/2 = 5.2M, (assuming that the index buffers are filled to 2/3, which is typical) you will probably have much of the index in memory and you will probably only need 1-2 calls to read data from the OS to find the row.

For writes, however, you will need 4 seek requests (as above) to find where to place the new index and normally 2 seeks to update the index and write the row.

Note that the above doesn't mean that your application will slowly degenerate by N log N! As long as everything is cached by the OS or SQL server things will only go marginally slower while the table gets bigger. After the data gets too big to be cached, things will start to go much slower until your applications is only bound by disk-seeks (which increase by N log N). To avoid this, increase the index cache as the data grows. See section 5.5.2 Tuning Server Parameters.

5.2.3 Speed of SELECT Queries

In general, when you want to make a slow SELECT ... WHERE faster, the first thing to check is whether or not you can add an index. See section 5.4.3 How MySQL Uses Indexes. All references between different tables should usually be done with indexes. You can use the EXPLAIN command to determine which indexes are used for a SELECT. See section 5.2.1 EXPLAIN Syntax (Get Information About a SELECT).

Some general tips:

5.2.4 How MySQL Optimizes WHERE Clauses

The WHERE optimizations are put in the SELECT part here because they are mostly used with SELECT, but the same optimizations apply for WHERE in DELETE and UPDATE statements.

Also note that this section is incomplete. MySQL does many optimizations, and we have not had time to document them all.

Some of the optimizations performed by MySQL are listed below:

Some examples of queries that are very fast: 

mysql> SELECT COUNT(*) FROM tbl_name;
mysql> SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name;
mysql> SELECT MAX(key_part2) FROM tbl_name
           WHERE key_part_1=constant;
mysql> SELECT ... FROM tbl_name
           ORDER BY key_part1,key_part2,... LIMIT 10;
mysql> SELECT ... FROM tbl_name
           ORDER BY key_part1 DESC,key_part2 DESC,... LIMIT 10;

The following queries are resolved using only the index tree (assuming the indexed columns are numeric): 

mysql> SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val;
mysql> SELECT COUNT(*) FROM tbl_name
           WHERE key_part1=val1 AND key_part2=val2;
mysql> SELECT key_part2 FROM tbl_name GROUP BY key_part1;

The following queries use indexing to retrieve the rows in sorted order without a separate sorting pass: 

mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,... ;
mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,... ;

5.2.5 How MySQL Optimizes DISTINCT

DISTINCT is converted to a GROUP BY on all columns, DISTINCT combined with ORDER BY will in many cases also need a temporary table.

When combining LIMIT # with DISTINCT, MySQL will stop as soon as it finds # unique rows.

If you don't use columns from all used tables, MySQL will stop the scanning of the not used tables as soon as it has found the first match. 

SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a;

In the case, assuming t1 is used before t2 (check with EXPLAIN), then MySQL will stop reading from t2 (for that particular row in t1) when the first row in t2 is found.

5.2.6 How MySQL Optimizes LEFT JOIN and RIGHT JOIN

A LEFT JOIN B in MySQL is implemented as follows:

RIGHT JOIN is implemented analogously as LEFT JOIN.

The table read order forced by LEFT JOIN and STRAIGHT JOIN will help the join optimizer (which calculates in which order tables should be joined) to do its work much more quickly, as there are fewer table permutations to check.

Note that the above means that if you do a query of type: 

SELECT * FROM a,b LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key

MySQL will do a full scan on b as the LEFT JOIN will force it to be read before d.

The fix in this case is to change the query to: 

SELECT * FROM b,a LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key

5.2.7 How MySQL Optimizes LIMIT

In some cases MySQL will handle the query differently when you are using LIMIT # and not using HAVING:

5.2.8 Speed of INSERT Queries

The time to insert a record consists approximately of:

where the numbers are somewhat proportional to the overall time. This does not take into consideration the initial overhead to open tables (which is done once for each concurrently running query).

The size of the table slows down the insertion of indexes by N log N (B-trees).

Some ways to speed up inserts:

To get some more speed for both LOAD DATA INFILE and INSERT, enlarge the key buffer. See section 5.5.2 Tuning Server Parameters.

5.2.9 Speed of UPDATE Queries

Update queries are optimized as a SELECT query with the additional overhead of a write. The speed of the write is dependent on the size of the data that is being updated and the number of indexes that are updated. Indexes that are not changed will not be updated.

Also, another way to get fast updates is to delay updates and then do many updates in a row later. Doing many updates in a row is much quicker than doing one at a time if you lock the table.

Note that, with dynamic record format, updating a record to a longer total length may split the record. So if you do this often, it is very important to OPTIMIZE TABLE sometimes. See section 4.5.1 OPTIMIZE TABLE Syntax.

5.2.10 Speed of DELETE Queries

If you want to delete all rows in the table, you should use TRUNCATE TABLE table_name. See section 6.4.6 TRUNCATE Syntax.

The time to delete a record is exactly proportional to the number of indexes. To delete records more quickly, you can increase the size of the index cache. See section 5.5.2 Tuning Server Parameters.

5.2.11 Other Optimization Tips

Unsorted tips for faster systems:

5.3 Locking Issues

5.3.1 How MySQL Locks Tables

You can find a discussion about different locking methods in the appendix. See section G.4 Locking methods.

All locking in MySQL is deadlock-free. This is managed by always requesting all needed locks at once at the beginning of a query and always locking the tables in the same order.

The locking method MySQL uses for WRITE locks works as follows:

The locking method MySQL uses for READ locks works as follows:

When a lock is released, the lock is made available to the threads in the write lock queue, then to the threads in the read lock queue.

This means that if you have many updates on a table, SELECT statements will wait until there are no more updates.

To work around this for the case where you want to do many INSERT and SELECT operations on a table, you can insert rows in a temporary table and update the real table with the records from the temporary table once in a while.

This can be done with the following code: 

mysql> LOCK TABLES real_table WRITE, insert_table WRITE;
mysql> insert into real_table select * from insert_table;
mysql> TRUNCATE TABLE insert_table;
mysql> UNLOCK TABLES;

You can use the LOW_PRIORITY options with INSERT, UPDATE or DELETE or HIGH_PRIORITY with SELECT if you want to prioritize retrieval in some specific cases. You can also start mysqld with --low-priority-updates to get the same behaveour.

Using SQL_BUFFER_RESULT can also help making table locks shorter. See section 6.4.1 SELECT Syntax.

You could also change the locking code in `mysys/thr_lock.c' to use a single queue. In this case, write locks and read locks would have the same priority, which might help some applications.

5.3.2 Table Locking Issues

The table locking code in MySQL is deadlock free.

MySQL uses table locking (instead of row locking or column locking) on all table types, except BDB tables, to achieve a very high lock speed. For large tables, table locking is MUCH better than row locking for most applications, but there are, of course, some pitfalls.

For BDB and InnoDB tables, MySQL only uses table locking if you explicitely lock the table with LOCK TABLES or execute a command that will modify every row in the table, like ALTER TABLE. For these table types we recommend you to not use LOCK TABLES at all.

In MySQL Version 3.23.7 and above, you can insert rows into MyISAM tables at the same time other threads are reading from the table. Note that currently this only works if there are no holes after deleted rows in the table at the time the insert is made. When all holes has been filled with new data, concurrent inserts will automatically be enabled again.

Table locking enables many threads to read from a table at the same time, but if a thread wants to write to a table, it must first get exclusive access. During the update, all other threads that want to access this particular table will wait until the update is ready.

As updates on tables normally are considered to be more important than SELECT, all statements that update a table have higher priority than statements that retrieve information from a table. This should ensure that updates are not 'starved' because one issues a lot of heavy queries against a specific table. (You can change this by using LOW_PRIORITY with the statement that does the update or HIGH_PRIORITY with the SELECT statement.)

Starting from MySQL Version 3.23.7 one can use the max_write_lock_count variable to force MySQL to temporary give all SELECT statements, that wait for a table, a higher priority after a specific number of inserts on a table.

Table locking is, however, not very good under the following senario:

Some possible solutions to this problem are:

5.4 Optimizing Database Structure

5.4.1 Design Choices

MySQL keeps row data and index data in separate files. Many (almost all) other databases mix row and index data in the same file. We believe that the MySQL choice is better for a very wide range of modern systems.

Another way to store the row data is to keep the information for each column in a separate area (examples are SDBM and Focus). This will cause a performance hit for every query that accesses more than one column. Because this degenerates so quickly when more than one column is accessed, we believe that this model is not good for general purpose databases.

The more common case is that the index and data are stored together (like in Oracle/Sybase et al). In this case you will find the row information at the leaf page of the index. The good thing with this layout is that it, in many cases, depending on how well the index is cached, saves a disk read. The bad things with this layout are:

5.4.2 Get Your Data as Small as Possible

One of the most basic optimization is to get your data (and indexes) to take as little space on the disk (and in memory) as possible. This can give huge improvements because disk reads are faster and normally less main memory will be used. Indexing also takes less resources if done on smaller columns.

MySQL supports a lot of different table types and row formats. Choosing the right table format may give you a big performance gain. See section 7 MySQL Table Types.

You can get better performance on a table and minimize storage space using the techniques listed below:

5.4.3 How MySQL Uses Indexes

Indexes are used to find rows with a specific value of one column fast. Without an index MySQL has to start with the first record and then read through the whole table until it finds the relevant rows. The bigger the table, the more this costs. If the table has an index for the columns in question, MySQL can quickly get a position to seek to in the middle of the data file without having to look at all the data. If a table has 1000 rows, this is at least 100 times faster than reading sequentially. Note that if you need to access almost all 1000 rows it is faster to read sequentially because we then avoid disk seeks.

All MySQL indexes (PRIMARY, UNIQUE, and INDEX) are stored in B-trees. Strings are automatically prefix- and end-space compressed. See section 6.5.7 CREATE INDEX Syntax.

Indexes are used to:

Suppose you issue the following SELECT statement: 

mysql> SELECT * FROM tbl_name WHERE col1=val1 AND col2=val2;

If a multiple-column index exists on col1 and col2, the appropriate rows can be fetched directly. If separate single-column indexes exist on col1 and col2, the optimizer tries to find the most restrictive index by deciding which index will find fewer rows and using that index to fetch the rows.

If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimizer to find rows. For example, if you have a three-column index on (col1,col2,col3), you have indexed search capabilities on (col1), (col1,col2), and (col1,col2,col3).

MySQL can't use a partial index if the columns don't form a leftmost prefix of the index. Suppose you have the SELECT statements shown below: 

mysql> SELECT * FROM tbl_name WHERE col1=val1;
mysql> SELECT * FROM tbl_name WHERE col2=val2;
mysql> SELECT * FROM tbl_name WHERE col2=val2 AND col3=val3;

If an index exists on (col1,col2,col3), only the first query shown above uses the index. The second and third queries do involve indexed columns, but (col2) and (col2,col3) are not leftmost prefixes of (col1,col2,col3).

MySQL also uses indexes for LIKE comparisons if the argument to LIKE is a constant string that doesn't start with a wild-card character. For example, the following SELECT statements use indexes: 

mysql> select * from tbl_name where key_col LIKE "Patrick%";
mysql> select * from tbl_name where key_col LIKE "Pat%_ck%";

In the first statement, only rows with "Patrick" <= key_col < "Patricl" are considered. In the second statement, only rows with "Pat" <= key_col < "Pau" are considered.

The following SELECT statements will not use indexes: 

mysql> select * from tbl_name where key_col LIKE "%Patrick%";
mysql> select * from tbl_name where key_col LIKE other_col;

In the first statement, the LIKE value begins with a wild-card character. In the second statement, the LIKE value is not a constant.

Searching using column_name IS NULL will use indexes if column_name is an index.

MySQL normally uses the index that finds the least number of rows. An index is used for columns that you compare with the following operators: =, >, >=, <, <=, BETWEEN, and a LIKE with a non-wild-card prefix like 'something%'.

Any index that doesn't span all AND levels in the WHERE clause is not used to optimize the query. In other words: To be able to use an index, a prefix of the index must be used in every AND group.

The following WHERE clauses use indexes: 

... WHERE index_part1=1 AND index_part2=2 AND other_column=3
... WHERE index=1 OR A=10 AND index=2      /* index = 1 OR index = 2 */
... WHERE index_part1='hello' AND index_part_3=5
          /* optimized like "index_part1='hello'" */
... WHERE index1=1 and index2=2 or index1=3 and index3=3;
          /* Can use index on index1 but not on index2 or index 3 */

These WHERE clauses do NOT use indexes: 

... WHERE index_part2=1 AND index_part3=2  /* index_part_1 is not used */
... WHERE index=1 OR A=10                  /* Index is not used in both AND parts */
... WHERE index_part1=1 OR index_part2=10  /* No index spans all rows */

Note that in some cases MySQL will not use an index, even if one would be available. Some of the cases where this happens are:

5.4.4 Column Indexes

All MySQL column types can be indexed. Use of indexes on the relevant columns is the best way to improve the performance of SELECT operations.

The maximum number of keys and the maximum index length is defined per table handler. See section 7 MySQL Table Types. You can with all table handlers have at least 16 keys and a total index length of at least 256 bytes.

For CHAR and VARCHAR columns, you can index a prefix of a column. This is much faster and requires less disk space than indexing the whole column. The syntax to use in the CREATE TABLE statement to index a column prefix looks like this: 

KEY index_name (col_name(length))

The example below creates an index for the first 10 characters of the name column: 

mysql> CREATE TABLE test (
           name CHAR(200) NOT NULL,
           KEY index_name (name(10)));

For BLOB and TEXT columns, you must index a prefix of the column. You cannot index the entire column.

In MySQL Version 3.23.23 or later, you can also create special FULLTEXT indexes. They are used for full-text search. Only the MyISAM table type supports FULLTEXT indexes. They can be created only from VARCHAR and TEXT columns. Indexing always happens over the entire column and partial indexing is not supported. See section 6.8 MySQL Full-text Search for details.

5.4.5 Multiple-Column Indexes

MySQL can create indexes on multiple columns. An index may consist of up to 15 columns. (On CHAR and VARCHAR columns you can also use a prefix of the column as a part of an index).

A multiple-column index can be considered a sorted array containing values that are created by concatenating the values of the indexed columns.

MySQL uses multiple-column indexes in such a way that queries are fast when you specify a known quantity for the first column of the index in a WHERE clause, even if you don't specify values for the other columns.

Suppose a table is created using the following specification: 

mysql> CREATE TABLE test (
           id INT NOT NULL,
           last_name CHAR(30) NOT NULL,
           first_name CHAR(30) NOT NULL,
           PRIMARY KEY (id),
           INDEX name (last_name,first_name));

Then the index name is an index over last_name and first_name. The index will be used for queries that specify values in a known range for last_name, or for both last_name and first_name. Therefore, the name index will be used in the following queries: 

mysql> SELECT * FROM test WHERE last_name="Widenius";

mysql> SELECT * FROM test WHERE last_name="Widenius"
                          AND first_name="Michael";

mysql> SELECT * FROM test WHERE last_name="Widenius"
                          AND (first_name="Michael" OR first_name="Monty");

mysql> SELECT * FROM test WHERE last_name="Widenius"
                          AND first_name >="M" AND first_name < "N";

However, the name index will NOT be used in the following queries: 

mysql> SELECT * FROM test WHERE first_name="Michael";

mysql> SELECT * FROM test WHERE last_name="Widenius"
                          OR first_name="Michael";

For more information on the manner in which MySQL uses indexes to improve query performance, see section 5.4.3 How MySQL Uses Indexes.

5.4.6 How MySQL Opens and Closes Tables

table_cache, max_connections, and max_tmp_tables affect the maximum number of files the server keeps open. If you increase one or both of these values, you may run up against a limit imposed by your operating system on the per-process number of open file descriptors. However, you can increase the limit on many systems. Consult your OS documentation to find out how to do this, because the method for changing the limit varies widely from system to system.

table_cache is related to max_connections. For example, for 200 concurrent running connections, you should have a table cache of at least 200 * n, where n is the maximum number of tables in a join. You also need to reserve some extra file descriptors for temporary tables and files.

The cache of open tables can grow to a maximum of table_cache (default 64; this can be changed with the -O table_cache=# option to mysqld). A table is never closed, except when the cache is full and another thread tries to open a table or if you use mysqladmin refresh or mysqladmin flush-tables.

When the table cache fills up, the server uses the following procedure to locate a cache entry to use:

A table is opened for each concurrent access. This means that if you have two threads accessing the same table or access the table twice in the same query (with AS) the table needs to be opened twice. The first open of any table takes two file descriptors; each additional use of the table takes only one file descriptor. The extra descriptor for the first open is used for the index file; this descriptor is shared among all threads.

You can check if your table cache is too small by checking the mysqld variable opened_tables. If this is quite big, even if you haven't done a lot of FLUSH TABLES, you should increase your table cache. See section 4.5.5.3 SHOW STATUS.

5.4.7 Drawbacks to Creating Large Numbers of Tables in the Same Database

If you have many files in a directory, open, close, and create operations will be slow. If you execute SELECT statements on many different tables, there will be a little overhead when the table cache is full, because for every table that has to be opened, another must be closed. You can reduce this overhead by making the table cache larger.

5.4.8 Why So Many Open tables?

When you run mysqladmin status, you'll see something like this: 

Uptime: 426 Running threads: 1 Questions: 11082 Reloads: 1 Open tables: 12

This can be somewhat perplexing if you only have 6 tables.

MySQL is multithreaded, so it may have many queries on the same table simultaneously. To minimize the problem with two threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory and one extra file descriptor for the data file. The index file descriptor is shared between all threads.

5.5 Optimizing the MySQL Server

5.5.1 System/Compile Time and Startup Parameter Tuning

We start with the system level things since some of these decisions have to be made very early. In other cases a fast look at this part may suffice because it not that important for the big gains. However, it is always nice to have a feeling about how much one could gain by changing things at this level.

The default OS to use is really important! To get the most use of multiple CPU machines one should use Solaris (because the threads works really nice) or Linux (because the 2.2 kernel has really good SMP support). Also on 32-bit machines Linux has a 2G file size limit by default. Hopefully this will be fixed soon when new filesystems are released (XFS/Reiserfs). If you have a desperate need for files bigger than 2G on Linux-intel 32 bit, you should get the LFS patch for the ext2 file system.

Because we have not run MySQL in production on that many platforms, we advice you to test your intended platform before choosing it, if possible.

Other tips:

5.5.2 Tuning Server Parameters

You can get the default buffer sizes used by the mysqld server with this command: 

shell> mysqld --help

This command produces a list of all mysqld options and configurable variables. The output includes the default values and looks something like this: 

Possible variables for option --set-variable (-O) are:
back_log              current value: 5
bdb_cache_size        current value: 1048540
binlog_cache_size     current_value: 32768
connect_timeout       current value: 5
delayed_insert_timeout  current value: 300
delayed_insert_limit  current value: 100
delayed_queue_size    current value: 1000
flush_time            current value: 0
interactive_timeout   current value: 28800
join_buffer_size      current value: 131072
key_buffer_size       current value: 1048540
lower_case_table_names  current value: 0
long_query_time       current value: 10
max_allowed_packet    current value: 1048576
max_binlog_cache_size current_value: 4294967295
max_connections       current value: 100
max_connect_errors    current value: 10
max_delayed_threads   current value: 20
max_heap_table_size   current value: 16777216
max_join_size         current value: 4294967295
max_sort_length       current value: 1024
max_tmp_tables        current value: 32
max_write_lock_count  current value: 4294967295
myisam_sort_buffer_size  current value: 8388608
net_buffer_length     current value: 16384
net_retry_count       current value: 10
net_read_timeout      current value: 30
net_write_timeout     current value: 60
query_buffer_size     current value: 0
record_buffer         current value: 131072
record_rnd_buffer     current value: 131072
slow_launch_time      current value: 2
sort_buffer           current value: 2097116
table_cache           current value: 64
thread_concurrency    current value: 10
tmp_table_size        current value: 1048576
thread_stack          current value: 131072
wait_timeout          current value: 28800

If there is a mysqld server currently running, you can see what values it actually is using for the variables by executing this command: 

shell> mysqladmin variables

You can find a full description for all variables in the SHOW VARIABLES section in this manual. See section 4.5.5.4 SHOW VARIABLES.

You can also see some statistics from a running server by issuing the command SHOW STATUS. See section 4.5.5.3 SHOW STATUS.

MySQL uses algorithms that are very scalable, so you can usually run with very little memory. If you, however, give MySQL more memory, you will normally also get better performance.

When tuning a MySQL server, the two most important variables to use are key_buffer_size and table_cache. You should first feel confident that you have these right before trying to change any of the other variables.

If you have much memory (>=256M) and many tables and want maximum performance with a moderate number of clients, you should use something like this: 

shell> safe_mysqld -O key_buffer=64M -O table_cache=256 \
           -O sort_buffer=4M -O record_buffer=1M &

If you have only 128M and only a few tables, but you still do a lot of sorting, you can use something like: 

shell> safe_mysqld -O key_buffer=16M -O sort_buffer=1M

If you have little memory and lots of connections, use something like this: 

shell> safe_mysqld -O key_buffer=512k -O sort_buffer=100k \
           -O record_buffer=100k &

or even: 

shell> safe_mysqld -O key_buffer=512k -O sort_buffer=16k \
           -O table_cache=32 -O record_buffer=8k -O net_buffer=1K &

If you are doing a GROUP BY or ORDER BY on files that are much bigger than your available memory you should increase the value of record_rnd_buffer to speed up the reading of rows after the sorting is done.

When you have installed MySQL, the `support-files' directory will contain some different my.cnf example files, `my-huge.cnf', `my-large.cnf', `my-medium.cnf', and `my-small.cnf', you can use as a base to optimize your system.

If there are very many connections, ``swapping problems'' may occur unless mysqld has been configured to use very little memory for each connection. mysqld performs better if you have enough memory for all connections, of course.

Note that if you change an option to mysqld, it remains in effect only for that instance of the server.

To see the effects of a parameter change, do something like this: 

shell> mysqld -O key_buffer=32m --help

Make sure that the --help option is last; otherwise, the effect of any options listed after it on the command line will not be reflected in the output.

5.5.3 How Compiling and Linking Affects the Speed of MySQL

Most of the following tests are done on Linux with the MySQL benchmarks, but they should give some indication for other operating systems and workloads.

You get the fastest executable when you link with -static.

On Linux, you will get the fastest code when compiling with pgcc and -O3. To compile `sql_yacc.cc' with these options, you need about 200M memory because gcc/pgcc needs a lot of memory to make all functions inline. You should also set CXX=gcc when configuring MySQL to avoid inclusion of the libstdc++ library (it is not needed). Note that with some versions of pgcc, the resulting code will only run on true Pentium processors, even if you use the compiler option that you want the resulting code to be working on all x586 type processors (like AMD).

By just using a better compiler and/or better compiler options you can get a 10-30 % speed increase in your application. This is particularly important if you compile the SQL server yourself!

We have tested both the Cygnus CodeFusion and Fujitsu compilers, but when we tested them, neither was sufficiently bug free to allow MySQL to be compiled with optimizations on.

When you compile MySQL you should only include support for the character sets that you are going to use. (Option --with-charset=xxx). The standard MySQL binary distributions are compiled with support for all character sets.

Here is a list of some measurements that we have done:

The MySQL-Linux distribution provided by MySQL AB used to be compiled with pgcc, but we had to go back to regular gcc because of a bug in pgcc that would generate the code that does not run on AMD. We will continue using gcc until that bug is resolved. In the meantime, if you have a non-AMD machine, you can get a faster binary by compiling with pgcc. The standard MySQL Linux binary is linked statically to get it faster and more portable.

5.5.4 How MySQL Uses Memory

The list below indicates some of the ways that the mysqld server uses memory. Where applicable, the name of the server variable relevant to the memory use is given:

ps and other system status programs may report that mysqld uses a lot of memory. This may be caused by thread-stacks on different memory addresses. For example, the Solaris version of ps counts the unused memory between stacks as used memory. You can verify this by checking available swap with swap -s. We have tested mysqld with commercial memory-leakage detectors, so there should be no memory leaks.

5.5.5 How MySQL uses DNS

When a new thread connects to mysqld, mysqld will span a new thread to handle the request. This thread will first check if the hostname is in the hostname cache. If not the thread will call gethostbyaddr_r() and gethostbyname_r() to resolve the hostname.

If the operating system doesn't support the above thread-safe calls, the thread will lock a mutex and call gethostbyaddr() and gethostbyname() instead. Note that in this case no other thread can resolve other hostnames that is not in the hostname cache until the first thread is ready.

You can disable DNS host lookup by starting mysqld with --skip-name-resolve. In this case you can however only use IP names in the MySQL privilege tables.

If you have a very slow DNS and many hosts, you can get more performance by either disabling DNS lookop with --skip-name-resolve or by increasing the HOST_CACHE_SIZE define (default: 128) and recompile mysqld.

You can disable the hostname cache with --skip-host-cache. You can clear the hostname cache with FLUSH HOSTS or mysqladmin flush-hosts.

If you don't want to allow connections over TCP/IP, you can do this by starting mysqld with --skip-networking.

5.5.6 SET Syntax

SET [OPTION] SQL_VALUE_OPTION= value, ...

SET OPTION sets various options that affect the operation of the server or your client. Any option you set remains in effect until the current session ends, or until you set the option to a different value.

CHARACTER SET character_set_name | DEFAULT
This maps all strings from and to the client with the given mapping. Currently the only option for character_set_name is cp1251_koi8, but you can easily add new mappings by editing the `sql/convert.cc' file in the MySQL source distribution. The default mapping can be restored by using a character_set_name value of DEFAULT. Note that the syntax for setting the CHARACTER SET option differs from the syntax for setting the other options.
PASSWORD = PASSWORD('some password')
Set the password for the current user. Any non-anonymous user can change his own password!
PASSWORD FOR user = PASSWORD('some password')
Set the password for a specific user on the current server host. Only a user with access to the mysql database can do this. The user should be given in user@hostname format, where user and hostname are exactly as they are listed in the User and Host columns of the mysql.user table entry. For example, if you had an entry with User and Host fields of 'bob' and '%.loc.gov', you would write: 
mysql> SET PASSWORD FOR bob@"%.loc.gov" = PASSWORD("newpass");

or

mysql> UPDATE mysql.user SET password=PASSWORD("newpass") where user="bob' and host="%.loc.gov";
SQL_AUTO_IS_NULL = 0 | 1
If set to 1 (default) then one can find the last inserted row for a table with an auto_increment row with the following construct: WHERE auto_increment_column IS NULL. This is used by some ODBC programs like Access.
AUTOCOMMIT= 0 | 1
If set to 1 all changes to a table will be done at once. To start a multi-command transaction, you have to use the BEGIN statement. See section 6.7.1 BEGIN/COMMIT/ROLLBACK Syntax. If set to 0 you have to use COMMIT / ROLLBACK to accept/revoke that transaction. See section 6.7.1 BEGIN/COMMIT/ROLLBACK Syntax. Note that when you change from not AUTOCOMMIT mode to AUTOCOMMIT mode, MySQL will do an automatic COMMIT on any open transactions.
SQL_BIG_TABLES = 0 | 1
If set to 1, all temporary tables are stored on disk rather than in memory. This will be a little slower, but you will not get the error The table tbl_name is full for big SELECT operations that require a large temporary table. The default value for a new connection is 0 (that is, use in-memory temporary tables).
SQL_BIG_SELECTS = 0 | 1
If set to 0, MySQL will abort if a SELECT is attempted that probably will take a very long time. This is useful when an inadvisable WHERE statement has been issued. A big query is defined as a SELECT that probably will have to examine more than max_join_size rows. The default value for a new connection is 1 (which will allow all SELECT statements).
SQL_BUFFER_RESULT = 0 | 1
SQL_BUFFER_RESULT will force the result from SELECT's to be put into a temporary table. This will help MySQL free the table locks early and will help in cases where it takes a long time to send the result set to the client.
SQL_LOW_PRIORITY_UPDATES = 0 | 1
If set to 1, all INSERT, UPDATE, DELETE, and and LOCK TABLE WRITE statements wait until there is no pending SELECT or LOCK TABLE READ on the affected table.
SQL_MAX_JOIN_SIZE = value | DEFAULT
Don't allow SELECTs that will probably need to examine more than value row combinations. By setting this value, you can catch SELECTs where keys are not used properly and that would probably take a long time. Setting this to a value other than DEFAULT will reset the SQL_BIG_SELECTS flag. If you set the SQL_BIG_SELECTS flag again, the SQL_MAX_JOIN_SIZE variable will be ignored. You can set a default value for this variable by starting mysqld with -O max_join_size=#.
SQL_SAFE_UPDATES = 0 | 1
If set to 1, MySQL will abort if an UPDATE or DELETE is attempted that doesn't use a key or LIMIT in the WHERE clause. This makes it possible to catch wrong updates when creating SQL commands by hand.
SQL_SELECT_LIMIT = value | DEFAULT
The maximum number of records to return from SELECT statements. If a SELECT has a LIMIT clause, the LIMIT takes precedence over the value of SQL_SELECT_LIMIT. The default value for a new connection is ``unlimited.'' If you have changed the limit, the default value can be restored by using a SQL_SELECT_LIMIT value of DEFAULT.
SQL_LOG_OFF = 0 | 1
If set to 1, no logging will be done to the standard log for this client, if the client has the process privilege. This does not affect the update log!
SQL_LOG_UPDATE = 0 | 1
If set to 0, no logging will be done to the update log for the client, if the client has the process privilege. This does not affect the standard log!
SQL_QUOTE_SHOW_CREATE = 0 | 1
If set to 1, SHOW CREATE TABLE will quote table and column names. This is on by default, for replication of tables with fancy column names to work. section 4.5.5.8 SHOW CREATE TABLE.
TIMESTAMP = timestamp_value | DEFAULT
Set the time for this client. This is used to get the original timestamp if you use the update log to restore rows. timestamp_value should be a UNIX Epoch timestamp, not a MySQL timestamp.
LAST_INSERT_ID = #
Set the value to be returned from LAST_INSERT_ID(). This is stored in the update log when you use LAST_INSERT_ID() in a command that updates a table.
INSERT_ID = #
Set the value to be used by the following INSERT or ALTER TABLE command when inserting an AUTO_INCREMENT value. This is mainly used with the update log.

5.6 Disk Issues

5.6.1 Using Symbolic Links

You can move tables and databases from the database directory to other locations and replace them with symbolic links to the new locations. You might want to do this, for example, to move a database to a file system with more free space or increase the speed of your system by spreading your tables to different disk.

The recommended may to do this, is to just symlink databases to different disk and only symlink tables as a last resort.

5.6.1.1 Using Symbolic Links for Databases

The way to symlink a database is to first create a directory on some disk where you have free space and then create a symlink to it from the MySQL database directory. 

shell> mkdir /dr1/databases/test
shell> ln -s /dr1/databases/test mysqld-datadir

MySQL doesn't support that you link one directory to multiple databases. Replacing a database directory with a symbolic link will work fine as long as you don't make a symbolic link between databases. Suppose you have a database db1 under the MySQL data directory, and then make a symlink db2 that points to db1: 

shell> cd /path/to/datadir
shell> ln -s db1 db2

Now, for any table tbl_a in db1, there also appears to be a table tbl_a in db2. If one thread updates db1.tbl_a and another thread updates db2.tbl_a, there will be problems.

If you really need this, you must change the following code in `mysys/mf_format.c': 

if (flag & 32 || (!lstat(to,&stat_buff) && S_ISLNK(stat_buff.st_mode)))

to 

if (1)

On Windows you can use internal symbolic links to directories by compiling MySQL with -DUSE_SYMDIR. This allows you to put different databases on different disks. See section 2.6.2.5 Splitting Data Across Different Disks on Windows.

5.6.1.2 Using Symbolic Links for Tables

Before MySQL 4.0 you should not symlink tables, if you are not very carefully with them. The problem is that if you run ALTER TABLE, REPAIR TABLE or OPTIMIZE TABLE on a symlinked table, the symlinks will be removed and replaced by the original files. This happens because the above command works by creating a temporary file in the database directory and when the command is complete, replace the original file with the temporary file.

You should not symlink tables on system that doesn't have a fully working realpath() call. (At least Linux and Solaris support realpath())

In MySQL 4.0 symlinks is only fully supported for MyISAM tables. For other table types you will probably get strange problems when doing any of the above mentioned commands.

The handling of symbolic links in MySQL 4.0 works the following way (this is mostly relevant only for MyISAM tables).

Things that are not yet supported:

6 MySQL Language Reference

MySQL has a very complex, but intuitive and easy to learn SQL interface. This chapter describes the various commands, types, and functions you will need to know in order to use MySQL efficiently and effectively. This chapter also serves as a reference to all functionality included in MySQL. In order to use this chapter effectively, you may find it useful to refer to the various indexes.

6.1 Language Structure

6.1.1 Literals: How to Write Strings and Numbers

This section describes the various ways to write strings and numbers in MySQL. It also covers the various nuances and ``gotchas'' that you may run into when dealing with these basic types in MySQL.

6.1.1.1 Strings

A string is a sequence of characters, surrounded by either single quote (`'') or double quote (`"') characters (only the single quote if you run in ANSI mode). Examples: 

'a string'
"another string"

Within a string, certain sequences have special meaning. Each of these sequences begins with a backslash (`\'), known as the escape character. MySQL recognizes the following escape sequences:

\0
An ASCII 0 (NUL) character.
\'
A single quote (`'') character.
\"
A double quote (`"') character.
\b
A backspace character.
\n
A newline character.
\r
A carriage return character.
\t
A tab character.
\z
ASCII(26) (Control-Z). This character can be encoded to allow you to go around the problem that ASCII(26) stands for END-OF-FILE on Windows. (ASCII(26) will cause problems if you try to use mysql database < filename).
\\
A backslash (`\') character.
\%
A `%' character. This is used to search for literal instances of `%' in contexts where `%' would otherwise be interpreted as a wild-card character. See section 6.3.2.1 String Comparison Functions.
\_
A `_' character. This is used to search for literal instances of `_' in contexts where `_' would otherwise be interpreted as a wild-card character. See section 6.3.2.1 String Comparison Functions.

Note that if you use `\%' or `\_' in some string contexts, these will return the strings `\%' and `\_' and not `%' and `_'.

There are several ways to include quotes within a string:

The SELECT statements shown below demonstrate how quoting and escaping work: 

mysql> SELECT 'hello', '"hello"', '""hello""', 'hel''lo', '\'hello';
+-------+---------+-----------+--------+--------+
| hello | "hello" | ""hello"" | hel'lo | 'hello |
+-------+---------+-----------+--------+--------+

mysql> SELECT "hello", "'hello'", "''hello''", "hel""lo", "\"hello";
+-------+---------+-----------+--------+--------+
| hello | 'hello' | ''hello'' | hel"lo | "hello |
+-------+---------+-----------+--------+--------+

mysql> SELECT "This\nIs\nFour\nlines";
+--------------------+
| This
Is
Four
lines |
+--------------------+

If you want to insert binary data into a BLOB column, the following characters must be represented by escape sequences:

NUL
ASCII 0. You should represent this by `\0' (a backslash and an ASCII `0' character).
\
ASCII 92, backslash. Represent this by `\\'.
'
ASCII 39, single quote. Represent this by `\''.
"
ASCII 34, double quote. Represent this by `\"'.

If you write C code, you can use the C API function mysql_escape_string() to escape characters for the INSERT statement. See section 8.4.2 C API Function Overview. In Perl, you can use the quote method of the DBI package to convert special characters to the proper escape sequences. See section 8.2.2 The DBI Interface.

You should use an escape function on any string that might contain any of the special characters listed above!

6.1.1.2 Numbers

Integers are represented as a sequence of digits. Floats use `.' as a decimal separator. Either type of number may be preceded by `-' to indicate a negative value.

Examples of valid integers: 

1221
0
-32

Examples of valid floating-point numbers: 

294.42
-32032.6809e+10
148.00

An integer may be used in a floating-point context; it is interpreted as the equivalent floating-point number.

6.1.1.3 Hexadecimal Values

MySQL supports hexadecimal values. In number context these act like an integer (64-bit precision). In string context these act like a binary string where each pair of hex digits is converted to a character: 

mysql> SELECT 0xa+0;
       -> 10
mysql> select 0x5061756c;
       -> Paul

Hexadecimal strings are often used by ODBC to give values for BLOB columns.

6.1.1.4 NULL Values

The NULL value means ``no data'' and is different from values such as 0 for numeric types or the empty string for string types. See section A.5.3 Problems with NULL Values.

NULL may be represented by \N when using the text file import or export formats (LOAD DATA INFILE, SELECT ... INTO OUTFILE). See section 6.4.8 LOAD DATA INFILE Syntax.

6.1.2 Database, Table, Index, Column, and Alias Names

Database, table, index, column, and alias names all follow the same rules in MySQL.

Note that the rules changed starting with MySQL Version 3.23.6 when we introduced quoting of identifiers (database, table, and column names) with ``'. `"' will also work to quote identifiers if you run in ANSI mode. See section 1.4.3 Running MySQL in ANSI Mode.

Identifier Max length Allowed characters
Database 64 Any character that is allowed in a directory name except `/' or `.'.
Table 64 Any character that is allowed in a file name, except `/' or `.'.
Column 64 All characters.
Alias 255 All characters.

Note that in addition to the above, you can't have ASCII(0) or ASCII(255) or the quoting character in an identifier.

Note that if the identifier is a restricted word or contains special characters you must always quote it with ` when you use it: 

SELECT * from `select` where `select`.id > 100;

In previous versions of MySQL, the name rules are as follows:

It is recommended that you do not use names like 1e, because an expression like 1e+1 is ambiguous. It may be interpreted as the expression 1e + 1 or as the number 1e+1.

In MySQL you can refer to a column using any of the following forms:

Column reference Meaning
col_name Column col_name from whichever table used in the query contains a column of that name.
tbl_name.col_name Column col_name from table tbl_name of the current database.
db_name.tbl_name.col_name Column col_name from table tbl_name of the database db_name. This form is available in MySQL Version 3.22 or later.
`column_name` A column that is a keyword or contains special characters.

You need not specify a tbl_name or db_name.tbl_name prefix for a column reference in a statement unless the reference would be ambiguous. For example, suppose tables t1 and t2 each contain a column c, and you retrieve c in a SELECT statement that uses both t1 and t2. In this case, c is ambiguous because it is not unique among the tables used in the statement, so you must indicate which table you mean by writing t1.c or t2.c. Similarly, if you are retrieving from a table t in database db1 and from a table t in database db2, you must refer to columns in those tables as db1.t.col_name and db2.t.col_name.

The syntax .tbl_name means the table tbl_name in the current database. This syntax is accepted for ODBC compatibility, because some ODBC programs prefix table names with a `.' character.

6.1.3 Case Sensitivity in Names

In MySQL, databases and tables correspond to directories and files within those directories. Consequently, the case sensitivity of the underlying operating system determines the case sensitivity of database and table names. This means database and table names are case sensitive in Unix and case insensitive in Windows. See section 1.4.1 MySQL Extensions to ANSI SQL92.

NOTE: Although database and table names are case insensitive for Windows, you should not refer to a given database or table using different cases within the same query. The following query would not work because it refers to a table both as my_table and as MY_TABLE: 

mysql> SELECT * FROM my_table WHERE MY_TABLE.col=1;

Column names are case insensitive in all cases.

Aliases on tables are case sensitive. The following query would not work because it refers to the alias both as a and as A: 

mysql> SELECT col_name FROM tbl_name AS a
           WHERE a.col_name = 1 OR A.col_name = 2;

Aliases on columns are case insensitive.

If you have a problem remembering the used cases for a table names, adopt a consistent convention, such as always creating databases and tables using lowercase names.

One way to avoid this problem is to start mysqld with -O lower_case_table_names=1. By default this option is 1 on Windows and 0 on Unix.

If lower_case_table_names is 1 MySQL will convert all table names to lower case on storage and lookup. Note that if you change this option, you need to first convert your old table names to lower case before starting mysqld.

6.1.4 User Variables

MySQL supports thread-specific variables with the @variablename syntax. A variable name may consist of alphanumeric characters from the current character set and also `_', `$', and `.' . The default character set is ISO-8859-1 Latin1; this may be changed with the --default-character-set option to mysqld. See section 4.6.1 The Character Set Used for Data and Sorting.

Variables don't have to be initialized. They contain NULL by default and can store an integer, real, or string value. All variables for a thread are automatically freed when the thread exits.

You can set a variable with the SET syntax: 

SET @variable= { integer expression | real expression | string expression }
[,@variable= ...].

You can also set a variable in an expression with the @variable:=expr syntax: 

select @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;
+----------------------+------+------+------+
| @t1:=(@t2:=1)+@t3:=4 | @t1  | @t2  | @t3  |
+----------------------+------+------+------+
|                    5 |    5 |    1 |    4 |
+----------------------+------+------+------+

(We had to use the := syntax here, because = was reserved for comparisons.)

User variables may be used where expressions are allowed. Note that this does not currently include use in contexts where a number is explicitly required, such as in the LIMIT clause of a SELECT statement, or the IGNORE number LINES clause of a LOAD DATA statement.

NOTE: In a SELECT statement, each expression is only evaluated when it's sent to the client. This means that in the HAVING, GROUP BY, or ORDER BY clause, you can't refer to an expression that involves variables that are set in the SELECT part. For example, the following statement will NOT work as expected: 

SELECT (@aa:=id) AS a, (@aa+3) AS b FROM table_name HAVING b=5;

The reason is that @aa will not contain the value of the current row, but the value of id for the previous accepted row.

6.1.5 Comment Syntax

The MySQL server supports the # to end of line, -- to end of line and /* in-line or multiple-line */ comment styles: 

mysql> select 1+1;     # This comment continues to the end of line
mysql> select 1+1;     -- This comment continues to the end of line
mysql> select 1 /* this is an in-line comment */ + 1;
mysql> select 1+
/*
this is a
multiple-line comment
*/
1;

Note that the -- comment style requires you to have at least one space after the --!

Although the server understands the comment syntax just described, there are some limitations on the way that the mysql client parses /* ... */ comments:

These limitations apply both when you run mysql interactively and when you put commands in a file and tell mysql to read its input from that file with mysql < some-file.

MySQL doesn't support the `--' ANSI SQL comment style. See section 1.4.4.8 `--' as the Start of a Comment.

6.1.6 Is MySQL Picky About Reserved Words?

A common problem stems from trying to create a table with column names that use the names of datatypes or functions built into MySQL, such as TIMESTAMP or GROUP. You're allowed to do it (for example, ABS is an allowed column name), but whitespace is not allowed between a function name and the `(' when using functions whose names are also column names.

The following words are explicitly reserved in MySQL. Most of them are forbidden by ANSI SQL92 as column and/or table names (for example, group). A few are reserved because MySQL needs them and is (currently) using a yacc parser:

action add aggregate all
alter after and as
asc avg avg_row_length auto_increment
between bigint bit binary
blob bool both by
cascade case char character
change check checksum column
columns comment constraint create
cross current_date current_time current_timestamp
data database databases date
datetime day day_hour day_minute
day_second dayofmonth dayofweek dayofyear
dec decimal default delayed
delay_key_write delete desc describe
distinct distinctrow double drop
end else escape escaped
enclosed enum explain exists
fields file first float
float4 float8 flush foreign
from for full function
global grant grants group
having heap high_priority hour
hour_minute hour_second hosts identified
ignore in index infile
inner insert insert_id int
integer interval int1 int2
int3 int4 int8 into
if is isam join
key keys kill last_insert_id
leading left length like
lines limit load local
lock logs long longblob
longtext low_priority max max_rows
match mediumblob mediumtext mediumint
middleint min_rows minute minute_second
modify month monthname myisam
natural numeric no not
null on optimize option
optionally or order outer
outfile pack_keys partial password
precision primary procedure process
processlist privileges read real
references reload regexp rename
replace restrict returns revoke
rlike row rows second
select set show shutdown
smallint soname sql_big_tables sql_big_selects
sql_low_priority_updates sql_log_off sql_log_update sql_select_limit
sql_small_result sql_big_result sql_warnings straight_join
starting status string table
tables temporary terminated text
then time timestamp tinyblob
tinytext tinyint trailing to
type use using unique
unlock unsigned update usage
values varchar variables varying
varbinary with write when
where year year_month zerofill

The following symbols (from the table above) are disallowed by ANSI SQL but allowed by MySQL as column/table names. This is because some of these names are very natural names and a lot of people have already used them.

6.2 Column Types

MySQL supports a number of column types, which may be grouped into three categories: numeric types, date and time types, and string (character) types. This section first gives an overview of the types available and summarizes the storage requirements for each column type, then provides a more detailed description of the properties of the types in each category. The overview is intentionally brief. The more detailed descriptions should be consulted for additional information about particular column types, such as the allowable formats in which you can specify values.

The column types supported by MySQL are listed below. The following code letters are used in the descriptions:

M
Indicates the maximum display size. The maximum legal display size is 255.
D
Applies to floating-point types and indicates the number of digits following the decimal point. The maximum possible value is 30, but should be no greater than M-2.

Square brackets (`[' and `]') indicate parts of type specifiers that are optional.

Note that if you specify ZEROFILL for a column, MySQL will automatically add the UNSIGNED attribute to the column.

TINYINT[(M)] [UNSIGNED] [ZEROFILL]
A very small integer. The signed range is -128 to 127. The unsigned range is 0 to 255.
SMALLINT[(M)] [UNSIGNED] [ZEROFILL]
A small integer. The signed range is -32768 to 32767. The unsigned range is 0 to 65535.
MEDIUMINT[(M)] [UNSIGNED] [ZEROFILL]
A medium-size integer. The signed range is -8388608 to 8388607. The unsigned range is 0 to 16777215.
INT[(M)] [UNSIGNED] [ZEROFILL]
A normal-size integer. The signed range is -2147483648 to 2147483647. The unsigned range is 0 to 4294967295.
INTEGER[(M)] [UNSIGNED] [ZEROFILL]
This is a synonym for INT.
BIGINT[(M)] [UNSIGNED] [ZEROFILL]
A large integer. The signed range is -9223372036854775808 to 9223372036854775807. The unsigned range is 0 to 18446744073709551615. Some things you should be aware about BIGINT columns:
FLOAT(precision) [ZEROFILL]
A floating-point number. Cannot be unsigned. precision can be <=24 for a single-precision floating-point number and between 25 and 53 for a double-precision floating-point number. These types are like the FLOAT and DOUBLE types described immediately below. FLOAT(X) has the same range as the corresponding FLOAT and DOUBLE types, but the display size and number of decimals is undefined. In MySQL Version 3.23, this is a true floating-point value. In earlier MySQL versions, FLOAT(precision) always has 2 decimals. Note that using FLOAT may give you some unexpected problems as all calculation in MySQL is done with double precision. See section A.5.6 Solving Problems with No Matching Rows. This syntax is provided for ODBC compatibility.
FLOAT[(M,D)] [ZEROFILL]
A small (single-precision) floating-point number. Cannot be unsigned. Allowable values are -3.402823466E+38 to -1.175494351E-38, 0, and 1.175494351E-38 to 3.402823466E+38. The M is the display width and D is the number of decimals. FLOAT without an argument or with an argument of <= 24 stands for a single-precision floating-point number.
DOUBLE[(M,D)] [ZEROFILL]
A normal-size (double-precision) floating-point number. Cannot be unsigned. Allowable values are -1.7976931348623157E+308 to -2.2250738585072014E-308, 0, and 2.2250738585072014E-308 to 1.7976931348623157E+308. The M is the display width and D is the number of decimals. DOUBLE without an argument or FLOAT(X) where 25 <= X <= 53 stands for a double-precision floating-point number.
DOUBLE PRECISION[(M,D)] [ZEROFILL]
REAL[(M,D)] [ZEROFILL]
These are synonyms for DOUBLE.
DECIMAL[(M[,D])] [ZEROFILL]
An unpacked floating-point number. Cannot be unsigned. Behaves like a CHAR column: ``unpacked'' means the number is stored as a string, using one character for each digit of the value. The decimal point and, for negative numbers, the `-' sign, are not counted in M (but space for these are reserved). If D is 0, values will have no decimal point or fractional part. The maximum range of DECIMAL values is the same as for DOUBLE, but the actual range for a given DECIMAL column may be constrained by the choice of M and D. If D is left out it's set to 0. If M is left out it's set to 10. Note that in MySQL Version 3.22 the M argument had to includes the space needed for the sign and the decimal point.
NUMERIC(M,D) [ZEROFILL]
This is a synonym for DECIMAL.
DATE
A date. The supported range is '1000-01-01' to '9999-12-31'. MySQL displays DATE values in 'YYYY-MM-DD' format, but allows you to assign values to DATE columns using either strings or numbers. See section 6.2.2.2 The DATETIME, DATE, and TIMESTAMP Types.
DATETIME
A date and time combination. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'. MySQL displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format, but allows you to assign values to DATETIME columns using either strings or numbers. See section 6.2.2.2 The DATETIME, DATE, and TIMESTAMP Types.
TIMESTAMP[(M)]
A timestamp. The range is '1970-01-01 00:00:00' to sometime in the year 2037. MySQL displays TIMESTAMP values in YYYYMMDDHHMMSS, YYMMDDHHMMSS, YYYYMMDD, or YYMMDD format, depending on whether M is 14 (or missing), 12, 8, or 6, but allows you to assign values to TIMESTAMP columns using either strings or numbers. A TIMESTAMP column is useful for recording the date and time of an INSERT or UPDATE operation because it is automatically set to the date and time of the most recent operation if you don't give it a value yourself. You can also set it to the current date and time by assigning it a NULL value. See section 6.2.2 Date and Time Types. A TIMESTAMP is always stored in 4 bytes. The M argument only affects how the TIMESTAMP column is displayed. Note that TIMESTAMP(X) columns where X is 8 or 14 are reported to be numbers while other TIMESTAMP(X) columns are reported to be strings. This is just to ensure that one can reliably dump and restore the table with these types! See section 6.2.2.2 The DATETIME, DATE, and TIMESTAMP Types.
TIME
A time. The range is '-838:59:59' to '838:59:59'. MySQL displays TIME values in 'HH:MM:SS' format, but allows you to assign values to TIME columns using either strings or numbers. See section 6.2.2.3 The TIME Type.
YEAR[(2|4)]
A year in 2- or 4-digit format (default is 4-digit). The allowable values are 1901 to 2155, 0000 in the 4-digit year format, and 1970-2069 if you use the 2-digit format (70-69). MySQL displays YEAR values in YYYY format, but allows you to assign values to YEAR columns using either strings or numbers. (The YEAR type is new in MySQL Version 3.22.). See section 6.2.2.4 The YEAR Type.
[NATIONAL] CHAR(M) [BINARY]
A fixed-length string that is always right-padded with spaces to the specified length when stored. The range of M is 1 to 255 characters. Trailing spaces are removed when the value is retrieved. CHAR values are sorted and compared in case-insensitive fashion according to the default character set unless the BINARY keyword is given. NATIONAL CHAR (short form NCHAR) is the ANSI SQL way to define that a CHAR column should use the default CHARACTER set. This is the default in MySQL. CHAR is a shorthand for CHARACTER. MySQL allows you to create a column of type CHAR(0). This is mainly useful when you have to be compliant with some old applications that depend on the existence of a column but that do not actually use the value. This is also quite nice when you need a column that only can take 2 values: A CHAR(0), that is not defined as NOT NULL, will only occupy one bit and can only take 2 values: NULL or "". See section 6.2.3.1 The CHAR and VARCHAR Types.
BIT
BOOL
CHAR
These three are synonyms for CHAR(1).
[NATIONAL] VARCHAR(M) [BINARY]
A variable-length string. NOTE: Trailing spaces are removed when the value is stored (this differs from the ANSI SQL specification). The range of M is 1 to 255 characters. VARCHAR values are sorted and compared in case-insensitive fashion unless the BINARY keyword is given. See section 6.5.3.1 Silent Column Specification Changes. VARCHAR is a shorthand for CHARACTER VARYING. See section 6.2.3.1 The CHAR and VARCHAR Types.
TINYBLOB
TINYTEXT
A BLOB or TEXT column with a maximum length of 255 (2^8 - 1) characters. See section 6.5.3.1 Silent Column Specification Changes. See section 6.2.3.2 The BLOB and TEXT Types.
BLOB
TEXT
A BLOB or TEXT column with a maximum length of 65535 (2^16 - 1) characters. See section 6.5.3.1 Silent Column Specification Changes. See section 6.2.3.2 The BLOB and TEXT Types.
MEDIUMBLOB
MEDIUMTEXT
A BLOB or TEXT column with a maximum length of 16777215 (2^24 - 1) characters. See section 6.5.3.1 Silent Column Specification Changes. See section 6.2.3.2 The BLOB and TEXT Types.
LONGBLOB
LONGTEXT
A BLOB or TEXT column with a maximum length of 4294967295 (2^32 - 1) characters. See section 6.5.3.1 Silent Column Specification Changes. Note that because the server/client protocol and MyISAM tables has currently a limit of 16M per communication packet / table row, you can't yet use this the whole range of this type. See section 6.2.3.2 The BLOB and TEXT Types.
ENUM('value1','value2',...)
An enumeration. A string object that can have only one value, chosen from the list of values 'value1', 'value2', ..., NULL or the special "" error value. An ENUM can have a maximum of 65535 distinct values. See section 6.2.3.3 The ENUM Type.
SET('value1','value2',...)
A set. A string object that can have zero or more values, each of which must be chosen from the list of values 'value1', 'value2', ... A SET can have a maximum of 64 members. See section 6.2.3.4 The SET Type.

6.2.1 Numeric Types

MySQL supports all of the ANSI/ISO SQL92 numeric types. These types include the exact numeric data types (NUMERIC, DECIMAL, INTEGER, and SMALLINT), as well as the approximate numeric data types (FLOAT, REAL, and DOUBLE PRECISION). The keyword INT is a synonym for INTEGER, and the keyword DEC is a synonym for DECIMAL.

The NUMERIC and DECIMAL types are implemented as the same type by MySQL, as permitted by the SQL92 standard. They are used for values for which it is important to preserve exact precision, for example with monetary data. When declaring a column of one of these types the precision and scale can be (and usually is) specified; for example: 

    salary DECIMAL(9,2)

In this example, 9 (precision) represents the number of significant decimal digits that will be stored for values, and 2 (scale) represents the number of digits that will be stored following the decimal point. In this case, therefore, the range of values that can be stored in the salary column is from -9999999.99 to 9999999.99. (MySQL can actually store numbers up to 9999999.99 in this column because it doesn't have to store the sign for positive numbers)

In ANSI/ISO SQL92, the syntax DECIMAL(p) is equivalent to DECIMAL(p,0). Similarly, the syntax DECIMAL is equivalent to DECIMAL(p,0), where the implementation is allowed to decide the value of p. MySQL does not currently support either of these variant forms of the DECIMAL/NUMERIC data types. This is not generally a serious problem, as the principal benefits of these types derive from the ability to control both precision and scale explicitly.

DECIMAL and NUMERIC values are stored as strings, rather than as binary floating-point numbers, in order to preserve the decimal precision of those values. One character is used for each digit of the value, the decimal point (if scale > 0), and the `-' sign (for negative numbers). If scale is 0, DECIMAL and NUMERIC values contain no decimal point or fractional part.

The maximum range of DECIMAL and NUMERIC values is the same as for DOUBLE, but the actual range for a given DECIMAL or NUMERIC column can be constrained by the precision or scale for a given column. When such a column is assigned a value with more digits following the decimal point than are allowed by the specified scale, the value is rounded to that scale. When a DECIMAL or NUMERIC column is assigned a value whose magnitude exceeds the range implied by the specified (or defaulted) precision and scale, MySQL stores the value representing the corresponding end point of that range.

As an extension to the ANSI/ISO SQL92 standard, MySQL also supports the integral types TINYINT, MEDIUMINT, and BIGINT as listed in the tables above. Another extension is supported by MySQL for optionally specifying the display width of an integral value in parentheses following the base keyword for the type (for example, INT(4)). This optional width specification is used to left-pad the display of values whose width is less than the width specified for the column, but does not constrain the range of values that can be stored in the column, nor the number of digits that will be displayed for values whose width exceeds that specified for the column. When used in conjunction with the optional extension attribute ZEROFILL, the default padding of spaces is replaced with zeroes. For example, for a column declared as INT(5) ZEROFILL, a value of 4 is retrieved as 00004. Note that if you store larger values than the display width in an integer column, you may experience problems when MySQL generates temporary tables for some complicated joins, as in these cases MySQL trusts that the data did fit into the original column width.

All integral types can have an optional (non-standard) attribute UNSIGNED. Unsigned values can be used when you want to allow only positive numbers in a column and you need a little bigger numeric range for the column.

The FLOAT type is used to represent approximate numeric data types. The ANSI/ISO SQL92 standard allows an optional specification of the precision (but not the range of the exponent) in bits following the keyword FLOAT in parentheses. The MySQL implementation also supports this optional precision specification. When the keyword FLOAT is used for a column type without a precision specification, MySQL uses four bytes to store the values. A variant syntax is also supported, with two numbers given in parentheses following the FLOAT keyword. With this option, the first number continues to represent the storage requirements for the value in bytes, and the second number specifies the number of digits to be stored and displayed following the decimal point (as with DECIMAL and NUMERIC). When MySQL is asked to store a number for such a column with more decimal digits following the decimal point than specified for the column, the value is rounded to eliminate the extra digits when the value is stored.

The REAL and DOUBLE PRECISION types do not accept precision specifications. As an extension to the ANSI/ISO SQL92 standard, MySQL recognizes DOUBLE as a synonym for the DOUBLE PRECISION type. In contrast with the standard's requirement that the precision for REAL be smaller than that used for DOUBLE PRECISION, MySQL implements both as 8-byte double-precision floating-point values (when not running in ``ANSI mode''). For maximum portability, code requiring storage of approximate numeric data values should use FLOAT or DOUBLE PRECISION with no specification of precision or number of decimal points.

When asked to store a value in a numeric column that is outside the column type's allowable range, MySQL clips the value to the appropriate endpoint of the range and stores the resulting value instead.

For example, the range of an INT column is -2147483648 to 2147483647. If you try to insert -9999999999 into an INT column, the value is clipped to the lower endpoint of the range, and -2147483648 is stored instead. Similarly, if you try to insert 9999999999, 2147483647 is stored instead.

If the INT column is UNSIGNED, the size of the column's range is the same but its endpoints shift up to 0 and 4294967295. If you try to store -9999999999 and 9999999999, the values stored in the column become 0 and 4294967296.

Conversions that occur due to clipping are reported as ``warnings'' for ALTER TABLE, LOAD DATA INFILE, UPDATE, and multi-row INSERT statements.

6.2.2 Date and Time Types

The date and time types are DATETIME, DATE, TIMESTAMP, TIME, and YEAR. Each of these has a range of legal values, as well as a ``zero'' value that is used when you specify a really illegal value. Note that MySQL allows you to store certain 'not strictly' legal date values, for example 1999-11-31. The reason for this is that we think it's the responsibility of the application to handle date checking, not the SQL servers. To make the date checking 'fast', MySQL only checks that the month is in the range of 0-12 and the day is in the range of 0-31. The above ranges are defined this way because MySQL allows you to store, in a DATE or DATETIME column, dates where the day or month-day is zero. This is extremely useful for applications that need to store a birth-date for which you don't know the exact date. In this case you simply store the date like 1999-00-00 or 1999-01-00. (You cannot expect to get a correct value from functions like DATE_SUB() or DATE_ADD for dates like these.)

Here are some general considerations to keep in mind when working with date and time types:

6.2.2.1 Y2K Issues and Date Types

MySQL itself is Y2K-safe (see section 1.1.9 Year 2000 Compliance), but input values presented to MySQL may not be. Any input containing 2-digit year values is ambiguous, because the century is unknown. Such values must be interpreted into 4-digit form because MySQL stores years internally using four digits.

For DATETIME, DATE, TIMESTAMP, and YEAR types, MySQL interprets dates with ambiguous year values using the following rules:

Remember that these rules provide only reasonable guesses as to what your data mean. If the heuristics used by MySQL don't produce the correct values, you should provide unambiguous input containing 4-digit year values.

ORDER BY will sort 2-digit YEAR/DATE/DATETIME types properly.

Note also that some functions like MIN() and MAX() will convert a TIMESTAMP/DATE to a number. This means that a timestamp with a 2-digit year will not work properly with these functions. The fix in this case is to convert the TIMESTAMP/DATE to 4-digit year format or use something like MIN(DATE_ADD(timestamp,INTERVAL 0 DAYS)).

6.2.2.2 The DATETIME, DATE, and TIMESTAMP Types

The DATETIME, DATE, and TIMESTAMP types are related. This section describes their characteristics, how they are similar, and how they differ.

The DATETIME type is used when you need values that contain both date and time information. MySQL retrieves and displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'. (``Supported'' means that although earlier values might work, there is no guarantee that they will.)

The DATE type is used when you need only a date value, without a time part. MySQL retrieves and displays DATE values in 'YYYY-MM-DD' format. The supported range is '1000-01-01' to '9999-12-31'.

The TIMESTAMP column type provides a type that you can use to automatically mark INSERT or UPDATE operations with the current date and time. If you have multiple TIMESTAMP columns, only the first one is updated automatically.

Automatic updating of the first TIMESTAMP column occurs under any of the following conditions:

TIMESTAMP columns other than the first may also be set to the current date and time. Just set the column to NULL or to NOW().

You can set any TIMESTAMP column to a value different than the current date and time by setting it explicitly to the desired value. This is true even for the first TIMESTAMP column. You can use this property if, for example, you want a TIMESTAMP to be set to the current date and time when you create a row, but not to be changed whenever the row is updated later:

On the other hand, you may find it just as easy to use a DATETIME column that you initialize to NOW() when the row is created and leave alone for subsequent updates.

TIMESTAMP values may range from the beginning of 1970 to sometime in the year 2037, with a resolution of one second. Values are displayed as numbers.

The format in which MySQL retrieves and displays TIMESTAMP values depends on the display size, as illustrated by the table below. The `full' TIMESTAMP format is 14 digits, but TIMESTAMP columns may be created with shorter display sizes:

Column type Display format
TIMESTAMP(14) YYYYMMDDHHMMSS
TIMESTAMP(12) YYMMDDHHMMSS
TIMESTAMP(10) YYMMDDHHMM
TIMESTAMP(8) YYYYMMDD
TIMESTAMP(6) YYMMDD
TIMESTAMP(4) YYMM
TIMESTAMP(2) YY

All TIMESTAMP columns have the same storage size, regardless of display size. The most common display sizes are 6, 8, 12, and 14. You can specify an arbitrary display size at table creation time, but values of 0 or greater than 14 are coerced to 14. Odd-valued sizes in the range from 1 to 13 are coerced to the next higher even number.

You can specify DATETIME, DATE, and TIMESTAMP values using any of a common set of formats:

Illegal DATETIME, DATE, or TIMESTAMP values are converted to the ``zero'' value of the appropriate type ('0000-00-00 00:00:00', '0000-00-00', or 00000000000000).

For values specified as strings that include date part delimiters, it is not necessary to specify two digits for month or day values that are less than 10. '1979-6-9' is the same as '1979-06-09'. Similarly, for values specified as strings that include time part delimiters, it is not necessary to specify two digits for hour, month, or second values that are less than 10. '1979-10-30 1:2:3' is the same as '1979-10-30 01:02:03'.

Values specified as numbers should be 6, 8, 12, or 14 digits long. If the number is 8 or 14 digits long, it is assumed to be in YYYYMMDD or YYYYMMDDHHMMSS format and that the year is given by the first 4 digits. If the number is 6 or 12 digits long, it is assumed to be in YYMMDD or YYMMDDHHMMSS format and that the year is given by the first 2 digits. Numbers that are not one of these lengths are interpreted as though padded with leading zeros to the closest length.

Values specified as non-delimited strings are interpreted using their length as given. If the string is 8 or 14 characters long, the year is assumed to be given by the first 4 characters. Otherwise the year is assumed to be given by the first 2 characters. The string is interpreted from left to right to find year, month, day, hour, minute, and second values, for as many parts as are present in the string. This means you should not use strings that have fewer than 6 characters. For example, if you specify '9903', thinking that will represent March, 1999, you will find that MySQL inserts a ``zero'' date into your table. This is because the year and month values are 99 and 03, but the day part is missing (zero), so the value is not a legal date.

TIMESTAMP columns store legal values using the full precision with which the value was specified, regardless of the display size. This has several implications:

You can to some extent assign values of one date type to an object of a different date type. However, there may be some alteration of the value or loss of information:

Be aware of certain pitfalls when specifying date values:

6.2.2.3 The TIME Type

MySQL retrieves and displays TIME values in 'HH:MM:SS' format (or 'HHH:MM:SS' format for large hours values). TIME values may range from '-838:59:59' to '838:59:59'. The reason the hours part may be so large is that the TIME type may be used not only to represent a time of day (which must be less than 24 hours), but also elapsed time or a time interval between two events (which may be much greater than 24 hours, or even negative).

You can specify TIME values in a variety of formats:

For TIME values specified as strings that include a time part delimiter, it is not necessary to specify two digits for hours, minutes, or seconds values that are less than 10. '8:3:2' is the same as '08:03:02'.

Be careful about assigning ``short'' TIME values to a TIME column. Without semicolon, MySQL interprets values using the assumption that the rightmost digits represent seconds. (MySQL interprets TIME values as elapsed time rather than as time of day.) For example, you might think of '1112' and 1112 as meaning '11:12:00' (12 minutes after 11 o'clock), but MySQL interprets them as '00:11:12' (11 minutes, 12 seconds). Similarly, '12' and 12 are interpreted as '00:00:12'. TIME values with semicolon, instead, are always treated as time of the day. That is '11:12' will mean '11:12:00', not '00:11:12'.

Values that lie outside the TIME range but are otherwise legal are clipped to the appropriate endpoint of the range. For example, '-850:00:00' and '850:00:00' are converted to '-838:59:59' and '838:59:59'.

Illegal TIME values are converted to '00:00:00'. Note that because '00:00:00' is itself a legal TIME value, there is no way to tell, from a value of '00:00:00' stored in a table, whether the original value was specified as '00:00:00' or whether it was illegal.

6.2.2.4 The YEAR Type

The YEAR type is a 1-byte type used for representing years.

MySQL retrieves and displays YEAR values in YYYY format. The range is 1901 to 2155.

You can specify YEAR values in a variety of formats:

Illegal YEAR values are converted to 0000.

6.2.3 String Types

The string types are CHAR, VARCHAR, BLOB, TEXT, ENUM, and SET. This section describes how these types work, their storage requirements, and how to use them in your queries.

6.2.3.1 The CHAR and VARCHAR Types

The CHAR and VARCHAR types are similar, but differ in the way they are stored and retrieved.

The length of a CHAR column is fixed to the length that you declare when you create the table. The length can be any value between 1 and 255. (As of MySQL Version 3.23, the length of CHAR may be 0 to 255.) When CHAR values are stored, they are right-padded with spaces to the specified length. When CHAR values are retrieved, trailing spaces are removed.

Values in VARCHAR columns are variable-length strings. You can declare a VARCHAR column to be any length between 1 and 255, just as for CHAR columns. However, in contrast to CHAR, VARCHAR values are stored using only as many characters as are needed, plus one byte to record the length. Values are not padded; instead, trailing spaces are removed when values are stored. (This space removal differs from the ANSI SQL specification.)

If you assign a value to a CHAR or VARCHAR column that exceeds the column's maximum length, the value is truncated to fit.

The table below illustrates the differences between the two types of columns by showing the result of storing various string values into CHAR(4) and VARCHAR(4) columns:

Value CHAR(4) Storage required VARCHAR(4) Storage required
'' ' ' 4 bytes '' 1 byte
'ab' 'ab ' 4 bytes 'ab' 3 bytes
'abcd' 'abcd' 4 bytes 'abcd' 5 bytes
'abcdefgh' 'abcd' 4 bytes 'abcd' 5 bytes

The values retrieved from the CHAR(4) and VARCHAR(4) columns will be the same in each case, because trailing spaces are removed from CHAR columns upon retrieval.

Values in CHAR and VARCHAR columns are sorted and compared in case-insensitive fashion, unless the BINARY attribute was specified when the table was created. The BINARY attribute means that column values are sorted and compared in case-sensitive fashion according to the ASCII order of the machine where the MySQL server is running. BINARY doesn't affect how the column is stored or retrieved.

The BINARY attribute is sticky. This means that if a column marked BINARY is used in an expression, the whole expression is compared as a BINARY value.

MySQL may silently change the type of a CHAR or VARCHAR column at table creation time. See section 6.5.3.1 Silent Column Specification Changes.

6.2.3.2 The BLOB and TEXT Types

A BLOB is a binary large object that can hold a variable amount of data. The four BLOB types TINYBLOB, BLOB, MEDIUMBLOB, and LONGBLOB differ only in the maximum length of the values they can hold. See section 6.2.6 Column Type Storage Requirements.

The four TEXT types TINYTEXT, TEXT, MEDIUMTEXT, and LONGTEXT correspond to the four BLOB types and have the same maximum lengths and storage requirements. The only difference between BLOB and TEXT types is that sorting and comparison is performed in case-sensitive fashion for BLOB values and case-insensitive fashion for TEXT values. In other words, a TEXT is a case-insensitive BLOB.

If you assign a value to a BLOB or TEXT column that exceeds the column type's maximum length, the value is truncated to fit.

In most respects, you can regard a TEXT column as a VARCHAR column that can be as big as you like. Similarly, you can regard a BLOB column as a VARCHAR BINARY column. The differences are:

MyODBC defines BLOB values as LONGVARBINARY and TEXT values as LONGVARCHAR.

Because BLOB and TEXT values may be extremely long, you may run up against some constraints when using them:

Note that each BLOB or TEXT value is represented internally by a separately allocated object. This is in contrast to all other column types, for which storage is allocated once per column when the table is opened.

6.2.3.3 The ENUM Type

An ENUM is a string object whose value normally is chosen from a list of allowed values that are enumerated explicitly in the column specification at table creation time.

The value may also be the empty string ("") or NULL under certain circumstances:

Each enumeration value has an index:

For example, a column specified as ENUM("one", "two", "three") can have any of the values shown below. The index of each value is also shown:

Value Index
NULL NULL
"" 0
"one" 1
"two" 2
"three" 3

An enumeration can have a maximum of 65535 elements.

Lettercase is irrelevant when you assign values to an ENUM column. However, values retrieved from the column later have lettercase matching the values that were used to specify the allowable values at table creation time.

If you retrieve an ENUM in a numeric context, the column value's index is returned. For example, you can retrieve numeric values from an ENUM column like this: 

mysql> SELECT enum_col+0 FROM tbl_name;

If you store a number into an ENUM, the number is treated as an index, and the value stored is the enumeration member with that index. (However, this will not work with LOAD DATA, which treats all input as strings.)

ENUM values are sorted according to the order in which the enumeration members were listed in the column specification. (In other words, ENUM values are sorted according to their index numbers.) For example, "a" sorts before "b" for ENUM("a", "b"), but "b" sorts before "a" for ENUM("b", "a"). The empty string sorts before non-empty strings, and NULL values sort before all other enumeration values.

If you want to get all possible values for an ENUM column, you should use: SHOW COLUMNS FROM table_name LIKE enum_column_name and parse the ENUM definition in the second column.

6.2.3.4 The SET Type

A SET is a string object that can have zero or more values, each of which must be chosen from a list of allowed values specified when the table is created. SET column values that consist of multiple set members are specified with members separated by commas (`,'). A consequence of this is that SET member values cannot themselves contain commas.

For example, a column specified as SET("one", "two") NOT NULL can have any of these values: 

""
"one"
"two"
"one,two"

A SET can have a maximum of 64 different members.

MySQL stores SET values numerically, with the low-order bit of the stored value corresponding to the first set member. If you retrieve a SET value in a numeric context, the value retrieved has bits set corresponding to the set members that make up the column value. For example, you can retrieve numeric values from a SET column like this: 

mysql> SELECT set_col+0 FROM tbl_name;

If a number is stored into a SET column, the bits that are set in the binary representation of the number determine the set members in the column value. Suppose a column is specified as SET("a","b","c","d"). Then the members have the following bit values:

SET member Decimal value Binary value
a 1 0001
b 2 0010
c 4 0100
d 8 1000

If you assign a value of 9 to this column, that is 1001 in binary, so the first and fourth SET value members "a" and "d" are selected and the resulting value is "a,d".

For a value containing more than one SET element, it does not matter what order the elements are listed in when you insert the value. It also does not matter how many times a given element is listed in the value. When the value is retrieved later, each element in the value will appear once, with elements listed according to the order in which they were specified at table creation time. For example, if a column is specified as SET("a","b","c","d"), then "a,d", "d,a", and "d,a,a,d,d" will all appear as "a,d" when retrieved.

SET values are sorted numerically. NULL values sort before non-NULL SET values.

Normally, you perform a SELECT on a SET column using the LIKE operator or the FIND_IN_SET() function: 

mysql> SELECT * FROM tbl_name WHERE set_col LIKE '%value%';
mysql> SELECT * FROM tbl_name WHERE FIND_IN_SET('value',set_col)>0;

But the following will also work: 

mysql> SELECT * FROM tbl_name WHERE set_col = 'val1,val2';
mysql> SELECT * FROM tbl_name WHERE set_col & 1;

The first of these statements looks for an exact match. The second looks for values containing the first set member.

If you want to get all possible values for a SET column, you should use: SHOW COLUMNS FROM table_name LIKE set_column_name and parse the SET definition in the second column.

6.2.4 Choosing the Right Type for a Column

For the most efficient use of storage, try to use the most precise type in all cases. For example, if an integer column will be used for values in the range between 1 and 99999, MEDIUMINT UNSIGNED is the best type.

Accurate representation of monetary values is a common problem. In MySQL, you should use the DECIMAL type. This is stored as a string, so no loss of accuracy should occur. If accuracy is not too important, the DOUBLE type may also be good enough.

For high precision, you can always convert to a fixed-point type stored in a BIGINT. This allows you to do all calculations with integers and convert results back to floating-point values only when necessary.

6.2.5 Using Column Types from Other Database Engines

To make it easier to use code written for SQL implementations from other vendors, MySQL maps column types as shown in the table below. These mappings make it easier to move table definitions from other database engines to MySQL:

Other vendor type MySQL type
BINARY(NUM) CHAR(NUM) BINARY
CHAR VARYING(NUM) VARCHAR(NUM)
FLOAT4 FLOAT
FLOAT8 DOUBLE
INT1 TINYINT
INT2 SMALLINT
INT3 MEDIUMINT
INT4 INT
INT8 BIGINT
LONG VARBINARY MEDIUMBLOB
LONG VARCHAR MEDIUMTEXT
MIDDLEINT MEDIUMINT
VARBINARY(NUM) VARCHAR(NUM) BINARY

Column type mapping occurs at table creation time. If you create a table with types used by other vendors and then issue a DESCRIBE tbl_name statement, MySQL reports the table structure using the equivalent MySQL types.

6.2.6 Column Type Storage Requirements

The storage requirements for each of the column types supported by MySQL are listed below by category.

6.2.6.1 Storage requirements for numeric types

Column type Storage required
TINYINT 1 byte
SMALLINT 2 bytes
MEDIUMINT 3 bytes
INT 4 bytes
INTEGER 4 bytes
BIGINT 8 bytes
FLOAT(X) 4 if X <= 24 or 8 if 25 <= X <= 53
FLOAT 4 bytes
DOUBLE 8 bytes
DOUBLE PRECISION 8 bytes
REAL 8 bytes
DECIMAL(M,D) M+2 bytes if D > 0, M+1 bytes if D = 0 (D+2, if M < D)
NUMERIC(M,D) M+2 bytes if D > 0, M+1 bytes if D = 0 (D+2, if M < D)

6.2.6.2 Storage requirements for date and time types

Column type Storage required
DATE 3 bytes
DATETIME 8 bytes
TIMESTAMP 4 bytes
TIME 3 bytes
YEAR 1 byte

6.2.6.3 Storage requirements for string types

Column type Storage required
CHAR(M) M bytes, 1 <= M <= 255
VARCHAR(M) L+1 bytes, where L <= M and 1 <= M <= 255
TINYBLOB, TINYTEXT L+1 bytes, where L < 2^8
BLOB, TEXT L+2 bytes, where L < 2^16
MEDIUMBLOB, MEDIUMTEXT L+3 bytes, where L < 2^24
LONGBLOB, LONGTEXT L+4 bytes, where L < 2^32
ENUM('value1','value2',...) 1 or 2 bytes, depending on the number of enumeration values (65535 values maximum)
SET('value1','value2',...) 1, 2, 3, 4 or 8 bytes, depending on the number of set members (64 members maximum)

VARCHAR and the BLOB and TEXT types are variable-length types, for which the storage requirements depend on the actual length of column values (represented by L in the preceding table), rather than on the type's maximum possible size. For example, a VARCHAR(10) column can hold a string with a maximum length of 10 characters. The actual storage required is the length of the string (L), plus 1 byte to record the length of the string. For the string 'abcd', L is 4 and the storage requirement is 5 bytes.

The BLOB and TEXT types require 1, 2, 3, or 4 bytes to record the length of the column value, depending on the maximum possible length of the type. See section 6.2.3.2 The BLOB and TEXT Types.

If a table includes any variable-length column types, the record format will also be variable-length. Note that when a table is created, MySQL may, under certain conditions, change a column from a variable-length type to a fixed-length type, or vice-versa. See section 6.5.3.1 Silent Column Specification Changes.

The size of an ENUM object is determined by the number of different enumeration values. One byte is used for enumerations with up to 255 possible values. Two bytes are used for enumerations with up to 65535 values. See section 6.2.3.3 The ENUM Type.

The size of a SET object is determined by the number of different set members. If the set size is N, the object occupies (N+7)/8 bytes, rounded up to 1, 2, 3, 4, or 8 bytes. A SET can have a maximum of 64 members. See section 6.2.3.4 The SET Type.

6.3 Functions for Use in SELECT and WHERE Clauses

A select_expression or where_definition in a SQL statement can consist of any expression using the functions described below.

An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for the operators and functions involved in the expression.

NOTE: There must be no whitespace between a function name and the parenthesis following it. This helps the MySQL parser distinguish between function calls and references to tables or columns that happen to have the same name as a function. Spaces around arguments are permitted, though.

You can force MySQL to accept spaces after the function name by starting mysqld with --ansi or using the CLIENT_IGNORE_SPACE to mysql_connect(), but in this case all function names will become reserved words. See section 1.4.3 Running MySQL in ANSI Mode.

For the sake of brevity, examples display the output from the mysql program in abbreviated form. So this: 

mysql> select MOD(29,9);
1 rows in set (0.00 sec)

+-----------+
| mod(29,9) |
+-----------+
|         2 |
+-----------+

is displayed like this: 

mysql> select MOD(29,9);
        -> 2

6.3.1 Non-Type-Specific Operators and Functions

6.3.1.1 Parenthesis

( ... )

Use parenthesis to force the order of evaluation in an expression. For example: 

mysql> select 1+2*3;
        -> 7
mysql> select (1+2)*3;
        -> 9

6.3.1.2 Comparison Operators

Comparison operations result in a value of 1 (TRUE), 0 (FALSE), or NULL. These functions work for both numbers and strings. Strings are automatically converted to numbers and numbers to strings as needed (as in Perl).

MySQL performs comparisons using the following rules:

By default, string comparisons are done in case-independent fashion using the current character set (ISO-8859-1 Latin1 by default, which also works excellently for English).

The examples below illustrate conversion of strings to numbers for comparison operations: 

mysql> SELECT 1 > '6x';
         -> 0
mysql> SELECT 7 > '6x';
         -> 1
mysql> SELECT 0 > 'x6';
         -> 0
mysql> SELECT 0 = 'x6';
         -> 1
=
Equal: 
mysql> select 1 = 0;
        -> 0
mysql> select '0' = 0;
        -> 1
mysql> select '0.0' = 0;
        -> 1
mysql> select '0.01' = 0;
        -> 0
mysql> select '.01' = 0.01;
        -> 1
<>
!=
Not equal: 
mysql> select '.01' <> '0.01';
        -> 1
mysql> select .01 <> '0.01';
        -> 0
mysql> select 'zapp' <> 'zappp';
        -> 1
<=
Less than or equal: 
mysql> select 0.1 <= 2;
        -> 1
<
Less than: 
mysql> select 2 < 2;
        -> 0
>=
Greater than or equal: 
mysql> select 2 >= 2;
        -> 1
>
Greater than: 
mysql> select 2 > 2;
        -> 0
<=>
Null safe equal: 
mysql> select 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
        -> 1 1 0
IS NULL
IS NOT NULL
Test whether or not a value is or is not NULL: 
mysql> select 1 IS NULL, 0 IS NULL, NULL IS NULL;
        -> 0 0 1
mysql> select 1 IS NOT NULL, 0 IS NOT NULL, NULL IS NOT NULL;
        -> 1 1 0
expr BETWEEN min AND max
If expr is greater than or equal to min and expr is less than or equal to max, BETWEEN returns 1, otherwise it returns 0. This is equivalent to the expression (min <= expr AND expr <= max) if all the arguments are of the same type. The first argument (expr) determines how the comparison is performed as follows: 
mysql> select 1 BETWEEN 2 AND 3;
        -> 0
mysql> select 'b' BETWEEN 'a' AND 'c';
        -> 1
mysql> select 2 BETWEEN 2 AND '3';
        -> 1
mysql> select 2 BETWEEN 2 AND 'x-3';
        -> 0
expr IN (value,...)
Returns 1 if expr is any of the values in the IN list, else returns 0. If all values are constants, then all values are evaluated according to the type of expr and sorted. The search for the item is then done using a binary search. This means IN is very quick if the IN value list consists entirely of constants. If expr is a case-sensitive string expression, the string comparison is performed in case-sensitive fashion: 
mysql> select 2 IN (0,3,5,'wefwf');
        -> 0
mysql> select 'wefwf' IN (0,3,5,'wefwf');
        -> 1
expr NOT IN (value,...)
Same as NOT (expr IN (value,...)).
ISNULL(expr)
If expr is NULL, ISNULL() returns 1, otherwise it returns 0: 
mysql> select ISNULL(1+1);
        -> 0
mysql> select ISNULL(1/0);
        -> 1
Note that a comparison of NULL values using = will always be false!
COALESCE(list)
Returns first non-NULL element in list: 
mysql> select COALESCE(NULL,1);
        -> 1
mysql> select COALESCE(NULL,NULL,NULL);
        -> NULL
INTERVAL(N,N1,N2,N3,...)
Returns 0 if N < N1, 1 if N < N2 and so on. All arguments are treated as integers. It is required that N1 < N2 < N3 < ... < Nn for this function to work correctly. This is because a binary search is used (very fast): 
mysql> select INTERVAL(23, 1, 15, 17, 30, 44, 200);
        -> 3
mysql> select INTERVAL(10, 1, 10, 100, 1000);
        -> 2
mysql> select INTERVAL(22, 23, 30, 44, 200);
        -> 0

If you are comparing case sensitive string with any of the standard operators (=, <>..., but not LIKE) end space will be ignored. 

mysql> select "a" ="A ";
        -> 1

6.3.1.3 Logical Operators

All logical functions return 1 (TRUE), 0 (FALSE) or NULL (unknown, which is in most cases the same as FALSE):

NOT
!
Logical NOT. Returns 1 if the argument is 0, otherwise returns 0. Exception: NOT NULL returns NULL: 
mysql> select NOT 1;
        -> 0
mysql> select NOT NULL;
        -> NULL
mysql> select ! (1+1);
        -> 0
mysql> select ! 1+1;
        -> 1
The last example returns 1 because the expression evaluates the same way as (!1)+1.
OR
||
Logical OR. Returns 1 if either argument is not 0 and not NULL: 
mysql> select 1 || 0;
        -> 1
mysql> select 0 || 0;
        -> 0
mysql> select 1 || NULL;
        -> 1
AND
&&
Logical AND. Returns 0 if either argument is 0 or NULL, otherwise returns 1: 
mysql> select 1 && NULL;
        -> 0
mysql> select 1 && 0;
        -> 0

6.3.1.4 Control Flow Functions

IFNULL(expr1,expr2)
If expr1 is not NULL, IFNULL() returns expr1, else it returns expr2. IFNULL() returns a numeric or string value, depending on the context in which it is used: 
mysql> select IFNULL(1,0);
        -> 1
mysql> select IFNULL(NULL,10);
        -> 10
mysql> select IFNULL(1/0,10);
        -> 10
mysql> select IFNULL(1/0,'yes');
        -> 'yes'
NULLIF(expr1,expr2)
If expr1 = expr2 is true, return NULL else return expr1. This is the same as CASE WHEN x = y THEN NULL ELSE x END: 
mysql> select NULLIF(1,1);
        -> NULL
mysql> select NULLIF(1,2);
        -> 1
Note that expr1 is evaluated twice in MySQL if the arguments are equal.
IF(expr1,expr2,expr3)
If expr1 is TRUE (expr1 <> 0 and expr1 <> NULL) then IF() returns expr2, else it returns expr3. IF() returns a numeric or string value, depending on the context in which it is used: 
mysql> select IF(1>2,2,3);
        -> 3
mysql> select IF(1<2,'yes','no');
        -> 'yes'
mysql> select IF(strcmp('test','test1'),'no','yes');
        -> 'no'
expr1 is evaluated as an integer value, which means that if you are testing floating-point or string values, you should do so using a comparison operation: 
mysql> select IF(0.1,1,0);
        -> 0
mysql> select IF(0.1<>0,1,0);
        -> 1
In the first case above, IF(0.1) returns 0 because 0.1 is converted to an integer value, resulting in a test of IF(0). This may not be what you expect. In the second case, the comparison tests the original floating-point value to see whether it is non-zero. The result of the comparison is used as an integer. The default return type of IF() (which may matter when it is stored into a temporary table) is calculated in MySQL Version 3.23 as follows:
Expression Return value
expr2 or expr3 returns string string
expr2 or expr3 returns a floating-point value floating-point
expr2 or expr3 returns an integer integer
CASE value WHEN [compare-value] THEN result [WHEN [compare-value] THEN result ...] [ELSE result] END
CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END
The first version returns the result where value=compare-value. The second version returns the result for the first condition, which is true. If there was no matching result value, then the result after ELSE is returned. If there is no ELSE part then NULL is returned: 
mysql> SELECT CASE 1 WHEN 1 THEN "one" WHEN 2 THEN "two" ELSE "more" END;
       -> "one"
mysql> SELECT CASE WHEN 1>0 THEN "true" ELSE "false" END;
       -> "true"
mysql> SELECT CASE BINARY "B" when "a" then 1 when "b" then 2 END;
       -> NULL

The type of the return value (INTEGER, DOUBLE or STRING) is the same as the type of the first returned value (the expression after the first THEN).

6.3.2 String Functions

String-valued functions return NULL if the length of the result would be greater than the max_allowed_packet server parameter. See section 5.5.2 Tuning Server Parameters.

For functions that operate on string positions, the first position is numbered 1.

ASCII(str)
Returns the ASCII code value of the leftmost character of the string str. Returns 0 if str is the empty string. Returns NULL if str is NULL: 
mysql> select ASCII('2');
        -> 50
mysql> select ASCII(2);
        -> 50
mysql> select ASCII('dx');
        -> 100
See also the ORD() function.
ORD(str)
If the leftmost character of the string str is a multi-byte character, returns the code of multi-byte character by returning the ASCII code value of the character in the format of: ((first byte ASCII code)*256+(second byte ASCII code))[*256+third byte ASCII code...]. If the leftmost character is not a multi-byte character, returns the same value as the like ASCII() function does: 
mysql> select ORD('2');
        -> 50
CONV(N,from_base,to_base)
Converts numbers between different number bases. Returns a string representation of the number N, converted from base from_base to base to_base. Returns NULL if any argument is NULL. The argument N is interpreted as an integer, but may be specified as an integer or a string. The minimum base is 2 and the maximum base is 36. If to_base is a negative number, N is regarded as a signed number. Otherwise, N is treated as unsigned. CONV works with 64-bit precision: 
mysql> select CONV("a",16,2);
        -> '1010'
mysql> select CONV("6E",18,8);
        -> '172'
mysql> select CONV(-17,10,-18);
        -> '-H'
mysql> select CONV(10+"10"+'10'+0xa,10,10);
        -> '40'
BIN(N)
Returns a string representation of the binary value of N, where N is a longlong (BIGINT) number. This is equivalent to CONV(N,10,2). Returns NULL if N is NULL: 
mysql> select BIN(12);
        -> '1100'
OCT(N)
Returns a string representation of the octal value of N, where N is a longlong number. This is equivalent to CONV(N,10,8). Returns NULL if N is NULL: 
mysql> select OCT(12);
        -> '14'
HEX(N)
Returns a string representation of the hexadecimal value of N, where N is a longlong (BIGINT) number. This is equivalent to CONV(N,10,16). Returns NULL if N is NULL: 
mysql> select HEX(255);
        -> 'FF'
CHAR(N,...)
CHAR() interprets the arguments as integers and returns a string consisting of the characters given by the ASCII code values of those integers. NULL values are skipped: 
mysql> select CHAR(77,121,83,81,'76');
        -> 'MySQL'
mysql> select CHAR(77,77.3,'77.3');
        -> 'MMM'
CONCAT(str1,str2,...)
Returns the string that results from concatenating the arguments. Returns NULL if any argument is NULL. May have more than 2 arguments. A numeric argument is converted to the equivalent string form: 
mysql> select CONCAT('My', 'S', 'QL');
        -> 'MySQL'
mysql> select CONCAT('My', NULL, 'QL');
        -> NULL
mysql> select CONCAT(14.3);
        -> '14.3'
CONCAT_WS(separator, str1, str2,...)
CONCAT_WS() stands for CONCAT With Separator and is a special form of CONCAT(). The first argument is the separator for the rest of the arguments. The separator can be a string as well as the rest of the arguments. If the separator is NULL, the result will be NULL. The function will skip any NULLs and empty strings, after the separator argument. The separator will be added between the strings to be concatenated: 
mysql> select CONCAT_WS(",","First name","Second name","Last Name");
       -> 'First name,Second name,Last Name'
mysql> select CONCAT_WS(",","First name",NULL,"Last Name");
       -> 'First name,Last Name'
LENGTH(str)
OCTET_LENGTH(str)
CHAR_LENGTH(str)
CHARACTER_LENGTH(str)
Returns the length of the string str: 
mysql> select LENGTH('text');
        -> 4
mysql> select OCTET_LENGTH('text');
        -> 4
Note that for CHAR_LENGTH(), multi-byte characters are only counted once.
LOCATE(substr,str)
POSITION(substr IN str)
Returns the position of the first occurrence of substring substr in string str. Returns 0 if substr is not in str: 
mysql> select LOCATE('bar', 'foobarbar');
        -> 4
mysql> select LOCATE('xbar', 'foobar');
        -> 0
This function is multi-byte safe.
LOCATE(substr,str,pos)
Returns the position of the first occurrence of substring substr in string str, starting at position pos. Returns 0 if substr is not in str: 
mysql> select LOCATE('bar', 'foobarbar',5);
        -> 7
This function is multi-byte safe.
INSTR(str,substr)
Returns the position of the first occurrence of substring substr in string str. This is the same as the two-argument form of LOCATE(), except that the arguments are swapped: 
mysql> select INSTR('foobarbar', 'bar');
        -> 4
mysql> select INSTR('xbar', 'foobar');
        -> 0
This function is multi-byte safe.
LPAD(str,len,padstr)
Returns the string str, left-padded with the string padstr until str is len characters long. If str is longer than len' then it will be shortened to len characters. 
mysql> select LPAD('hi',4,'??');
        -> '??hi'
RPAD(str,len,padstr)
Returns the string str, right-padded with the string padstr until str is len characters long. If str is longer than len' then it will be shortened to len characters. 
mysql> select RPAD('hi',5,'?');
        -> 'hi???'
LEFT(str,len)
Returns the leftmost len characters from the string str: 
mysql> select LEFT('foobarbar', 5);
        -> 'fooba'
This function is multi-byte safe.
RIGHT(str,len)
Returns the rightmost len characters from the string str: 
mysql> select RIGHT('foobarbar', 4);
        -> 'rbar'
This function is multi-byte safe.
SUBSTRING(str,pos,len)
SUBSTRING(str FROM pos FOR len)
MID(str,pos,len)
Returns a substring len characters long from string str, starting at position pos. The variant form that uses FROM is ANSI SQL92 syntax: 
mysql> select SUBSTRING('Quadratically',5,6);
        -> 'ratica'
This function is multi-byte safe.
SUBSTRING(str,pos)
SUBSTRING(str FROM pos)
Returns a substring from string str starting at position pos: 
mysql> select SUBSTRING('Quadratically',5);
        -> 'ratically'
mysql> select SUBSTRING('foobarbar' FROM 4);
        -> 'barbar'
This function is multi-byte safe.
SUBSTRING_INDEX(str,delim,count)
Returns the substring from string str before count occurrences of the delimiter delim. If count is positive, everything to the left of the final delimiter (counting from the left) is returned. If count is negative, everything to the right of the final delimiter (counting from the right) is returned: 
mysql> select SUBSTRING_INDEX('www.mysql.com', '.', 2);
        -> 'www.mysql'
mysql> select SUBSTRING_INDEX('www.mysql.com', '.', -2);
        -> 'mysql.com'
This function is multi-byte safe.
LTRIM(str)
Returns the string str with leading space characters removed: 
mysql> select LTRIM('  barbar');
        -> 'barbar'
RTRIM(str)
Returns the string str with trailing space characters removed: 
mysql> select RTRIM('barbar   ');
        -> 'barbar'
This function is multi-byte safe.
TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)
Returns the string str with all remstr prefixes and/or suffixes removed. If none of the specifiers BOTH, LEADING or TRAILING are given, BOTH is assumed. If remstr is not specified, spaces are removed: 
mysql> select TRIM('  bar   ');
        -> 'bar'
mysql> select TRIM(LEADING 'x' FROM 'xxxbarxxx');
        -> 'barxxx'
mysql> select TRIM(BOTH 'x' FROM 'xxxbarxxx');
        -> 'bar'
mysql> select TRIM(TRAILING 'xyz' FROM 'barxxyz');
        -> 'barx'
This function is multi-byte safe.
SOUNDEX(str)
Returns a soundex string from str. Two strings that sound almost the same should have identical soundex strings. A standard soundex string is 4 characters long, but the SOUNDEX() function returns an arbitrarily long string. You can use SUBSTRING() on the result to get a standard soundex string. All non-alphanumeric characters are ignored in the given string. All international alpha characters outside the A-Z range are treated as vowels: 
mysql> select SOUNDEX('Hello');
        -> 'H400'
mysql> select SOUNDEX('Quadratically');
        -> 'Q36324'
SPACE(N)
Returns a string consisting of N space characters: 
mysql> select SPACE(6);
        -> '      '
REPLACE(str,from_str,to_str)
Returns the string str with all all occurrences of the string from_str replaced by the string to_str: 
mysql> select REPLACE('www.mysql.com', 'w', 'Ww');
        -> 'WwWwWw.mysql.com'
This function is multi-byte safe.
REPEAT(str,count)
Returns a string consisting of the string str repeated count times. If count <= 0, returns an empty string. Returns NULL if str or count are NULL: 
mysql> select REPEAT('MySQL', 3);
        -> 'MySQLMySQLMySQL'
REVERSE(str)
Returns the string str with the order of the characters reversed: 
mysql> select REVERSE('abc');
        -> 'cba'
This function is multi-byte safe.
INSERT(str,pos,len,newstr)
Returns the string str, with the substring beginning at position pos and len characters long replaced by the string newstr: 
mysql> select INSERT('Quadratic', 3, 4, 'What');
        -> 'QuWhattic'
This function is multi-byte safe.
ELT(N,str1,str2,str3,...)
Returns str1 if N = 1, str2 if N = 2, and so on. Returns NULL if N is less than 1 or greater than the number of arguments. ELT() is the complement of FIELD(): 
mysql> select ELT(1, 'ej', 'Heja', 'hej', 'foo');
        -> 'ej'
mysql> select ELT(4, 'ej', 'Heja', 'hej', 'foo');
        -> 'foo'
FIELD(str,str1,str2,str3,...)
Returns the index of str in the str1, str2, str3, ... list. Returns 0 if str is not found. FIELD() is the complement of ELT(): 
mysql> select FIELD('ej', 'Hej', 'ej', 'Heja', 'hej', 'foo');
        -> 2
mysql> select FIELD('fo', 'Hej', 'ej', 'Heja', 'hej', 'foo');
        -> 0
FIND_IN_SET(str,strlist)
Returns a value 1 to N if the string str is in the list strlist consisting of N substrings. A string list is a string composed of substrings separated by `,' characters. If the first argument is a constant string and the second is a column of type SET, the FIND_IN_SET() function is optimized to use bit arithmetic! Returns 0 if str is not in strlist or if strlist is the empty string. Returns NULL if either argument is NULL. This function will not work properly if the first argument contains a `,': 
mysql> SELECT FIND_IN_SET('b','a,b,c,d');
        -> 2
MAKE_SET(bits,str1,str2,...)
Returns a set (a string containing substrings separated by `,' characters) consisting of the strings that have the corresponding bit in bits set. str1 corresponds to bit 0, str2 to bit 1, etc. NULL strings in str1, str2, ... are not appended to the result: 
mysql> SELECT MAKE_SET(1,'a','b','c');
        -> 'a'
mysql> SELECT MAKE_SET(1 | 4,'hello','nice','world');
        -> 'hello,world'
mysql> SELECT MAKE_SET(0,'a','b','c');
        -> ''
EXPORT_SET(bits,on,off,[separator,[number_of_bits]])
Returns a string where for every bit set in 'bit', you get an 'on' string and for every reset bit you get an 'off' string. Each string is separated with 'separator' (default ',') and only 'number_of_bits' (default 64) of 'bits' is used: 
mysql> select EXPORT_SET(5,'Y','N',',',4)
        -> Y,N,Y,N
LCASE(str)
LOWER(str)
Returns the string str with all characters changed to lowercase according to the current character set mapping (the default is ISO-8859-1 Latin1): 
mysql> select LCASE('QUADRATICALLY');
        -> 'quadratically'
This function is multi-byte safe.
UCASE(str)
UPPER(str)
Returns the string str with all characters changed to uppercase according to the current character set mapping (the default is ISO-8859-1 Latin1): 
mysql> select UCASE('Hej');
        -> 'HEJ'
This function is multi-byte safe.
LOAD_FILE(file_name)
Reads the file and returns the file contents as a string. The file must be on the server, you must specify the full pathname to the file, and you must have the file privilege. The file must be readable by all and be smaller than max_allowed_packet. If the file doesn't exist or can't be read due to one of the above reasons, the function returns NULL: 
mysql> UPDATE table_name
           SET blob_column=LOAD_FILE("/tmp/picture")
           WHERE id=1;

If you are not using MySQL Version 3.23, you have to do the reading of the file inside your application and create an INSERT statement to update the database with the file information. One way to do this, if you are using the MySQL++ library, can be found at http://www.mysql.com/documentation/mysql++/mysql++-examples.html.

MySQL automatically converts numbers to strings as necessary, and vice-versa: 

mysql> SELECT 1+"1";
        -> 2
mysql> SELECT CONCAT(2,' test');
        -> '2 test'

If you want to convert a number to a string explicitly, pass it as the argument to CONCAT().

If a string function is given a binary string as an argument, the resulting string is also a binary string. A number converted to a string is treated as a binary string. This only affects comparisons.

6.3.2.1 String Comparison Functions

Normally, if any expression in a string comparison is case sensitive, the comparison is performed in case-sensitive fashion.

expr LIKE pat [ESCAPE 'escape-char']
Pattern matching using SQL simple regular expression comparison. Returns 1 (TRUE) or 0 (FALSE). With LIKE you can use the following two wild-card characters in the pattern:
% Matches any number of characters, even zero characters
_ Matches exactly one character 
mysql> select 'David!' LIKE 'David_';
        -> 1
mysql> select 'David!' LIKE '%D%v%';
        -> 1
To test for literal instances of a wild-card character, precede the character with the escape character. If you don't specify the ESCAPE character, `\' is assumed:
\% Matches one % character
\_ Matches one _ character 
mysql> select 'David!' LIKE 'David\_';
        -> 0
mysql> select 'David_' LIKE 'David\_';
        -> 1
To specify a different escape character, use the ESCAPE clause: 
mysql> select 'David_' LIKE 'David|_' ESCAPE '|';
        -> 1
The following two statements illustrate that string comparisons are case insensitive unless one of the operands is a binary string: 
mysql> select 'abc' LIKE 'ABC';
        -> 1
mysql> SELECT 'abc' LIKE BINARY 'ABC';
        -> 0
LIKE is allowed on numeric expressions! (This is a MySQL extension to the ANSI SQL LIKE.) 
mysql> select 10 LIKE '1%';
        -> 1
Note: Because MySQL uses the C escape syntax in strings (for example, `\n'), you must double any `\' that you use in your LIKE strings. For example, to search for `\n', specify it as `\\n'. To search for `\', specify it as `\\\\' (the backslashes are stripped once by the parser and another time when the pattern match is done, leaving a single backslash to be matched).
expr NOT LIKE pat [ESCAPE 'escape-char']
Same as NOT (expr LIKE pat [ESCAPE 'escape-char']).
expr REGEXP pat
expr RLIKE pat
Performs a pattern match of a string expression expr against a pattern pat. The pattern can be an extended regular expression. See section I Description of MySQL regular expression syntax. Returns 1 if expr matches pat, otherwise returns 0. RLIKE is a synonym for REGEXP, provided for mSQL compatibility. Note: Because MySQL uses the C escape syntax in strings (for example, `\n'), you must double any `\' that you use in your REGEXP strings. As of MySQL Version 3.23.4, REGEXP is case insensitive for normal (not binary) strings: 
mysql> select 'Monty!' REGEXP 'm%y%%';
        -> 0
mysql> select 'Monty!' REGEXP '.*';
        -> 1
mysql> select 'new*\n*line' REGEXP 'new\\*.\\*line';
        -> 1
mysql> select "a" REGEXP "A", "a" REGEXP BINARY "A";
        -> 1  0
mysql> select "a" REGEXP "^[a-d]";
        -> 1
REGEXP and RLIKE use the current character set (ISO-8859-1 Latin1 by default) when deciding the type of a character.
expr NOT REGEXP pat
expr NOT RLIKE pat
Same as NOT (expr REGEXP pat).
STRCMP(expr1,expr2)
STRCMP() returns 0 if the strings are the same, -1 if the first argument is smaller than the second according to the current sort order, and 1 otherwise: 
mysql> select STRCMP('text', 'text2');
        -> -1
mysql> select STRCMP('text2', 'text');
        -> 1
mysql> select STRCMP('text', 'text');
        -> 0
MATCH (col1,col2,...) AGAINST (expr)
MATCH ... AGAINST() is used for full-text search and returns relevance - similarity measure between the text in columns (col1,col2,...) and the query expr. Relevance is a positive floating-point number. Zero relevance means no similarity. For MATCH ... AGAINST() to work, a FULLTEXT index must be created first. See section 6.5.3 CREATE TABLE Syntax. MATCH ... AGAINST() is available in MySQL Version 3.23.23 or later. For details and usage examples see section 6.8 MySQL Full-text Search.

6.3.2.2 Case Sensitivity

BINARY
The BINARY operator casts the string following it to a binary string. This is an easy way to force a column comparison to be case sensitive even if the column isn't defined as BINARY or BLOB: 
mysql> select "a" = "A";
        -> 1
mysql> select BINARY "a" = "A";
        -> 0
BINARY was introduced in MySQL Version 3.23.0. Note that in some context MySQL will not be able to use the index efficiently when you cast an indexed column to BINARY.

If you want to compare a blob case-insensitively you can always convert the blob to upper case before doing the comparison: 

SELECT 'A' LIKE UPPER(blob_col) FROM table_name;

We plan to soon introduce casting between different character sets to make string comparison even more flexible.

6.3.3 Numeric Functions

6.3.3.1 Arithmetic Operations

The usual arithmetic operators are available. Note that in the case of `-', `+', and `*', the result is calculated with BIGINT (64-bit) precision if both arguments are integers!

+
Addition: 
mysql> select 3+5;
        -> 8
-
Subtraction: 
mysql> select 3-5;
        -> -2
*
Multiplication: 
mysql> select 3*5;
        -> 15
mysql> select 18014398509481984*18014398509481984.0;
        -> 324518553658426726783156020576256.0
mysql> select 18014398509481984*18014398509481984;
        -> 0
The result of the last expression is incorrect because the result of the integer multiplication exceeds the 64-bit range of BIGINT calculations.
/
Division: 
mysql> select 3/5;
        -> 0.60
Division by zero produces a NULL result: 
mysql> select 102/(1-1);
        -> NULL
A division will be calculated with BIGINT arithmetic only if performed in a context where its result is converted to an integer!

6.3.3.2 Mathematical Functions

All mathematical functions return NULL in case of an error.

-
Unary minus. Changes the sign of the argument: 
mysql> select - 2;
        -> -2
Note that if this operator is used with a BIGINT, the return value is a BIGINT! This means that you should avoid using - on integers that may have the value of -2^63!
ABS(X)
Returns the absolute value of X: 
mysql> select ABS(2);
        -> 2
mysql> select ABS(-32);
        -> 32
This function is safe to use with BIGINT values.
SIGN(X)
Returns the sign of the argument as -1, 0, or 1, depending on whether X is negative, zero, or positive: 
mysql> select SIGN(-32);
        -> -1
mysql> select SIGN(0);
        -> 0
mysql> select SIGN(234);
        -> 1
MOD(N,M)
%
Modulo (like the % operator in C). Returns the remainder of N divided by M: 
mysql> select MOD(234, 10);
        -> 4
mysql> select 253 % 7;
        -> 1
mysql> select MOD(29,9);
        -> 2
This function is safe to use with BIGINT values.
FLOOR(X)
Returns the largest integer value not greater than X: 
mysql> select FLOOR(1.23);
        -> 1
mysql> select FLOOR(-1.23);
        -> -2
Note that the return value is converted to a BIGINT!
CEILING(X)
Returns the smallest integer value not less than X: 
mysql> select CEILING(1.23);
        -> 2
mysql> select CEILING(-1.23);
        -> -1
Note that the return value is converted to a BIGINT!
ROUND(X)
Returns the argument X, rounded to the nearest integer: 
mysql> select ROUND(-1.23);
        -> -1
mysql> select ROUND(-1.58);
        -> -2
mysql> select ROUND(1.58);
        -> 2
Note that the behavior of ROUND() when the argument is half way between two integers depends on the C library implementation. Some round to the nearest even number, always up, always down, or always towards zero. If you need one kind of rounding, you should use a well-defined function like TRUNCATE() or FLOOR() instead.
ROUND(X,D)
Returns the argument X, rounded to a number with D decimals. If D is 0, the result will have no decimal point or fractional part: 
mysql> select ROUND(1.298, 1);
        -> 1.3
mysql> select ROUND(1.298, 0);
        -> 1
EXP(X)
Returns the value of e (the base of natural logarithms) raised to the power of X: 
mysql> select EXP(2);
        -> 7.389056
mysql> select EXP(-2);
        -> 0.135335
LOG(X)
Returns the natural logarithm of X: 
mysql> select LOG(2);
        -> 0.693147
mysql> select LOG(-2);
        -> NULL
If you want the log of a number X to some arbitary base B, use the formula LOG(X)/LOG(B).
LOG10(X)
Returns the base-10 logarithm of X: 
mysql> select LOG10(2);
        -> 0.301030
mysql> select LOG10(100);
        -> 2.000000
mysql> select LOG10(-100);
        -> NULL
POW(X,Y)
POWER(X,Y)
Returns the value of X raised to the power of Y: 
mysql> select POW(2,2);
        -> 4.000000
mysql> select POW(2,-2);
        -> 0.250000
SQRT(X)
Returns the non-negative square root of X: 
mysql> select SQRT(4);
        -> 2.000000
mysql> select SQRT(20);
        -> 4.472136
PI()
Returns the value of PI. The default shown number of decimals is 5, but MySQL internally uses the full double precession for PI. 
mysql> select PI();
        -> 3.141593
mysql> SELECT PI()+0.000000000000000000;
        -> 3.141592653589793116
COS(X)
Returns the cosine of X, where X is given in radians: 
mysql> select COS(PI());
        -> -1.000000
SIN(X)
Returns the sine of X, where X is given in radians: 
mysql> select SIN(PI());
        -> 0.000000
TAN(X)
Returns the tangent of X, where X is given in radians: 
mysql> select TAN(PI()+1);
        -> 1.557408
ACOS(X)
Returns the arc cosine of X, that is, the value whose cosine is X. Returns NULL if X is not in the range -1 to 1: 
mysql> select ACOS(1);
        -> 0.000000
mysql> select ACOS(1.0001);
        -> NULL
mysql> select ACOS(0);
        -> 1.570796
ASIN(X)
Returns the arc sine of X, that is, the value whose sine is X. Returns NULL if X is not in the range -1 to 1: 
mysql> select ASIN(0.2);
        -> 0.201358
mysql> select ASIN('foo');
        -> 0.000000
ATAN(X)
Returns the arc tangent of X, that is, the value whose tangent is X: 
mysql> select ATAN(2);
        -> 1.107149
mysql> select ATAN(-2);
        -> -1.107149
ATAN(Y,X)
ATAN2(Y,X)
Returns the arc tangent of the two variables X and Y. It is similar to calculating the arc tangent of Y / X, except that the signs of both arguments are used to determine the quadrant of the result: 
mysql> select ATAN(-2,2);
        -> -0.785398
mysql> select ATAN2(PI(),0);
        -> 1.570796
COT(X)
Returns the cotangent of X: 
mysql> select COT(12);
        -> -1.57267341
mysql> select COT(0);
        -> NULL
RAND()
RAND(N)
Returns a random floating-point value in the range 0 to 1.0. If an integer argument N is specified, it is used as the seed value: 
mysql> select RAND();
        -> 0.5925
mysql> select RAND(20);
        -> 0.1811
mysql> select RAND(20);
        -> 0.1811
mysql> select RAND();
        -> 0.2079
mysql> select RAND();
        -> 0.7888
You can't use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times. In MySQL Version 3.23, you can, however, do: SELECT * FROM table_name ORDER BY RAND() This is useful to get a random sample of a set SELECT * FROM table1,table2 WHERE a=b AND c<d ORDER BY RAND() LIMIT 1000. Note that a RAND() in a WHERE clause will be re-evaluated every time the WHERE is executed.
LEAST(X,Y,...)
With two or more arguments, returns the smallest (minimum-valued) argument. The arguments are compared using the following rules: 
mysql> select LEAST(2,0);
        -> 0
mysql> select LEAST(34.0,3.0,5.0,767.0);
        -> 3.0
mysql> select LEAST("B","A","C");
        -> "A"
In MySQL versions prior to Version 3.22.5, you can use MIN() instead of LEAST.
GREATEST(X,Y,...)
Returns the largest (maximum-valued) argument. The arguments are compared using the same rules as for LEAST: 
mysql> select GREATEST(2,0);
        -> 2
mysql> select GREATEST(34.0,3.0,5.0,767.0);
        -> 767.0
mysql> select GREATEST("B","A","C");
        -> "C"
In MySQL versions prior to Version 3.22.5, you can use MAX() instead of GREATEST.
DEGREES(X)
Returns the argument X, converted from radians to degrees: 
mysql> select DEGREES(PI());
        -> 180.000000
RADIANS(X)
Returns the argument X, converted from degrees to radians: 
mysql> select RADIANS(90);
        -> 1.570796
TRUNCATE(X,D)
Returns the number X, truncated to D decimals. If D is 0, the result will have no decimal point or fractional part: 
mysql> select TRUNCATE(1.223,1);
        -> 1.2
mysql> select TRUNCATE(1.999,1);
        -> 1.9
mysql> select TRUNCATE(1.999,0);
        -> 1
Note that as decimal numbers are normally not stored as exact numbers in computers, but as double values, you may be fooled by the following result: 
mysql> select TRUNCATE(10.28*100,0);
       -> 1027
The above happens because 10.28 is actually stored as something like 10.2799999999999999.

6.3.4 Date and Time Functions

See section 6.2.2 Date and Time Types for a description of the range of values each type has and the valid formats in which date and time values may be specified.

Here is an example that uses date functions. The query below selects all records with a date_col value from within the last 30 days: 

mysql> SELECT something FROM table
           WHERE TO_DAYS(NOW()) - TO_DAYS(date_col) <= 30;
DAYOFWEEK(date)
Returns the weekday index for date (1 = Sunday, 2 = Monday, ... 7 = Saturday). These index values correspond to the ODBC standard: 
mysql> select DAYOFWEEK('1998-02-03');
        -> 3
WEEKDAY(date)
Returns the weekday index for date (0 = Monday, 1 = Tuesday, ... 6 = Sunday): 
mysql> select WEEKDAY('1997-10-04 22:23:00');
        -> 5
mysql> select WEEKDAY('1997-11-05');
        -> 2
DAYOFMONTH(date)
Returns the day of the month for date, in the range 1 to 31: 
mysql> select DAYOFMONTH('1998-02-03');
        -> 3
DAYOFYEAR(date)
Returns the day of the year for date, in the range 1 to 366: 
mysql> select DAYOFYEAR('1998-02-03');
        -> 34
MONTH(date)
Returns the month for date, in the range 1 to 12: 
mysql> select MONTH('1998-02-03');
        -> 2
DAYNAME(date)
Returns the name of the weekday for date: 
mysql> select DAYNAME("1998-02-05");
        -> 'Thursday'
MONTHNAME(date)
Returns the name of the month for date: 
mysql> select MONTHNAME("1998-02-05");
        -> 'February'
QUARTER(date)
Returns the quarter of the year for date, in the range 1 to 4: 
mysql> select QUARTER('98-04-01');
        -> 2
WEEK(date)
WEEK(date,first)
With a single argument, returns the week for date, in the range 0 to 53 (yes, there may be the beginnings of a week 53), for locations where Sunday is the first day of the week. The two-argument form of WEEK() allows you to specify whether the week starts on Sunday or Monday. The week starts on Sunday if the second argument is 0, on Monday if the second argument is 1: 
mysql> select WEEK('1998-02-20');
        -> 7
mysql> select WEEK('1998-02-20',0);
        -> 7
mysql> select WEEK('1998-02-20',1);
        -> 8
mysql> select WEEK('1998-12-31',1);
        -> 53
YEAR(date)
Returns the year for date, in the range 1000 to 9999: 
mysql> select YEAR('98-02-03');
        -> 1998
YEARWEEK(date)
YEARWEEK(date,first)
Returns year and week for a date. The second arguments works exactly like the second argument to WEEK(). Note that the year may be different from the year in the date argument for the first and the last week of the year: 
mysql> select YEARWEEK('1987-01-01');
        -> 198653
HOUR(time)
Returns the hour for time, in the range 0 to 23: 
mysql> select HOUR('10:05:03');
        -> 10
MINUTE(time)
Returns the minute for time, in the range 0 to 59: 
mysql> select MINUTE('98-02-03 10:05:03');
        -> 5
SECOND(time)
Returns the second for time, in the range 0 to 59: 
mysql> select SECOND('10:05:03');
        -> 3
PERIOD_ADD(P,N)
Adds N months to period P (in the format YYMM or YYYYMM). Returns a value in the format YYYYMM. Note that the period argument P is not a date value: 
mysql> select PERIOD_ADD(9801,2);
        -> 199803
PERIOD_DIFF(P1,P2)
Returns the number of months between periods P1 and P2. P1 and P2 should be in the format YYMM or YYYYMM. Note that the period arguments P1 and P2 are not date values: 
mysql> select PERIOD_DIFF(9802,199703);
        -> 11
DATE_ADD(date,INTERVAL expr type)
DATE_SUB(date,INTERVAL expr type)
ADDDATE(date,INTERVAL expr type)
SUBDATE(date,INTERVAL expr type)
These functions perform date arithmetic. They are new for MySQL Version 3.22. ADDDATE() and SUBDATE() are synonyms for DATE_ADD() and DATE_SUB(). In MySQL Version 3.23, you can use + and - instead of DATE_ADD() and DATE_SUB() if the expression on the right side is a date or datetime column. (See example) date is a DATETIME or DATE value specifying the starting date. expr is an expression specifying the interval value to be added or subtracted from the starting date. expr is a string; it may start with a `-' for negative intervals. type is a keyword indicating how the expression should be interpreted. The related function EXTRACT(type FROM date) returns the 'type' interval from the date. The following table shows how the type and expr arguments are related:
type value Expected expr format
SECOND SECONDS
MINUTE MINUTES
HOUR HOURS
DAY DAYS
MONTH MONTHS
YEAR YEARS
MINUTE_SECOND "MINUTES:SECONDS"
HOUR_MINUTE "HOURS:MINUTES"
DAY_HOUR "DAYS HOURS"
YEAR_MONTH "YEARS-MONTHS"
HOUR_SECOND "HOURS:MINUTES:SECONDS"
DAY_MINUTE "DAYS HOURS:MINUTES"
DAY_SECOND "DAYS HOURS:MINUTES:SECONDS"
MySQL allows any punctuation delimiter in the expr format. Those shown in the table are the suggested delimiters. If the date argument is a DATE value and your calculations involve only YEAR, MONTH, and DAY parts (that is, no time parts), the result is a DATE value. Otherwise the result is a DATETIME value: 
mysql> SELECT "1997-12-31 23:59:59" + INTERVAL 1 SECOND;
        -> 1998-01-01 00:00:00
mysql> SELECT INTERVAL 1 DAY + "1997-12-31";
        -> 1998-01-01
mysql> SELECT "1998-01-01" - INTERVAL 1 SECOND;
       -> 1997-12-31 23:59:59
mysql> SELECT DATE_ADD("1997-12-31 23:59:59",
                       INTERVAL 1 SECOND);
        -> 1998-01-01 00:00:00
mysql> SELECT DATE_ADD("1997-12-31 23:59:59",
                       INTERVAL 1 DAY);
        -> 1998-01-01 23:59:59
mysql> SELECT DATE_ADD("1997-12-31 23:59:59",
                       INTERVAL "1:1" MINUTE_SECOND);
        -> 1998-01-01 00:01:00
mysql> SELECT DATE_SUB("1998-01-01 00:00:00",
                       INTERVAL "1 1:1:1" DAY_SECOND);
        -> 1997-12-30 22:58:59
mysql> SELECT DATE_ADD("1998-01-01 00:00:00",
                       INTERVAL "-1 10" DAY_HOUR);
        -> 1997-12-30 14:00:00
mysql> SELECT DATE_SUB("1998-01-02", INTERVAL 31 DAY);
        -> 1997-12-02
If you specify an interval value that is too short (does not include all the interval parts that would be expected from the type keyword), MySQL assumes you have left out the leftmost parts of the interval value. For example, if you specify a type of DAY_SECOND, the value of expr is expected to have days, hours, minutes, and seconds parts. If you specify a value like "1:10", MySQL assumes that the days and hours parts are missing and the value represents minutes and seconds. In other words, "1:10" DAY_SECOND is interpreted in such a way that it is equivalent to "1:10" MINUTE_SECOND. This is analogous to the way that MySQL interprets TIME values as representing elapsed time rather than as time of day. Note that if you add or subtract a date value against something that contains a time part, the date value will be automatically converted to a datetime value: 
mysql> select date_add("1999-01-01", interval 1 day);
       -> 1999-01-02
mysql> select date_add("1999-01-01", interval 1 hour);
       -> 1999-01-01 01:00:00
If you use really incorrect dates, the result is NULL. If you add MONTH, YEAR_MONTH, or YEAR and the resulting date has a day that is larger than the maximum day for the new month, the day is adjusted to the maximum days in the new month: 
mysql> select DATE_ADD('1998-01-30', Interval 1 month);
        -> 1998-02-28
Note from the preceding example that the word INTERVAL and the type keyword are not case sensitive.
EXTRACT(type FROM date)
The EXTRACT() function uses the same kinds of interval type specifiers as DATE_ADD() or DATE_SUB(), but extracts parts from the date rather than performing date arithmetic. 
mysql> SELECT EXTRACT(YEAR FROM "1999-07-02");
       -> 1999
mysql> SELECT EXTRACT(YEAR_MONTH FROM "1999-07-02 01:02:03");
       -> 199907
mysql> SELECT EXTRACT(DAY_MINUTE FROM "1999-07-02 01:02:03");
       -> 20102
TO_DAYS(date)
Given a date date, returns a daynumber (the number of days since year 0): 
mysql> select TO_DAYS(950501);
        -> 728779
mysql> select TO_DAYS('1997-10-07');
        -> 729669
TO_DAYS() is not intended for use with values that precede the advent of the Gregorian calendar (1582), because it doesn't take into account the days that were lost when the calendar was changed.
FROM_DAYS(N)
Given a daynumber N, returns a DATE value: 
mysql> select FROM_DAYS(729669);
        -> '1997-10-07'
FROM_DAYS() is not intended for use with values that precede the advent of the Gregorian calendar (1582), because it doesn't take into account the days that were lost when the calendar was changed.
DATE_FORMAT(date,format)
Formats the date value according to the format string. The following specifiers may be used in the format string:
%M Month name (January..December)
%W Weekday name (Sunday..Saturday)
%D Day of the month with English suffix (1st, 2nd, 3rd, etc.)
%Y Year, numeric, 4 digits
%y Year, numeric, 2 digits
%X Year for the week where Sunday is the first day of the week, numeric, 4 digits, used with '%V'
%x Year for the week, where Monday is the first day of the week, numeric, 4 digits, used with '%v'
%a Abbreviated weekday name (Sun..Sat)
%d Day of the month, numeric (00..31)
%e Day of the month, numeric (0..31)
%m Month, numeric (01..12)
%c Month, numeric (1..12)
%b Abbreviated month name (Jan..Dec)
%j Day of year (001..366)
%H Hour (00..23)
%k Hour (0..23)
%h Hour (01..12)
%I Hour (01..12)
%l Hour (1..12)
%i Minutes, numeric (00..59)
%r Time, 12-hour (hh:mm:ss [AP]M)
%T Time, 24-hour (hh:mm:ss)
%S Seconds (00..59)
%s Seconds (00..59)
%p AM or PM
%w Day of the week (0=Sunday..6=Saturday)
%U Week (0..53), where Sunday is the first day of the week
%u Week (0..53), where Monday is the first day of the week
%V Week (1..53), where Sunday is the first day of the week. Used with '%X'
%v Week (1..53), where Monday is the first day of the week. Used with '%x'
%% A literal `%'.
All other characters are just copied to the result without interpretation: 
mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%W %M %Y');
        -> 'Saturday October 1997'
mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%H:%i:%s');
        -> '22:23:00'
mysql> select DATE_FORMAT('1997-10-04 22:23:00',
                          '%D %y %a %d %m %b %j');
        -> '4th 97 Sat 04 10 Oct 277'
mysql> select DATE_FORMAT('1997-10-04 22:23:00',
                          '%H %k %I %r %T %S %w');
        -> '22 22 10 10:23:00 PM 22:23:00 00 6'
mysql> select DATE_FORMAT('1999-01-01', '%X %V');
        -> '1998 52'
As of MySQL Version 3.23, the `%' character is required before format specifier characters. In earlier versions of MySQL, `%' was optional.
TIME_FORMAT(time,format)
This is used like the DATE_FORMAT() function above, but the format string may contain only those format specifiers that handle hours, minutes, and seconds. Other specifiers produce a NULL value or 0.
CURDATE()
CURRENT_DATE
Returns today's date as a value in 'YYYY-MM-DD' or YYYYMMDD format, depending on whether the function is used in a string or numeric context: 
mysql> select CURDATE();
        -> '1997-12-15'
mysql> select CURDATE() + 0;
        -> 19971215
CURTIME()
CURRENT_TIME
Returns the current time as a value in 'HH:MM:SS' or HHMMSS format, depending on whether the function is used in a string or numeric context: 
mysql> select CURTIME();
        -> '23:50:26'
mysql> select CURTIME() + 0;
        -> 235026
NOW()
SYSDATE()
CURRENT_TIMESTAMP
Returns the current date and time as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context: 
mysql> select NOW();
        -> '1997-12-15 23:50:26'
mysql> select NOW() + 0;
        -> 19971215235026
UNIX_TIMESTAMP()
UNIX_TIMESTAMP(date)
If called with no argument, returns a Unix timestamp (seconds since '1970-01-01 00:00:00' GMT). If UNIX_TIMESTAMP() is called with a date argument, it returns the value of the argument as seconds since '1970-01-01 00:00:00' GMT. date may be a DATE string, a DATETIME string, a TIMESTAMP, or a number in the format YYMMDD or YYYYMMDD in local time: 
mysql> select UNIX_TIMESTAMP();
        -> 882226357
mysql> select UNIX_TIMESTAMP('1997-10-04 22:23:00');
        -> 875996580
When UNIX_TIMESTAMP is used on a TIMESTAMP column, the function will receive the value directly, with no implicit ``string-to-unix-timestamp'' conversion. If you give UNIX_TIMESTAMP() a wrong or out-of-range date, it will return 0.
FROM_UNIXTIME(unix_timestamp)
Returns a representation of the unix_timestamp argument as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context: 
mysql> select FROM_UNIXTIME(875996580);
        -> '1997-10-04 22:23:00'
mysql> select FROM_UNIXTIME(875996580) + 0;
        -> 19971004222300
FROM_UNIXTIME(unix_timestamp,format)
Returns a string representation of the Unix timestamp, formatted according to the format string. format may contain the same specifiers as those listed in the entry for the DATE_FORMAT() function: 
mysql> select FROM_UNIXTIME(UNIX_TIMESTAMP(),
                            '%Y %D %M %h:%i:%s %x');
        -> '1997 23rd December 03:43:30 x'
SEC_TO_TIME(seconds)
Returns the seconds argument, converted to hours, minutes, and seconds, as a value in 'HH:MM:SS' or HHMMSS format, depending on whether the function is used in a string or numeric context: 
mysql> select SEC_TO_TIME(2378);
        -> '00:39:38'
mysql> select SEC_TO_TIME(2378) + 0;
        -> 3938
TIME_TO_SEC(time)
Returns the time argument, converted to seconds: 
mysql> select TIME_TO_SEC('22:23:00');
        -> 80580
mysql> select TIME_TO_SEC('00:39:38');
        -> 2378

6.3.5 Other Functions

6.3.5.1 Bit Functions

MySQL uses BIGINT (64-bit) arithmetic for bit operations, so these operators have a maximum range of 64 bits.

|
Bitwise OR: 
mysql> select 29 | 15;
        -> 31
&
Bitwise AND: 
mysql> select 29 & 15;
        -> 13
<<
Shifts a longlong (BIGINT) number to the left: 
mysql> select 1 << 2;
        -> 4
>>
Shifts a longlong (BIGINT) number to the right: 
mysql> select 4 >> 2;
        -> 1
~
Invert all bits: 
mysql> select 5 & ~1;
        -> 4
BIT_COUNT(N)
Returns the number of bits that are set in the argument N: 
mysql> select BIT_COUNT(29);
        -> 4

6.3.5.2 Miscellaneous Functions

DATABASE()
Returns the current database name: 
mysql> select DATABASE();
        -> 'test'
If there is no current database, DATABASE() returns the empty string.
USER()
SYSTEM_USER()
SESSION_USER()
Returns the current MySQL user name: 
mysql> select USER();
        -> 'davida@localhost'
In MySQL Version 3.22.11 or later, this includes the client hostname as well as the user name. You can extract just the user name part like this (which works whether or not the value includes a hostname part): 
mysql> select substring_index(USER(),"@",1);
        -> 'davida'
PASSWORD(str)
Calculates a password string from the plaintext password str. This is the function that is used for encrypting MySQL passwords for storage in the Password column of the user grant table: 
mysql> select PASSWORD('badpwd');
        -> '7f84554057dd964b'
PASSWORD() encryption is non-reversible. PASSWORD() does not perform password encryption in the same way that Unix passwords are encrypted. You should not assume that if your Unix password and your MySQL password are the same, PASSWORD() will result in the same encrypted value as is stored in the Unix password file. See ENCRYPT().
ENCRYPT(str[,salt])
Encrypt str using the Unix crypt() system call. The salt argument should be a string with two characters. (As of MySQL Version 3.22.16, salt may be longer than two characters.): 
mysql> select ENCRYPT("hello");
        -> 'VxuFAJXVARROc'
If crypt() is not available on your system, ENCRYPT() always returns NULL. ENCRYPT() ignores all but the first 8 characters of str, at least on some systems. This will be determined by the behavior of the underlying crypt() system call.
ENCODE(str,pass_str)
Encrypt str using pass_str as the password. To decrypt the result, use DECODE(). The results is a binary string of the same length as string. If you want to save it in a column, use a BLOB column type.
DECODE(crypt_str,pass_str)
Descrypts the encrypted string crypt_str using pass_str as the password. crypt_str should be a string returned from ENCODE().
MD5(string)
Calculates a MD5 checksum for the string. Value is returned as a 32 long hex number that may, for example, be used as a hash key: 
mysql> select MD5("testing");
        -> 'ae2b1fca515949e5d54fb22b8ed95575'
This is an "RSA Data Security, Inc. MD5 Message-Digest Algorithm".
LAST_INSERT_ID([expr])
Returns the last automatically generated value that was inserted into an AUTO_INCREMENT column. See section 8.4.3.126 mysql_insert_id(). 
mysql> select LAST_INSERT_ID();
        -> 195
The last ID that was generated is maintained in the server on a per-connection basis. It will not be changed by another client. It will not even be changed if you update another AUTO_INCREMENT column with a non-magic value (that is, a value that is not NULL and not 0). If you insert many rows at the same time with an insert statement, LAST_INSERT_ID() returns the value for the first inserted row. The reason for this is so that you it makes it possible to easily reproduce the same INSERT statement against some other server. If expr is given as an argument to LAST_INSERT_ID(), then the value of the argument is returned by the function, is set as the next value to be returned by LAST_INSERT_ID() and used as the next auto_increment value. This can be used to simulate sequences: First create the table: 
mysql> create table sequence (id int not null);
mysql> insert into sequence values (0);
Then the table can be used to generate sequence numbers like this: 
mysql> update sequence set id=LAST_INSERT_ID(id+1);
You can generate sequences without calling LAST_INSERT_ID(), but the utility of using the function this way is that the ID value is maintained in the server as the last automatically generated value. You can retrieve the new ID as you would read any normal AUTO_INCREMENT value in MySQL. For example, LAST_INSERT_ID() (without an argument) will return the new ID. The C API function mysql_insert_id() can also be used to get the value. Note that as mysql_insert_id() is only updated after INSERT and UPDATE statements, you can't use this function to retrieve the value for LAST_INSERT_ID(expr) after executing other SQL statements like SELECT or SET.
FORMAT(X,D)
Formats the number X to a format like '#,###,###.##', rounded to D decimals. If D is 0, the result will have no decimal point or fractional part: 
mysql> select FORMAT(12332.123456, 4);
        -> '12,332.1235'
mysql> select FORMAT(12332.1,4);
        -> '12,332.1000'
mysql> select FORMAT(12332.2,0);
        -> '12,332'
VERSION()
Returns a string indicating the MySQL server version: 
mysql> select VERSION();
        -> '3.23.13-log'
Note that if your version ends with -log this means that logging is enabled.
CONNECTION_ID()
Returns the connection id (thread_id) for the connection. Every connection has its own unique id: 
mysql> select CONNECTION_ID();
        -> 1
GET_LOCK(str,timeout)
Tries to obtain a lock with a name given by the string str, with a timeout of timeout seconds. Returns 1 if the lock was obtained successfully, 0 if the attempt timed out, or NULL if an error occurred (such as running out of memory or the thread was killed with mysqladmin kill). A lock is released when you execute RELEASE_LOCK(), execute a new GET_LOCK(), or the thread terminates. This function can be used to implement application locks or to simulate record locks. It blocks requests by other clients for locks with the same name; clients that agree on a given lock string name can use the string to perform cooperative advisory locking: 
mysql> select GET_LOCK("lock1",10);
        -> 1
mysql> select GET_LOCK("lock2",10);
        -> 1
mysql> select RELEASE_LOCK("lock2");
        -> 1
mysql> select RELEASE_LOCK("lock1");
        -> NULL
Note that the second RELEASE_LOCK() call returns NULL because the lock "lock1" was automatically released by the second GET_LOCK() call.
RELEASE_LOCK(str)
Releases the lock named by the string str that was obtained with GET_LOCK(). Returns 1 if the lock was released, 0 if the lock wasn't locked by this thread (in which case the lock is not released), and NULL if the named lock didn't exist. The lock will not exist if it was never obtained by a call to GET_LOCK() or if it already has been released. The DO statement is convinient to use with RELEASE_LOCK(). See section 6.4.9 DO Syntax.
BENCHMARK(count,expr)
The BENCHMARK() function executes the expression expr repeatedly count times. It may be used to time how fast MySQL processes the expression. The result value is always 0. The intended use is in the mysql client, which reports query execution times: 
mysql> select BENCHMARK(1000000,encode("hello","goodbye"));
+----------------------------------------------+
| BENCHMARK(1000000,encode("hello","goodbye")) |
+----------------------------------------------+
|                                            0 |
+----------------------------------------------+
1 row in set (4.74 sec)
The time reported is elapsed time on the client end, not CPU time on the server end. It may be advisable to execute BENCHMARK() several times, and interpret the result with regard to how heavily loaded the server machine is.
INET_NTOA(expr)
Returns the network address (4 or 8 byte) for the numeric expression: 
mysql> select INET_NTOA(3520061480);
       ->  "209.207.224.40"
INET_ATON(expr)
Returns an integer that represents the numeric value for a network address. Addresses may be 4 or 8 byte addresses: 
mysql> select INET_ATON("209.207.224.40");
       ->  3520061480
The generated number is always in network byte order; For example the above number is calculated as 209*255^3 + 207*255^2 + 224*255 +40.
MASTER_POS_WAIT(log_name, log_pos)
Blocks until the slave reaches the specified position in the master log during replication. If master information is not initialized, returns NULL. If the slave is not running, will block and wait until it is started and goes to or past the specified position. If the slave is already past the specified position, returns immediately. The return value is the number of log events it had to wait to get to the specified position, or NULL in case of error. Useful for control of master-slave synchronization, but was originally written to facilitate replication testing.

6.4 Data Manipulation: SELECT, INSERT, UPDATE, DELETE

6.4.1 SELECT Syntax

SELECT [STRAIGHT_JOIN] [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
       [HIGH_PRIORITY]
       [DISTINCT | DISTINCTROW | ALL]
    select_expression,...
    [INTO {OUTFILE | DUMPFILE} 'file_name' export_options]
    [FROM table_references
        [WHERE where_definition]
        [GROUP BY {unsigned_integer | col_name | formula} [ASC | DESC], ...]
        [HAVING where_definition]
        [ORDER BY {unsigned_integer | col_name | formula} [ASC | DESC] ,...]
        [LIMIT [offset,] rows]
        [PROCEDURE procedure_name]
        [FOR UPDATE | LOCK IN SHARE MODE]]

SELECT is used to retrieve rows selected from one or more tables. select_expression indicates the columns you want to retrieve. SELECT may also be used to retrieve rows computed without reference to any table. For example: 

mysql> SELECT 1 + 1;
         -> 2

All keywords used must be given in exactly the order shown above. For example, a HAVING clause must come after any GROUP BY clause and before any ORDER BY clause.

6.4.1.1 JOIN Syntax

MySQL supports the following JOIN syntaxes for use in SELECT statements: 

table_reference, table_reference
table_reference [CROSS] JOIN table_reference
table_reference INNER JOIN table_reference join_condition
table_reference STRAIGHT_JOIN table_reference
table_reference LEFT [OUTER] JOIN table_reference join_condition
table_reference LEFT [OUTER] JOIN table_reference
table_reference NATURAL [LEFT [OUTER]] JOIN table_reference
{ oj table_reference LEFT OUTER JOIN table_reference ON conditional_expr }
table_reference RIGHT [OUTER] JOIN table_reference join_condition
table_reference RIGHT [OUTER] JOIN table_reference
table_reference NATURAL [RIGHT [OUTER]] JOIN table_reference

Where table_reference is defined as: 

table_name [[AS] alias] [USE INDEX (key_list)] [IGNORE INDEX (key_list)]

and join_condition is defined as: 

ON conditional_expr |
USING (column_list)

You should never have any conditions in the ON part that are used to restrict which rows you have in the result set. If you want to restrict which rows should be in the result, you have to do this in the WHERE clause.

Note that in versions before Version 3.23.17, the INNER JOIN didn't take a join_condition!

The last LEFT OUTER JOIN syntax shown above exists only for compatibility with ODBC:

Some examples: 

mysql> select * from table1,table2 where table1.id=table2.id;
mysql> select * from table1 LEFT JOIN table2 ON table1.id=table2.id;
mysql> select * from table1 LEFT JOIN table2 USING (id);
mysql> select * from table1 LEFT JOIN table2 ON table1.id=table2.id
           LEFT JOIN table3 ON table2.id=table3.id;
mysql> select * from table1 USE INDEX (key1,key2) WHERE key1=1 and key2=2 AND
       key3=3;
mysql> select * from table1 IGNORE INDEX (key3) WHERE key1=1 and key2=2 AND
       key3=3;

See section 5.2.6 How MySQL Optimizes LEFT JOIN and RIGHT JOIN.

6.4.2 INSERT Syntax

    INSERT [LOW_PRIORITY | DELAYED] [IGNORE]
        [INTO] tbl_name [(col_name,...)]
        VALUES (expression,...),(...),...
or  INSERT [LOW_PRIORITY | DELAYED] [IGNORE]
        [INTO] tbl_name [(col_name,...)]
        SELECT ...
or  INSERT [LOW_PRIORITY | DELAYED] [IGNORE]
        [INTO] tbl_name
        SET col_name=expression, col_name=expression, ...

INSERT inserts new rows into an existing table. The INSERT ... VALUES form of the statement inserts rows based on explicitly specified values. The INSERT ... SELECT form inserts rows selected from another table or tables. The INSERT ... VALUES form with multiple value lists is supported in MySQL Version 3.22.5 or later. The col_name=expression syntax is supported in MySQL Version 3.22.10 or later.

tbl_name is the table into which rows should be inserted. The column name list or the SET clause indicates which columns the statement specifies values for:

If you use INSERT ... SELECT or an INSERT ... VALUES statement with multiple value lists, you can use the C API function mysql_info() to get information about the query. The format of the information string is shown below: 

Records: 100 Duplicates: 0 Warnings: 0

Duplicates indicates the number of rows that couldn't be inserted because they would duplicate some existing unique index value. Warnings indicates the number of attempts to insert column values that were problematic in some way. Warnings can occur under any of the following conditions:

6.4.2.1 INSERT ... SELECT Syntax

INSERT [LOW_PRIORITY] [IGNORE] [INTO] tbl_name [(column list)] SELECT ...

With INSERT ... SELECT statement you can quickly insert many rows into a table from one or many tables. 

INSERT INTO tblTemp2 (fldID) SELECT tblTemp1.fldOrder_ID FROM tblTemp1 WHERE
tblTemp1.fldOrder_ID > 100;

The following conditions hold for an INSERT ... SELECT statement:

You can of course also use REPLACE instead of INSERT to overwrite old rows.

6.4.3 INSERT DELAYED syntax

INSERT DELAYED ...

The DELAYED option for the INSERT statement is a MySQL-specific option that is very useful if you have clients that can't wait for the INSERT to complete. This is a common problem when you use MySQL for logging and you also periodically run SELECT and UPDATE statements that take a long time to complete. DELAYED was introduced in MySQL Version 3.22.15. It is a MySQL extension to ANSI SQL92.

INSERT DELAYED only works with ISAM and MyISAM tables. Note that as MyISAM tables supports concurrent SELECT and INSERT, if there is no free blocks in the middle of the data file, you very seldom need to use INSERT DELAYED with MyISAM. See section 7.1 MyISAM Tables.

When you use INSERT DELAYED, the client will get an OK at once and the row will be inserted when the table is not in use by any other thread.

Another major benefit of using INSERT DELAYED is that inserts from many clients are bundled together and written in one block. This is much faster than doing many separate inserts.

Note that currently the queued rows are only stored in memory until they are inserted into the table. This means that if you kill mysqld the hard way (kill -9) or if mysqld dies unexpectedly, any queued rows that weren't written to disk are lost!

The following describes in detail what happens when you use the DELAYED option to INSERT or REPLACE. In this description, the ``thread'' is the thread that received an INSERT DELAYED command and ``handler'' is the thread that handles all INSERT DELAYED statements for a particular table.

Note that INSERT DELAYED is slower than a normal INSERT if the table is not in use. There is also the additional overhead for the server to handle a separate thread for each table on which you use INSERT DELAYED. This means that you should only use INSERT DELAYED when you are really sure you need it!

6.4.4 UPDATE Syntax

UPDATE [LOW_PRIORITY] [IGNORE] tbl_name
    SET col_name1=expr1, [col_name2=expr2, ...]
    [WHERE where_definition]
    [LIMIT #]

UPDATE updates columns in existing table rows with new values. The SET clause indicates which columns to modify and the values they should be given. The WHERE clause, if given, specifies which rows should be updated. Otherwise all rows are updated. If the ORDER BY clause is specified, the rows will be updated in the order that is specified.

If you specify the keyword LOW_PRIORITY, execution of the UPDATE is delayed until no other clients are reading from the table.

If you specify the keyword IGNORE, the update statement will not abort even if we get duplicate key errors during the update. Rows that would cause conflicts will not be updated.

If you access a column from tbl_name in an expression, UPDATE uses the current value of the column. For example, the following statement sets the age column to one more than its current value: 

mysql> UPDATE persondata SET age=age+1;

UPDATE assignments are evaluated from left to right. For example, the following statement doubles the age column, then increments it: 

mysql> UPDATE persondata SET age=age*2, age=age+1;

If you set a column to the value it currently has, MySQL notices this and doesn't update it.

UPDATE returns the number of rows that were actually changed. In MySQL Version 3.22 or later, the C API function mysql_info() returns the number of rows that were matched and updated and the number of warnings that occurred during the UPDATE.

In MySQL Version 3.23, you can use LIMIT # to ensure that only a given number of rows are changed.

6.4.5 DELETE Syntax

DELETE [LOW_PRIORITY] FROM tbl_name
    [WHERE where_definition]
    [LIMIT rows]

DELETE deletes rows from tbl_name that satisfy the condition given by where_definition, and returns the number of records deleted.

If you issue a DELETE with no WHERE clause, all rows are deleted. If you do this in AUTOCOMMIT mode, this works as TRUNCATE. See section 6.4.6 TRUNCATE Syntax. One problem with this is that DELETE will return zero as the number of affected records, but this will be fixed in 4.0.

If you really want to know how many records are deleted when you are deleting all rows, and are willing to suffer a speed penalty, you can use a DELETE statement of this form: 

mysql> DELETE FROM tbl_name WHERE 1>0;

Note that this is MUCH slower than DELETE FROM tbl_name with no WHERE clause, because it deletes rows one at a time.

If you specify the keyword LOW_PRIORITY, execution of the DELETE is delayed until no other clients are reading from the table.

Deleted records are maintained in a linked list and subsequent INSERT operations reuse old record positions. To reclaim unused space and reduce file sizes, use the OPTIMIZE TABLE statement or the myisamchk utility to reorganize tables. OPTIMIZE TABLE is easier, but myisamchk is faster. See section 4.5.1 OPTIMIZE TABLE Syntax and section 4.4.6.10 Table Optimization.

The MySQL-specific LIMIT rows option to DELETE tells the server the maximum number of rows to be deleted before control is returned to the client. This can be used to ensure that a specific DELETE command doesn't take too much time. You can simply repeat the DELETE command until the number of affected rows is less than the LIMIT value.

6.4.6 TRUNCATE Syntax

TRUNCATE TABLE table_name

Is in 3.23 and the same thing as DELETE FROM table_name. See section 6.4.5 DELETE Syntax. The differences are:

TRUNCATE is an Oracle SQL extension.

6.4.7 REPLACE Syntax

    REPLACE [LOW_PRIORITY | DELAYED]
        [INTO] tbl_name [(col_name,...)]
        VALUES (expression,...),(...),...
or  REPLACE [LOW_PRIORITY | DELAYED]
        [INTO] tbl_name [(col_name,...)]
        SELECT ...
or  REPLACE [LOW_PRIORITY | DELAYED]
        [INTO] tbl_name
        SET col_name=expression, col_name=expression,...

REPLACE works exactly like INSERT, except that if an old record in the table has the same value as a new record on a unique index, the old record is deleted before the new record is inserted. See section 6.4.2 INSERT Syntax.

In other words, you can't access the values of the old row from a REPLACE statement. In some old MySQL version it looked like you could do this, but that was a bug that has been corrected.

When one uses a REPLACE command, mysql_affected_rows() will return 2 if the new row replaced and old row. This is because in this case one row was inserted and then the duplicate was deleted.

The above makes it easy to check if REPLACE added or replaced a row.

6.4.8 LOAD DATA INFILE Syntax

LOAD DATA [LOW_PRIORITY | CONCURRENT] [LOCAL] INFILE 'file_name.txt'
    [REPLACE | IGNORE]
    INTO TABLE tbl_name
    [FIELDS
        [TERMINATED BY '\t']
        [[OPTIONALLY] ENCLOSED BY '']
        [ESCAPED BY '\\' ]
    ]
    [LINES TERMINATED BY '\n']
    [IGNORE number LINES]
    [(col_name,...)]

The LOAD DATA INFILE statement reads rows from a text file into a table at a very high speed. If the LOCAL keyword is specified, the file is read from the client host. If LOCAL is not specified, the file must be located on the server. (LOCAL is available in MySQL Version 3.22.6 or later.)

For security reasons, when reading text files located on the server, the files must either reside in the database directory or be readable by all. Also, to use LOAD DATA INFILE on server files, you must have the file privilege on the server host. See section 4.2.6 Privileges Provided by MySQL.

If you specify the keyword LOW_PRIORITY, execution of the LOAD DATA statement is delayed until no other clients are reading from the table.

If you specify the keyword CONCURRENT with a MyISAM table, then other threads can retrieve data from the table while LOAD DATA is executing. Using this option will of course affect the performance of LOAD DATA a bit even if no other thread is using the table at the same time.

Using LOCAL will be a bit slower than letting the server access the files directly, because the contents of the file must travel from the client host to the server host. On the other hand, you do not need the file privilege to load local files.

If you are using MySQL before Version 3.23.24 you can't read from a FIFO with LOAD DATA INFILE. If you need to read from a FIFO (for example the output from gunzip), use LOAD DATA LOCAL INFILE instead.

You can also load data files by using the mysqlimport utility; it operates by sending a LOAD DATA INFILE command to the server. The --local option causes mysqlimport to read data files from the client host. You can specify the --compress option to get better performance over slow networks if the client and server support the compressed protocol.

When locating files on the server host, the server uses the following rules:

Note that these rules mean a file given as `./myfile.txt' is read from the server's data directory, whereas a file given as `myfile.txt' is read from the database directory of the current database. For example, the following LOAD DATA statement reads the file `data.txt' from the database directory for db1 because db1 is the current database, even though the statement explicitly loads the file into a table in the db2 database: 

mysql> USE db1;
mysql> LOAD DATA INFILE "data.txt" INTO TABLE db2.my_table;

The REPLACE and IGNORE keywords control handling of input records that duplicate existing records on unique key values. If you specify REPLACE, new rows replace existing rows that have the same unique key value. If you specify IGNORE, input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, an error occurs when a duplicate key value is found, and the rest of the text file is ignored.

If you load data from a local file using the LOCAL keyword, the server has no way to stop transmission of the file in the middle of the operation, so the default bahavior is the same as if IGNORE is specified.

If you use LOAD DATA INFILE on an empty MyISAM table, all non-unique indexes are created in a separate batch (like in REPAIR). This normally makes LOAD DATA INFILE much faster when you have many indexes.

LOAD DATA INFILE is the complement of SELECT ... INTO OUTFILE. See section 6.4.1 SELECT Syntax. To write data from a database to a file, use SELECT ... INTO OUTFILE. To read the file back into the database, use LOAD DATA INFILE. The syntax of the FIELDS and LINES clauses is the same for both commands. Both clauses are optional, but FIELDS must precede LINES if both are specified.

If you specify a FIELDS clause, each of its subclauses (TERMINATED BY, [OPTIONALLY] ENCLOSED BY, and ESCAPED BY) is also optional, except that you must specify at least one of them.

If you don't specify a FIELDS clause, the defaults are the same as if you had written this: 

FIELDS TERMINATED BY '\t' ENCLOSED BY '' ESCAPED BY '\\'

If you don't specify a LINES clause, the default is the same as if you had written this: 

LINES TERMINATED BY '\n'

In other words, the defaults cause LOAD DATA INFILE to act as follows when reading input:

Conversely, the defaults cause SELECT ... INTO OUTFILE to act as follows when writing output:

Note that to write FIELDS ESCAPED BY '\\', you must specify two backslashes for the value to be read as a single backslash.

The IGNORE number LINES option can be used to ignore a header of column names at the start of the file: 

mysql> LOAD DATA INFILE "/tmp/file_name" into table test IGNORE 1 LINES;

When you use SELECT ... INTO OUTFILE in tandem with LOAD DATA INFILE to write data from a database into a file and then read the file back into the database later, the field and line handling options for both commands must match. Otherwise, LOAD DATA INFILE will not interpret the contents of the file properly. Suppose you use SELECT ... INTO OUTFILE to write a file with fields delimited by commas: 

mysql> SELECT * INTO OUTFILE 'data.txt'
           FIELDS TERMINATED BY ','
           FROM ...;

To read the comma-delimited file back in, the correct statement would be: 

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
           FIELDS TERMINATED BY ',';

If instead you tried to read in the file with the statement shown below, it wouldn't work because it instructs LOAD DATA INFILE to look for tabs between fields: 

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
           FIELDS TERMINATED BY '\t';

The likely result is that each input line would be interpreted as a single field.

LOAD DATA INFILE can be used to read files obtained from external sources, too. For example, a file in dBASE format will have fields separated by commas and enclosed in double quotes. If lines in the file are terminated by newlines, the command shown below illustrates the field and line handling options you would use to load the file: 

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE tbl_name
           FIELDS TERMINATED BY ',' ENCLOSED BY '"'
           LINES TERMINATED BY '\n';

Any of the field or line handling options may specify an empty string (''). If not empty, the FIELDS [OPTIONALLY] ENCLOSED BY and FIELDS ESCAPED BY values must be a single character. The FIELDS TERMINATED BY and LINES TERMINATED BY values may be more than one character. For example, to write lines that are terminated by carriage return-linefeed pairs, or to read a file containing such lines, specify a LINES TERMINATED BY '\r\n' clause.

For example, to read a file of jokes, that are separated with a line of %%, into a SQL table you can do: 

create table jokes (a int not null auto_increment primary key, joke text
not null);
load data infile "/tmp/jokes.txt" into table jokes fields terminated by ""
lines terminated by "\n%%\n" (joke);

FIELDS [OPTIONALLY] ENCLOSED BY controls quoting of fields. For output (SELECT ... INTO OUTFILE), if you omit the word OPTIONALLY, all fields are enclosed by the ENCLOSED BY character. An example of such output (using a comma as the field delimiter) is shown below: 

"1","a string","100.20"
"2","a string containing a , comma","102.20"
"3","a string containing a \" quote","102.20"
"4","a string containing a \", quote and comma","102.20"

If you specify OPTIONALLY, the ENCLOSED BY character is used only to enclose CHAR and VARCHAR fields: 

1,"a string",100.20
2,"a string containing a , comma",102.20
3,"a string containing a \" quote",102.20
4,"a string containing a \", quote and comma",102.20

Note that occurrences of the ENCLOSED BY character within a field value are escaped by prefixing them with the ESCAPED BY character. Also note that if you specify an empty ESCAPED BY value, it is possible to generate output that cannot be read properly by LOAD DATA INFILE. For example, the output just shown above would appear as shown below if the escape character is empty. Observe that the second field in the fourth line contains a comma following the quote, which (erroneously) appears to terminate the field: 

1,"a string",100.20
2,"a string containing a , comma",102.20
3,"a string containing a " quote",102.20
4,"a string containing a ", quote and comma",102.20

For input, the ENCLOSED BY character, if present, is stripped from the ends of field values. (This is true whether or not OPTIONALLY is specified; OPTIONALLY has no effect on input interpretation.) Occurrences of the ENCLOSED BY character preceded by the ESCAPED BY character are interpreted as part of the current field value. In addition, duplicated ENCLOSED BY characters occurring within fields are interpreted as single ENCLOSED BY characters if the field itself starts with that character. For example, if ENCLOSED BY '"' is specified, quotes are handled as shown below: 

"The ""BIG"" boss"  -> The "BIG" boss
The "BIG" boss      -> The "BIG" boss
The ""BIG"" boss    -> The ""BIG"" boss

FIELDS ESCAPED BY controls how to write or read special characters. If the FIELDS ESCAPED BY character is not empty, it is used to prefix the following characters on output:

If the FIELDS ESCAPED BY character is empty, no characters are escaped. It is probably not a good idea to specify an empty escape character, particularly if field values in your data contain any of the characters in the list just given.

For input, if the FIELDS ESCAPED BY character is not empty, occurrences of that character are stripped and the following character is taken literally as part of a field value. The exceptions are an escaped `0' or `N' (for example, \0 or \N if the escape character is `\'). These sequences are interpreted as ASCII 0 (a zero-valued byte) and NULL. See below for the rules on NULL handling.

For more information about `\'-escape syntax, see section 6.1.1 Literals: How to Write Strings and Numbers.

In certain cases, field and line handling options interact:

Handling of NULL values varies, depending on the FIELDS and LINES options you use:

Some cases are not supported by LOAD DATA INFILE:

The following example loads all columns of the persondata table: 

mysql> LOAD DATA INFILE 'persondata.txt' INTO TABLE persondata;

No field list is specified, so LOAD DATA INFILE expects input rows to contain a field for each table column. The default FIELDS and LINES values are used.

If you wish to load only some of a table's columns, specify a field list: 

mysql> LOAD DATA INFILE 'persondata.txt'
           INTO TABLE persondata (col1,col2,...);

You must also specify a field list if the order of the fields in the input file differs from the order of the columns in the table. Otherwise, MySQL cannot tell how to match up input fields with table columns.

If a row has too few fields, the columns for which no input field is present are set to default values. Default value assignment is described in section 6.5.3 CREATE TABLE Syntax.

An empty field value is interpreted differently than if the field value is missing:

Note that these are the same values that result if you assign an empty string explicitly to a string, numeric, or date or time type explicitly in an INSERT or UPDATE statement.

TIMESTAMP columns are only set to the current date and time if there is a NULL value for the column, or (for the first TIMESTAMP column only) if the TIMESTAMP column is left out from the field list when a field list is specified.

If an input row has too many fields, the extra fields are ignored and the number of warnings is incremented.

LOAD DATA INFILE regards all input as strings, so you can't use numeric values for ENUM or SET columns the way you can with INSERT statements. All ENUM and SET values must be specified as strings!

If you are using the C API, you can get information about the query by calling the API function mysql_info() when the LOAD DATA INFILE query finishes. The format of the information string is shown below: 

Records: 1  Deleted: 0  Skipped: 0  Warnings: 0

Warnings occur under the same circumstances as when values are inserted via the INSERT statement (see section 6.4.2 INSERT Syntax), except that LOAD DATA INFILE also generates warnings when there are too few or too many fields in the input row. The warnings are not stored anywhere; the number of warnings can only be used as an indication if everything went well. If you get warnings and want to know exactly why you got them, one way to do this is to use SELECT ... INTO OUTFILE into another file and compare this to your original input file.

If you need LOAD DATA to read from a pipe, you can use the following trick: 

mkfifo /mysql/db/x/x
chmod 666 /mysql/db/x/x
cat < /dev/tcp/10.1.1.12/4711 > /nt/mysql/db/x/x
mysql -e "LOAD DATA INFILE 'x' INTO TABLE x" x

If you are using a version of MySQL older than 3.23.25 you can only do the above with LOAD DATA LOCAL INFILE.

For more information about the efficiency of INSERT versus LOAD DATA INFILE and speeding up LOAD DATA INFILE, See section 5.2.8 Speed of INSERT Queries.

6.4.9 DO Syntax

DO expression, [expression, ...]

Execute the expression but don't return any results. This is a shorthand of SELECT expression, expression, but has the advantage that it's slightly faster when you don't care about the result.

This is mainly useful with functions that has side effects, like RELEASE_LOCK.

6.5 Data Definition: CREATE, DROP, ALTER

6.5.1 CREATE DATABASE Syntax

CREATE DATABASE [IF NOT EXISTS] db_name

CREATE DATABASE creates a database with the given name. Rules for allowable database names are given in section 6.1.2 Database, Table, Index, Column, and Alias Names. An error occurs if the database already exists and you didn't specify IF NOT EXISTS.

Databases in MySQL are implemented as directories containing files that correspond to tables in the database. Because there are no tables in a database when it is initially created, the CREATE DATABASE statement only creates a directory under the MySQL data directory.

You can also create databases with mysqladmin. See section 4.8 MySQL Client-Side Scripts and Utilities.

6.5.2 DROP DATABASE Syntax

DROP DATABASE [IF EXISTS] db_name

DROP DATABASE drops all tables in the database and deletes the database. If you do a DROP DATABASE on a symbolic linked database, both the link and the original database is deleted. Be VERY careful with this command!

DROP DATABASE returns the number of files that were removed from the database directory. Normally, this is three times the number of tables, because normally each table corresponds to a `.MYD' file, a `.MYI' file, and a `.frm' file.

The DROP DATABASE command removes from the given database directory all files with the following extensions:

.BAK .DAT .HSH .ISD
.ISM .ISM .MRG .MYD
.MYI .db .frm

All subdirectories that consists of 2 digits (RAID directories) are also removed.

In MySQL Version 3.22 or later, you can use the keywords IF EXISTS to prevent an error from occurring if the database doesn't exist.

You can also drop databases with mysqladmin. See section 4.8 MySQL Client-Side Scripts and Utilities.

6.5.3 CREATE TABLE Syntax

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name [(create_definition,...)]
[table_options] [select_statement]

create_definition:
  col_name type [NOT NULL | NULL] [DEFAULT default_value] [AUTO_INCREMENT]
            [PRIMARY KEY] [reference_definition]
  or    PRIMARY KEY (index_col_name,...)
  or    KEY [index_name] (index_col_name,...)
  or    INDEX [index_name] (index_col_name,...)
  or    UNIQUE [INDEX] [index_name] (index_col_name,...)
  or    FULLTEXT [INDEX] [index_name] (index_col_name,...)
  or    [CONSTRAINT symbol] FOREIGN KEY index_name (index_col_name,...)
            [reference_definition]
  or    CHECK (expr)

type:
        TINYINT[(length)] [UNSIGNED] [ZEROFILL]
  or    SMALLINT[(length)] [UNSIGNED] [ZEROFILL]
  or    MEDIUMINT[(length)] [UNSIGNED] [ZEROFILL]
  or    INT[(length)] [UNSIGNED] [ZEROFILL]
  or    INTEGER[(length)] [UNSIGNED] [ZEROFILL]
  or    BIGINT[(length)] [UNSIGNED] [ZEROFILL]
  or    REAL[(length,decimals)] [UNSIGNED] [ZEROFILL]
  or    DOUBLE[(length,decimals)] [UNSIGNED] [ZEROFILL]
  or    FLOAT[(length,decimals)] [UNSIGNED] [ZEROFILL]
  or    DECIMAL(length,decimals) [UNSIGNED] [ZEROFILL]
  or    NUMERIC(length,decimals) [UNSIGNED] [ZEROFILL]
  or    CHAR(length) [BINARY]
  or    VARCHAR(length) [BINARY]
  or    DATE
  or    TIME
  or    TIMESTAMP
  or    DATETIME
  or    TINYBLOB
  or    BLOB
  or    MEDIUMBLOB
  or    LONGBLOB
  or    TINYTEXT
  or    TEXT
  or    MEDIUMTEXT
  or    LONGTEXT
  or    ENUM(value1,value2,value3,...)
  or    SET(value1,value2,value3,...)

index_col_name:
        col_name [(length)]

reference_definition:
        REFERENCES tbl_name [(index_col_name,...)]
                   [MATCH FULL | MATCH PARTIAL]
                   [ON DELETE reference_option]
                   [ON UPDATE reference_option]

reference_option:
        RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT

table_options:
	TYPE = {BDB | HEAP | ISAM | InnoDB | MERGE | MYISAM }
or	AUTO_INCREMENT = #
or	AVG_ROW_LENGTH = #
or	CHECKSUM = {0 | 1}
or	COMMENT = "string"
or	MAX_ROWS = #
or	MIN_ROWS = #
or	PACK_KEYS = {0 | 1}
or	PASSWORD = "string"
or	DELAY_KEY_WRITE = {0 | 1}
or      ROW_FORMAT= { default | dynamic | fixed | compressed }
or	RAID_TYPE= {1 | STRIPED | RAID0 } RAID_CHUNKS=#  RAID_CHUNKSIZE=#
or	UNION = (table_name,[table_name...])
or      DATA DIRECTORY="directory"
or      INDEX DIRECTORY="directory"

select_statement:
	[IGNORE | REPLACE] SELECT ...  (Some legal select statement)

CREATE TABLE creates a table with the given name in the current database. Rules for allowable table names are given in section 6.1.2 Database, Table, Index, Column, and Alias Names. An error occurs if there is no current database or if the table already exists.

In MySQL Version 3.22 or later, the table name can be specified as db_name.tbl_name. This works whether or not there is a current database.

In MySQL Version 3.23, you can use the TEMPORARY keyword when you create a table. A temporary table will automatically be deleted if a connection dies and the name is per connection. This means that two different connections can both use the same temporary table name without conflicting with each other or with an existing table of the same name. (The existing table is hidden until the temporary table is deleted).

In MySQL Version 3.23 or later, you can use the keywords IF NOT EXISTS so that an error does not occur if the table already exists. Note that there is no verification that the table structures are identical.

Each table tbl_name is represented by some files in the database directory. In the case of MyISAM-type tables you will get:

File Purpose
tbl_name.frm Table definition (form) file
tbl_name.MYD Data file
tbl_name.MYI Index file

For more information on the properties of the various column types, see section 6.2 Column Types:

6.5.3.1 Silent Column Specification Changes

In some cases, MySQL silently changes a column specification from that given in a CREATE TABLE statement. (This may also occur with ALTER TABLE.):

If you want to see whether or not MySQL used a column type other than the one you specified, issue a DESCRIBE tbl_name statement after creating or altering your table.

Certain other column type changes may occur if you compress a table using myisampack. See section 7.1.2.3 Compressed Table Characteristics.

6.5.4 ALTER TABLE Syntax

ALTER [IGNORE] TABLE tbl_name alter_spec [, alter_spec ...]

alter_specification:
        ADD [COLUMN] create_definition [FIRST | AFTER column_name ]
  or    ADD [COLUMN] (create_definition, create_definition,...)
  or    ADD INDEX [index_name] (index_col_name,...)
  or    ADD PRIMARY KEY (index_col_name,...)
  or    ADD UNIQUE [index_name] (index_col_name,...)
  or    ADD FULLTEXT [index_name] (index_col_name,...)
  or	ADD [CONSTRAINT symbol] FOREIGN KEY index_name (index_col_name,...)
            [reference_definition]
  or    ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
  or    CHANGE [COLUMN] old_col_name create_definition
  or    MODIFY [COLUMN] create_definition
  or    DROP [COLUMN] col_name
  or    DROP PRIMARY KEY
  or    DROP INDEX index_name
  or    RENAME [TO] new_tbl_name
  or    ORDER BY col
  or    table_options

ALTER TABLE allows you to change the structure of an existing table. For example, you can add or delete columns, create or destroy indexes, change the type of existing columns, or rename columns or the table itself. You can also change the comment for the table and type of the table. See section 6.5.3 CREATE TABLE Syntax.

If you use ALTER TABLE to change a column specification but DESCRIBE tbl_name indicates that your column was not changed, it is possible that MySQL ignored your modification for one of the reasons described in section 6.5.3.1 Silent Column Specification Changes. For example, if you try to change a VARCHAR column to CHAR, MySQL will still use VARCHAR if the table contains other variable-length columns.

ALTER TABLE works by making a temporary copy of the original table. The alteration is performed on the copy, then the original table is deleted and the new one is renamed. This is done in such a way that all updates are automatically redirected to the new table without any failed updates. While ALTER TABLE is executing, the original table is readable by other clients. Updates and writes to the table are stalled until the new table is ready.

Note that if you use any other option to ALTER TABLE than RENAME, MySQL will always create a temporary table, even if the data wouldn't strictly need to be copied (like when you change the name of a column). We plan to fix this in the future, but as one doesn't normally do ALTER TABLE that often this isn't that high on our TODO.

Here is an example that shows some of the uses of ALTER TABLE. We begin with a table t1 that is created as shown below: 

mysql> CREATE TABLE t1 (a INTEGER,b CHAR(10));

To rename the table from t1 to t2: 

mysql> ALTER TABLE t1 RENAME t2;

To change column a from INTEGER to TINYINT NOT NULL (leaving the name the same), and to change column b from CHAR(10) to CHAR(20) as well as renaming it from b to c: 

mysql> ALTER TABLE t2 MODIFY a TINYINT NOT NULL, CHANGE b c CHAR(20);

To add a new TIMESTAMP column named d: 

mysql> ALTER TABLE t2 ADD d TIMESTAMP;

To add an index on column d, and make column a the primary key: 

mysql> ALTER TABLE t2 ADD INDEX (d), ADD PRIMARY KEY (a);

To remove column c: 

mysql> ALTER TABLE t2 DROP COLUMN c;

To add a new AUTO_INCREMENT integer column named c: 

mysql> ALTER TABLE t2 ADD c INT UNSIGNED NOT NULL AUTO_INCREMENT,
           ADD INDEX (c);

Note that we indexed c, because AUTO_INCREMENT columns must be indexed, and also that we declare c as NOT NULL, because indexed columns cannot be NULL.

When you add an AUTO_INCREMENT column, column values are filled in with sequence numbers for you automatically. You can set the first sequence number by executing SET INSERT_ID=# before ALTER TABLE or using the AUTO_INCREMENT = # table option. See section 5.5.6 SET Syntax.

With MyISAM tables, if you don't change the AUTO_INCREMENT column, the sequence number will not be affected. If you drop an AUTO_INCREMENT column and then add another AUTO_INCREMENT column, the numbers will start from 1 again.

See section A.6.1 Problems with ALTER TABLE..

6.5.5 RENAME TABLE Syntax

RENAME TABLE tbl_name TO new_table_name[, tbl_name2 TO new_table_name2,...]

The rename is done atomically, which means that no other thread can access any of the tables while the rename is running. This makes it possible to replace a table with an empty one: 

CREATE TABLE new_table (...);
RENAME TABLE old_table TO backup_table, new_table TO old_table;

The rename is done from left to right, which means that if you want to swap two tables names, you have to: 

RENAME TABLE old_table    TO backup_table,
             new_table    TO old_table,
             backup_table TO new_table;

As long as two databases are on the same disk you can also rename from one database to another: 

RENAME TABLE current_database.table_name TO other_database.table_name;

When you execute RENAME, you can't have any locked tables or active transactions. You must also have the ALTER and DROP privilege on the original table and CREATE and INSERT privilege on the new table.

If MySQL encounters any errors in a multiple table rename, it will do a reverse rename for all renamed tables to get everything back to the original state.

6.5.6 DROP TABLE Syntax

DROP TABLE [IF EXISTS] tbl_name [, tbl_name,...] [RESTRICT | CASCADE]

DROP TABLE removes one or more tables. All table data and the table definition are removed, so be careful with this command!

In MySQL Version 3.22 or later, you can use the keywords IF EXISTS to prevent an error from occurring for tables that don't exist.

RESTRICT and CASCADE are allowed to make porting easier. For the moment they don't do anything.

NOTE: DROP TABLE is not transaction-safe and will automatically commit any active transactions.

6.5.7 CREATE INDEX Syntax

CREATE [UNIQUE|FULLTEXT] INDEX index_name ON tbl_name (col_name[(length)],... )

The CREATE INDEX statement doesn't do anything in MySQL prior to Version 3.22. In Version 3.22 or later, CREATE INDEX is mapped to an ALTER TABLE statement to create indexes. See section 6.5.4 ALTER TABLE Syntax.

Normally, you create all indexes on a table at the time the table itself is created with CREATE TABLE. See section 6.5.3 CREATE TABLE Syntax. CREATE INDEX allows you to add indexes to existing tables.

A column list of the form (col1,col2,...) creates a multiple-column index. Index values are formed by concatenating the values of the given columns.

For CHAR and VARCHAR columns, indexes can be created that use only part of a column, using col_name(length) syntax. (On BLOB and TEXT columns the length is required). The statement shown below creates an index using the first 10 characters of the name column: 

mysql> CREATE INDEX part_of_name ON customer (name(10));

Because most names usually differ in the first 10 characters, this index should not be much slower than an index created from the entire name column. Also, using partial columns for indexes can make the index file much smaller, which could save a lot of disk space and might also speed up INSERT operations!

Note that you can only add an index on a column that can have NULL values or on a BLOB/TEXT column if you are using MySQL Version 3.23.2 or newer and are using the MyISAM table type.

For more information about how MySQL uses indexes, see section 5.4.3 How MySQL Uses Indexes.

FULLTEXT indexes can index only VARCHAR and TEXT columns, and only in MyISAM tables. FULLTEXT indexes are available in MySQL Version 3.23.23 and later. section 6.8 MySQL Full-text Search.

6.5.8 DROP INDEX Syntax

DROP INDEX index_name ON tbl_name

DROP INDEX drops the index named index_name from the table tbl_name. DROP INDEX doesn't do anything in MySQL prior to Version 3.22. In Version 3.22 or later, DROP INDEX is mapped to an ALTER TABLE statement to drop the index. See section 6.5.4 ALTER TABLE Syntax.

6.6 Basic MySQL User Utility Commands

6.6.1 USE Syntax

USE db_name

The USE db_name statement tells MySQL to use the db_name database as the default database for subsequent queries. The database remains current until the end of the session or until another USE statement is issued: 

mysql> USE db1;
mysql> SELECT count(*) FROM mytable;      # selects from db1.mytable
mysql> USE db2;
mysql> SELECT count(*) FROM mytable;      # selects from db2.mytable

Making a particular database current by means of the USE statement does not preclude you from accessing tables in other databases. The example below accesses the author table from the db1 database and the editor table from the db2 database: 

mysql> USE db1;
mysql> SELECT author_name,editor_name FROM author,db2.editor
           WHERE author.editor_id = db2.editor.editor_id;

The USE statement is provided for Sybase compatibility.

6.6.2 DESCRIBE Syntax (Get Information About Columns)

{DESCRIBE | DESC} tbl_name {col_name | wild}

DESCRIBE is a shortcut for SHOW COLUMNS FROM. See section 4.5.5.1 Retrieving information about Database, Tables, Columns, and Indexes.

DESCRIBE provides information about a table's columns. col_name may be a column name or a string containing the SQL `%' and `_' wild-card characters.

If the column types are different than you expect them to be based on a CREATE TABLE statement, note that MySQL sometimes changes column types. See section 6.5.3.1 Silent Column Specification Changes.

This statement is provided for Oracle compatibility.

The SHOW statement provides similar information. See section 4.5.5 SHOW Syntax.

6.7 MySQL Transactional and Locking Commands

6.7.1 BEGIN/COMMIT/ROLLBACK Syntax

By default, MySQL runs in autocommit mode. This means that as soon as you execute an update, MySQL will store the update on disk.

If you are using transactions safe tables (like BDB, InnoDB, you can put MySQL into non-autocommit mode with the following command: 

SET AUTOCOMMIT=0

After this you must use COMMIT to store your changes to disk or ROLLBACK if you want to ignore the changes you have made since the beginning of your transaction.

If you want to switch from AUTOCOMMIT mode for one series of statements, you can use the BEGIN or BEGIN WORK statement: 

BEGIN;
SELECT @A:=SUM(salary) FROM table1 WHERE type=1;
UPDATE table2 SET summmary=@A WHERE type=1;
COMMIT;

Note that if you are using non-transaction-safe tables, the changes will be stored at once, independent of the status of the autocommit mode.

If you do a ROLLBACK when you have updated a non-transactional table you will get an error (ER_WARNING_NOT_COMPLETE_ROLLBACK) as a warning. All transactional safe tables will be restored but any non-transactional table will not change.

If you are using BEGIN or SET AUTOCOMMIT=0, you should use the MySQL binary log for backups instead of the older update log. Transactions are stored in the binary log in one chunk, upon COMMIT, to ensure that transactions which are rolled back are not stored. See section 4.9.4 The Binary Update Log.

The following commands automatically end a transaction (as if you had done a COMMIT before executing the command):

ALTER TABLE BEGIN CREATE INDEX
DROP DATABASE DROP TABLE RENAME TABLE
TRUNCATE

You can change the isolation level for transactions with SET TRANSACTION ISOLATION LEVEL .... See section 6.7.3 SET TRANSACTION Syntax.

6.7.2 LOCK TABLES/UNLOCK TABLES Syntax

LOCK TABLES tbl_name [AS alias] {READ | [READ LOCAL] | [LOW_PRIORITY] WRITE}
            [, tbl_name {READ | [LOW_PRIORITY] WRITE} ...]
...
UNLOCK TABLES

LOCK TABLES locks tables for the current thread. UNLOCK TABLES releases any locks held by the current thread. All tables that are locked by the current thread are automatically unlocked when the thread issues another LOCK TABLES, or when the connection to the server is closed.

The main reasons to use LOCK TABLES are for emulating transactions or getting more speed when updating tables. This is explained in more detail later.

If a thread obtains a READ lock on a table, that thread (and all other threads) can only read from the table. If a thread obtains a WRITE lock on a table, then only the thread holding the lock can READ from or WRITE to the table. Other threads are blocked.

The difference between READ LOCAL and READ is that READ LOCAL allows non-conflicting INSERT statements to execute while the lock is held. This can't however be used if you are going to manipulate the database files outside MySQL while you hold the lock.

When you use LOCK TABLES, you must lock all tables that you are going to use and you must use the same alias that you are going to use in your queries! If you are using a table multiple times in a query (with aliases), you must get a lock for each alias!

WRITE locks normally have higher priority than READ locks, to ensure that updates are processed as soon as possible. This means that if one thread obtains a READ lock and then another thread requests a WRITE lock, subsequent READ lock requests will wait until the WRITE thread has gotten the lock and released it. You can use LOW_PRIORITY WRITE locks to allow other threads to obtain READ locks while the thread is waiting for the WRITE lock. You should only use LOW_PRIORITY WRITE locks if you are sure that there will eventually be a time when no threads will have a READ lock.

LOCK TABLES works as follows:

  1. Sort all tables to be locked in a internally defined order (from the user standpoint the order is undefined).
  2. If a table is locked with a read and a write lock, put the write lock before the read lock.
  3. Lock one table at a time until the thread gets all locks.

This policy ensures that table locking is deadlock free. There is however other things one needs to be aware of with this schema:

If you are using a LOW_PRIORITY_WRITE lock for a table, this means only that MySQL will wait for this particlar lock until there is no threads that wants a READ lock. When the thread has got the WRITE lock and is waiting to get the lock for the next table in the lock table list, all other threads will wait for the WRITE lock to be released. If this becomes a serious problem with your application, you should consider converting some of your tables to transactions safe tables.

You can safely kill a thread that is waiting for a table lock with KILL. See section 4.5.4 KILL Syntax.

Note that you should NOT lock any tables that you are using with INSERT DELAYED. This is because that in this case the INSERT is done by a separate thread.

Normally, you don't have to lock tables, as all single UPDATE statements are atomic; no other thread can interfere with any other currently executing SQL statement. There are a few cases when you would like to lock tables anyway:

By using incremental updates (UPDATE customer SET value=value+new_value) or the LAST_INSERT_ID() function, you can avoid using LOCK TABLES in many cases.

You can also solve some cases by using the user-level lock functions GET_LOCK() and RELEASE_LOCK(). These locks are saved in a hash table in the server and implemented with pthread_mutex_lock() and pthread_mutex_unlock() for high speed. See section 6.3.5.2 Miscellaneous Functions.

See section 5.3.1 How MySQL Locks Tables, for more information on locking policy.

You can lock all tables in all databases with read locks with the FLUSH TABLES WITH READ LOCK command. See section 4.5.3 FLUSH Syntax. This is very convenient way to get backups if you have a file system, like Veritas, that can take snapshots in time.

NOTE: LOCK TABLES is not transaction-safe and will automatically commit any active transactions before attempting to lock the tables.

6.7.3 SET TRANSACTION Syntax

SET [GLOBAL | SESSION] TRANSACTION ISOLATION LEVEL
[READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE]

Sets the transaction isolation level for the global, whole session or the next transaction.

The default behavior is to set the isolation level for the next (not started) transaction.

If you set the GLOBAL privilege it will affect all new created threads. You will need the PROCESS privilege to do do this.

Setting the SESSION privilege will affect the following and all future transactions.

You can set the default isolation level for mysqld with --transaction-isolation=.... See section 4.1.1 mysqld Command-line Options.

6.8 MySQL Full-text Search

Since Version 3.23.23, MySQL has support for full-text indexing and searching. Full-text indexes in MySQL are an index of type FULLTEXT. FULLTEXT indexes can be created from VARCHAR and TEXT columns at CREATE TABLE time or added later with ALTER TABLE or CREATE INDEX. For large datasets, adding FULLTEXT index with ALTER TABLE (or CREATE INDEX) would be much faster than inserting rows into the empty table with a FULLTEXT index.

Full-text search is performed with the MATCH function. 

mysql> CREATE TABLE articles (
    ->   id INT UNSIGNED AUTO_INCREMENT NOT NULL PRIMARY KEY, 
    ->   title VARCHAR(200),
    ->   body TEXT,
    ->   FULLTEXT (title,body)
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO articles VALUES
    -> (0,'MySQL Tutorial', 'DBMS stands for DataBase Management ...'),
    -> (0,'How To Use MySQL Efficiently', 'After you went through a ...'),
    -> (0,'Optimizing MySQL','In this tutorial we will show how to ...'),
    -> (0,'1001 MySQL Trick','1. Never run mysqld as root. 2. Normalize ...'),
    -> (0,'MySQL vs. YourSQL', 'In the following database comparison we ...'),
    -> (0,'MySQL Security', 'When configured properly, MySQL could be ...');
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM articles WHERE MATCH (title,body) AGAINST ('database');
+----+-------------------+---------------------------------------------+
| id | title             | body                                        |
+----+-------------------+---------------------------------------------+
|  5 | MySQL vs. YourSQL | In the following database comparison we ... |
|  1 | MySQL Tutorial    | DBMS stands for DataBase Management ...     |
+----+-------------------+---------------------------------------------+
2 rows in set (0.00 sec)

The function MATCH matches a natural language query AGAINST a text collection (which is simply the set of columns covered by a FULLTEXT index). For every row in a table it returns relevance - a similarity measure between the text in that row (in the columns that are part of the collection) and the query. When it is used in a WHERE clause (see example above) the rows returned are automatically sorted with relevance decreasing. Relevance is a non-negative floating-point number. Zero relevance means no similarity. Relevance is computed based on the number of words in the row, the number of unique words in that row, the total number of words in the collection, and the number of documents (rows) that contain a particular word.

The above is a basic example of using MATCH function. Rows are returned with relevance decreasing. 

mysql> SELECT id,MATCH (title,body) AGAINST ('Tutorial') FROM articles;
+----+-----------------------------------------+
| id | MATCH (title,body) AGAINST ('Tutorial') |
+----+-----------------------------------------+
|  1 |                        0.64840710366884 |
|  2 |                                       0 |
|  3 |                        0.66266459031789 |
|  4 |                                       0 |
|  5 |                                       0 |
|  6 |                                       0 |
+----+-----------------------------------------+
5 rows in set (0.00 sec)

This example shows how to retrieve the relevances. As neither WHERE nor ORDER BY clauses are present, returned rows are not ordered. 

mysql> SELECT id, body, MATCH (title,body) AGAINST (
    -> 'Security implications of running MySQL as root') AS score
    -> FROM articles WHERE MATCH (title,body) AGAINST
    -> ('Security implications of running MySQL as root');
+----+-----------------------------------------------+-----------------+
| id | body                                          | score           |
+----+-----------------------------------------------+-----------------+
|  4 | 1. Never run mysqld as root. 2. Normalize ... | 1.5055546709332 |
|  6 | When configured properly, MySQL could be ...  |   1.31140957288 |
+----+-----------------------------------------------+-----------------+
2 rows in set (0.00 sec)

This is more complex example - the query returns the relevance and still sorts the rows with relevance decreasing. To achieve it one should specify MATCH twice. Note, that this will cause no additional overhead, as MySQL optimizer will notice that these two MATCH calls are identical and will call full-text search code only once.

MySQL uses a very simple parser to split text into words. A ``word'' is any sequence of letters, numbers, `'', and `_'. Any ``word'' that is present in the stopword list or just too short (3 characters or less) is ignored.

Every correct word in the collection and in the query is weighted, according to its significance in the query or collection. This way, a word that is present in many documents will have lower weight (and may even have a zero weight), because it has lower semantic value in this particular collection. Otherwise, if the word is rare, it will receive a higher weight. The weights of the words are then combined to compute the relevance of the row.

Such a technique works best with large collections (in fact, it was carefully tuned this way). For very small tables, word distribution does not reflect adequately their semantical value, and this model may sometimes produce bizarre results. 

mysql> SELECT * FROM articles WHERE MATCH (title,body) AGAINST ('MySQL');
Empty set (0.00 sec)

Search for the word MySQL produces no results in the above example. Word MySQL is present in more than half of rows, and as such, is effectively treated as a stopword (that is, with semantical value zero). It is, really, the desired behavior - a natural language query should not return every second row in 1GB table.

A word that matches half of rows in a table is less likely to locate relevant documents. In fact, it will most likely find plenty of irrelevant documents. We all know this happens far too often when we are trying to find something on the Internet with a search engine. It is with this reasoning that such rows have been assigned a low semantical value in this particular dataset.

6.8.1 Fulltext restrictions

6.8.2 Fine-tuning MySQL Full-text Search

Unfortunately, full-text search has no user-tunable parameters yet, although adding some is very high on the TODO. However, if you have a MySQL source distribution (See section 2.3 Installing a MySQL Source Distribution.), you can somewhat alter the full-text search behavior.

Note that full-text search was carefully tuned for the best searching effectiveness. Modifying the default behavior will, in most cases, only make the search results worse. Do not alter the MySQL sources unless you know what you are doing!

6.8.3 New Features of Full-text Search to Appear in MySQL 4.0

This section includes a list of the fulltext features that are already implemented in the 4.0 tree. It explains More functions for full-text search entry of section 1.6.1 Things that should be in 4.0.

6.8.4 Full-text Search TODO

7 MySQL Table Types

As of MySQL Version 3.23.6, you can choose between three basic table formats (ISAM, HEAP and MyISAM. Newer MySQL may support additional table type (BDB, or InnoDB), depending on how you compile it. When you create a new table, you can tell MySQL which table type it should use for the table. MySQL will always create a .frm file to hold the table and column definitions. Depending on the table type, the index and data will be stored in other files.

Note that to use InnoDB tables you have to use at least the innodb_data_file_path startup option. See section 7.6.2 InnoDB startup options.

The default table type in MySQL is MyISAM. If you are trying to use a table type that is not compiled-in or activated, MySQL will instead create a table of type MyISAM. This is a very useful feature when you want to copy tables between different SQL servers that supports different table types (like copying tables to a slave that is optimized for speed by not having transactional tables). This automatic table changing can however also be very confusing for new MySQL users. We plan to fix this by introducing warnings in MySQL 4.0 and giving a warning when a table type is automatically changed.

You can convert tables between different types with the ALTER TABLE statement. See section 6.5.4 ALTER TABLE Syntax.

Note that MySQL supports two different kinds of tables. Transaction-safe tables (BDB, InnoDB and not transaction-safe tables (HEAP, ISAM, MERGE, and MyISAM).

Advantages of transaction-safe tables (TST):

Advantages of not transaction-safe tables (NTST):

You can combine TST and NTST tables in the same statements to get the best of both worlds.

7.1 MyISAM Tables

MyISAM is the default table type in MySQL Version 3.23. It's based on the ISAM code and has a lot of useful extensions.

The index is stored in a file with the .MYI (MYIndex) extension, and the data is stored in a file with the .MYD (MYData) extension. You can check/repair MyISAM tables with the myisamchk utility. See section 4.4.6.7 Using myisamchk for Crash Recovery. You can compress MyISAM tables with myisampack to take up much less space. See section 4.7.4 myisampack, The MySQL Compressed Read-only Table Generator.

The following is new in MyISAM:

MyISAM also supports the following things, which MySQL will be able to use in the near future:

Note that index files are usually much smaller with MyISAM than with ISAM. This means that MyISAM will normally use less system resources than ISAM, but will need more CPU when inserting data into a compressed index.

The following options to mysqld can be used to change the behavior of MyISAM tables. See section 4.5.5.4 SHOW VARIABLES.

Option Meaning
--myisam-recover=# Automatic recover of crashed tables.
-O myisam_sort_buffer_size=# Buffer used when recovering tables.
--delay-key-write-for-all-tables Don't flush key buffers between writes for any MyISAM table
-O myisam_max_extra_sort_file_size=# Used to help MySQL to decide when to use the slow but safe key cache index create method. NOTE that this parameter is given in megabytes!
-O myisam_max_sort_file_size=# Don't use the fast sort index method to created index if the temporary file would get bigger than this. NOTE that this paramter is given in megabytes!

The automatic recovery is activated if you start mysqld with --myisam-recover=#. See section 4.1.1 mysqld Command-line Options. On open, the table is checked if it's marked as crashed or if the open count variable for the table is not 0 and you are running with --skip-locking. If either of the above is true the following happens.

If the recover wouldn't be able to recover all rows from a previous completed statement and you didn't specify FORCE as an option to myisam-recover, then the automatic repair will abort with an error message in the error file: 

Error: Couldn't repair table: test.g00pages

If you in this case had used the FORCE option you would instead have got a warning in the error file: 

Warning: Found 344 of 354 rows when repairing ./test/g00pages

Note that if you run automatic recover with the BACKUP option, you should have a cron script that automatically moves file with names like `tablename-datetime.BAK' from the database directories to a backup media.

See section 4.1.1 mysqld Command-line Options.

7.1.1 Space Needed for Keys

MySQL can support different index types, but the normal type is ISAM or MyISAM. These use a B-tree index, and you can roughly calculate the size for the index file as (key_length+4)/0.67, summed over all keys. (This is for the worst case when all keys are inserted in sorted order and we don't have any compressed keys.)

String indexes are space compressed. If the first index part is a string, it will also be prefix compressed. Space compression makes the index file smaller than the above figures if the string column has a lot of trailing space or is a VARCHAR column that is not always used to the full length. Prefix compression is used on keys that start with a string. Prefix compression helps if there are many strings with an identical prefix.

In MyISAM tables, you can also prefix compress numbers by specifying PACK_KEYS=1 when you create the table. This helps when you have many integer keys that have an identical prefix when the numbers are stored high-byte first.

7.1.2 MyISAM Table Formats

MyISAM supports 3 different table types. Two of them are chosen automatically depending on the type of columns you are using. The third, compressed tables, can only be created with the myisampack tool.

7.1.2.1 Static (Fixed-length) Table Characteristics

This is the default format. It's used when the table contains no VARCHAR, BLOB, or TEXT columns.

This format is the simplest and most secure format. It is also the fastest of the on-disk formats. The speed comes from the easy way data can be found on disk. When looking up something with an index and static format it is very simple. Just multiply the row number by the row length.

Also, when scanning a table it is very easy to read a constant number of records with each disk read.

The security is evidenced if your computer crashes when writing to a fixed-size MyISAM file, in which case myisamchk can easily figure out where each row starts and ends. So it can usually reclaim all records except the partially written one. Note that in MySQL all indexes can always be reconstructed:

7.1.2.2 Dynamic Table Characteristics

This format is used if the table contains any VARCHAR, BLOB, or TEXT columns or if the table was created with ROW_FORMAT=dynamic.

This format is a little more complex because each row has to have a header that says how long it is. One record can also end up at more than one location when it is made longer at an update.

You can use OPTIMIZE table or myisamchk to defragment a table. If you have static data that you access/change a lot in the same table as some VARCHAR or BLOB columns, it might be a good idea to move the dynamic columns to other tables just to avoid fragmentation:

7.1.2.3 Compressed Table Characteristics

This is a read-only type that is generated with the optional myisampack tool (pack_isam for ISAM tables):

7.1.3 MyISAM table problems.

The file format that MySQL uses to store data has been extensively tested, but there are always circumstances that may cause database tables to become corrupted.

7.1.3.1 Corrupted MyISAM tables.

Even if the MyISAM table format is very reliable (all changes to a table is written before the SQL statements returns) , you can still get corrupted tables if some of the following things happens:

Typial typical symptoms for a corrupt table is:

You can check if a table is ok with the command CHECK TABLE. See section 4.4.4 CHECK TABLE Syntax.

You can repair a corrupted table with REPAIR TABLE. See section 4.4.5 REPAIR TABLE Syntax. You can also repair a table, when mysqld is not running with the myisamchk command. myisamchk syntax.

If your tables get corrupted a lot you should try to find the reason for this! See section A.4.1 What To Do If MySQL Keeps Crashing.

In this case the most important thing to know is if the table got corrupted if the mysqld died (one can easily verify this by checking if there is a recent row restarted mysqld in the mysqld error file). If this isn't the case, then you should try to make a test case of this. See section G.1.6 Making a test case when you experience table corruption.

7.1.3.2 Clients is using or hasn't closed the table properly

Each MyISAM .MYI file has in the header a counter that can be used to check if a table has been closed properly.

If you get the following warning from CHECK TABLE or myisamchk: 

# clients is using or hasn't closed the table properly

this means that this counter has come out of sync. This doesn't mean that the table is corrupted, but means that you should at least do a check on the table to verify that it's ok.

The counter works as follows:

In other words, the only ways this can go out of sync are:

7.2 MERGE Tables

MERGE tables are new in MySQL Version 3.23.25. The code is still in gamma, but should be resonable stable.

A MERGE table is a collection of identical MyISAM tables that can be used as one. You can only SELECT, DELETE, and UPDATE from the collection of tables. If you DROP the MERGE table, you are only dropping the MERGE specification.

Note that DELETE FROM merge_table used without a WHERE will only clear the mapping for the table, not delete everything in the mapped tables. (We plan to fix this in 4.0).

With identical tables we mean that all tables are created with identical column and key information. You can't put a MERGE over tables where the columns are packed differently, doesn't have exactly the same columns or have the keys in different order. Some of the tables can however be compressed with myisampack. See section 4.7.4 myisampack, The MySQL Compressed Read-only Table Generator.

When you create a MERGE table, you will get a .frm table definition file and a .MRG table list file. The .MRG just contains a list of the index files (.MYI files) that should be used as one. All used tables must be in the same database as the MERGE table itself.

For the moment you need to have SELECT, UPDATE, and DELETE privileges on the tables you map to a MERGE table.

MERGE tables can help you solve the following problems:

The disadvantages with MERGE tables are:

The following example shows you how to use MERGE tables: 

CREATE TABLE t1 (a INT AUTO_INCREMENT PRIMARY KEY, message CHAR(20));
CREATE TABLE t2 (a INT AUTO_INCREMENT PRIMARY KEY, message CHAR(20));
INSERT INTO t1 (message) VALUES ("Testing"),("table"),("t1");
INSERT INTO t2 (message) VALUES ("Testing"),("table"),("t2");
CREATE TABLE total (a INT NOT NULL, message CHAR(20), KEY(a)) TYPE=MERGE UNION=(t1,t2);

Note that we didn't create a UNIQUE or PRIMARY KEY in the total table as the key isn't going to be unique in the total table.

Note that you can also manipulate the .MRG file directly from the outside of the MySQL server: 

shell> cd /mysql-data-directory/current-database
shell> ls -1 t1.MYI t2.MYI > total.MRG
shell> mysqladmin flush-tables

Now you can do things like: 

mysql> select * from total;
+---+---------+
| a | message |
+---+---------+
| 1 | Testing |
| 2 | table   |
| 3 | t1      |
| 1 | Testing |
| 2 | table   |
| 3 | t2      |
+---+---------+

To remap a MERGE table you can do one of the following:

7.3 ISAM Tables

You can also use the deprecated ISAM table type. This will disappear rather soon because MyISAM is a better implementation of the same thing. ISAM uses a B-tree index. The index is stored in a file with the .ISM extension, and the data is stored in a file with the .ISD extension. You can check/repair ISAM tables with the isamchk utility. See section 4.4.6.7 Using myisamchk for Crash Recovery.

ISAM has the following features/properties:

Most of the things true for MyISAM tables are also true for ISAM tables. See section 7.1 MyISAM Tables. The major differences compared to MyISAM tables are:

If you want to convert an ISAM table to a MyISAM table so that you can use utilities such as mysqlcheck, use an ALTER TABLE statement: 

mysql> ALTER TABLE tbl_name TYPE = MYISAM;

7.4 HEAP Tables

HEAP tables use a hashed index and are stored in memory. This makes them very fast, but if MySQL crashes you will lose all data stored in them. HEAP is very useful for temporary tables!

The MySQL internal HEAP tables use 100% dynamic hashing without overflow areas. There is no extra space needed for free lists. HEAP tables also don't have problems with delete + inserts, which normally is common with hashed tables: 

mysql> CREATE TABLE test TYPE=HEAP SELECT ip,SUM(downloads) as down
        FROM log_table GROUP BY ip;
mysql> SELECT COUNT(ip),AVG(down) FROM test;
mysql> DROP TABLE test;

Here are some things you should consider when you use HEAP tables:

The memory needed for one row in a HEAP table is: 

SUM_OVER_ALL_KEYS(max_length_of_key + sizeof(char*) * 2)
+ ALIGN(length_of_row+1, sizeof(char*))

sizeof(char*) is 4 on 32-bit machines and 8 on 64-bit machines.

7.5 BDB or Berkeley_DB Tables

7.5.1 Overview of BDB Tables

Support for BDB tables is included in the MySQL source distribution starting from Version 3.23.34 and is activated in the MySQL-Max binary.

BerkeleyDB, available at http://www.sleepycat.com/ has provided MySQL with a transactional table handler. By using BerkeleyDB tables, your tables may have a greater chance of surviving crashes, and also provides COMMIT and ROLLBACK on transactions. The MySQL source distribution comes with a BDB distribution that has a couple of small patches to make it work more smoothly with MySQL. You can't use a non-patched BDB version with MySQL.

We at MySQL AB are working in close cooperation with Sleepycat to keep the quality of the MySQL/BDB interface high.

When it comes to supporting BDB tables, we are committed to help our users to locate the problem and help creating a reproducable test case for any problems involving BDB tables. Any such test case will be forwarded to Sleepycat who in turn will help us find and fix the problem. As this is a two stage operation, any problems with BDB tables may take a little longer for us to fix than for other table handlers. However, as the BerkeleyDB code itself has been used by many other applications than MySQL, we don't envision any big problems with this. See section 1.3.5.6 Support for other table handlers.

7.5.2 Installing BDB

If you have downloaded a binary version of MySQL that includes support for BerkeleyDB, simply follow the instructions for installing a binary version of MySQL. See section M.1 Installing a MySQL Binary Distribution. See section 4.7.5 mysqld-max, An extended mysqld server.

To compile MySQL with Berkeley DB support, download MySQL Version 3.23.34 or newer and configure MySQL with the --with-berkeley-db option. See section 2.3 Installing a MySQL Source Distribution. 

cd /path/to/source/of/mysql-3.23.34
./configure --with-berkeley-db

Please refer to the manual provided with the BDB distribution for more updated information.

Even though Berkeley DB is in itself very tested and reliable, the MySQL interface is still considered beta quality. We are actively improving and optimizing it to get it stable very soon.

7.5.3 BDB startup options

If you are running with AUTOCOMMIT=0 then your changes in BDB tables will not be updated until you execute COMMIT. Instead of commit you can execute ROLLBACK to forget your changes. See section 6.7.1 BEGIN/COMMIT/ROLLBACK Syntax.

If you are running with AUTOCOMMIT=1 (the default), your changes will be committed immediately. You can start an extended transaction with the BEGIN WORK SQL command, after which your changes will not be committed until you execute COMMIT (or decide to ROLLBACK the changes).

The following options to mysqld can be used to change the behavior of BDB tables:

Option Meaning
--bdb-home=directory Base directory for BDB tables. This should be the same directory you use for --datadir.
--bdb-lock-detect=# Berkeley lock detect. One of (DEFAULT, OLDEST, RANDOM, or YOUNGEST).
--bdb-logdir=directory Berkeley DB log file directory.
--bdb-no-sync Don't synchronously flush logs.
--bdb-no-recover Don't start Berkeley DB in recover mode.
--bdb-shared-data Start Berkeley DB in multi-process mode (Don't use DB_PRIVATE when initializing Berkeley DB)
--bdb-tmpdir=directory Berkeley DB tempfile name.
--skip-bdb Don't use berkeley db.
-O bdb_max_lock=1000 Set the maximum number of locks possible. See section 4.5.5.4 SHOW VARIABLES.

If you use --skip-bdb, MySQL will not initialize the Berkeley DB library and this will save a lot of memory. Of course, you cannot use BDB tables if you are using this option.

Normally you should start mysqld without --bdb-no-recover if you intend to use BDB tables. This may, however, give you problems when you try to start mysqld if the BDB log files are corrupted. See section 2.4.2 Problems Starting the MySQL Server.

With bdb_max_lock you can specify the maximum number of locks (10000 by default) you can have active on a BDB table. You should increase this if you get errors of type bdb: Lock table is out of available locks or Got error 12 from ... when you have do long transactions or when mysqld has to examine a lot of rows to calculate the query.

You may also want to change binlog_cache_size and max_binlog_cache_size if you are using big multi-line transactions. See section 6.7.1 BEGIN/COMMIT/ROLLBACK Syntax.

7.5.4 Some characteristic of BDB tables:

7.5.5 Some things we need to fix for BDB in the near future:

7.5.6 Operating systems supported by BDB

If you after having built MySQL with support for BDB tables get the following error in the log file when you start mysqld: 

bdb: architecture lacks fast mutexes: applications cannot be threaded
Can't init dtabases

This means that BDB tables are not supported for your architecture. In this case you have to rebuild MySQL without BDB table support.

NOTE: The following list is not complete; We will update this as we get more information about this.

Currently we know that BDB tables works with the following operating system.

It doesn't work with the following operating systems:

7.5.7 Errors You May Get When Using BDB Tables

7.6 InnoDB Tables

7.6.1 InnoDB tables overview

InnoDB tables are included in the MySQL source distribution starting from 3.23.34a and are activated in the MySQL -max binary.

If you have downloaded a binary version of MySQL that includes support for InnoDB (mysqld-max), simply follow the instructions for installing a binary version of MySQL. See section M.1 Installing a MySQL Binary Distribution. See section 4.7.5 mysqld-max, An extended mysqld server.

To compile MySQL with InnoDB support, download MySQL-3.23.37 or newer and configure MySQL with the --with-innodb option. See section 2.3 Installing a MySQL Source Distribution. 

cd /path/to/source/of/mysql-3.23.37
./configure --with-innodb

To get InnoDB to work you have to specify where the data for InnoDB tables should be stored by specifying the innodb_data_file_path option on the command line or in an MySQL option file. See section 7.6.2 InnoDB startup options. If you have configured MySQL for InnoDB but you have not specified the above option, mysqld will print at start: 

Can't initialize InnoDB as 'innodb_data_file_path' is not set

InnoDB provides MySQL with a transaction-safe table handler with commit, rollback, and crash recovery capabilities. InnoDB does locking on row level, and also provides an Oracle-style consistent non-locking read in SELECTS, which increases transaction concurrency. There is not need for lock escalation in InnoDB, because row level locks in InnoDB fit in very small space.

InnoDB has been designed for maximum performance when processing large data volumes. Its CPU efficiency is probably not matched by any other disk-based relational database engine.

You can find the latest information about InnoDB at http://www.innodb.com. The most up-to-date version of the InnoDB manual is always placed there, and you can also order commercial support for InnoDB.

Technically, InnoDB is a database backend placed under MySQL. InnoDB has its own buffer pool for caching data and indexes in main memory. InnoDB stores its tables and indexes in a tablespace, which may consist of several files. This is different from, for example, MyISAM tables where each table is stored as a separate file.

InnoDB is distributed under the GNU GPL License Version 2 (of June 1991). In the source distribution of MySQL, InnoDB appears as a subdirectory.

7.6.2 InnoDB startup options

Beginning from MySQL-3.23.37 the prefix of the options is changed from innobase_... to innodb_....

To use InnoDB tables you MUST specify configuration parameters in the MySQL configuration file in the [mysqld] section of the configuration file `my.cnf'. See section 4.1.2 my.cnf Option Files.

The only required parameter to use InnoDB is innodb_data_file_path, but you should set others if you want to get a better performance.

Suppose you have a Windows NT machine with 128 MB RAM and a single 10 GB hard disk. Below is an example of possible configuration parameters in `my.cnf' for InnoDB: 

innodb_data_file_path = ibdata1:2000M;ibdata2:2000M
innodb_data_home_dir = c:\ibdata
set-variable = innodb_mirrored_log_groups=1
innodb_log_group_home_dir = c:\iblogs
set-variable = innodb_log_files_in_group=3
set-variable = innodb_log_file_size=30M
set-variable = innodb_log_buffer_size=8M
innodb_flush_log_at_trx_commit=1
innodb_log_arch_dir = c:\iblogs
innodb_log_archive=0
set-variable = innodb_buffer_pool_size=80M
set-variable = innodb_additional_mem_pool_size=10M
set-variable = innodb_file_io_threads=4
set-variable = innodb_lock_wait_timeout=50

Note that data files must be < 4G, and < 2G on some file systems! The total size of data files has to be >= 10 MB. InnoDB does not create directories: you have to create them yourself.

Suppose you have a Linux machine with 512 MB RAM and three 20 GB hard disks (at directory paths `/', `/dr2' and `/dr3'). Below is an example of possible configuration parameters in `my.cnf' for InnoDB: 

innodb_data_file_path = ibdata/ibdata1:2000M;dr2/ibdata/ibdata2:2000M
innodb_data_home_dir = /
set-variable = innodb_mirrored_log_groups=1
innodb_log_group_home_dir = /dr3
set-variable = innodb_log_files_in_group=3
set-variable = innodb_log_file_size=50M
set-variable = innodb_log_buffer_size=8M
innodb_flush_log_at_trx_commit=1
innodb_log_arch_dir = /dr3/iblogs
innodb_log_archive=0
set-variable = innodb_buffer_pool_size=400M
set-variable = innodb_additional_mem_pool_size=20M
set-variable = innodb_file_io_threads=4
set-variable = innodb_lock_wait_timeout=50

Note that we have placed the two data files on different disks. The reason for the name innodb_data_file_path is that you can also specify paths to your data files, and innodb_data_home_dir is just textually catenated before your data file paths, adding a possible slash or backslash in between. InnoDB will fill the tablespace formed by the data files from bottom up. In some cases it will improve the performance of the database if all data is not placed on the same physical disk. Putting log files on a different disk from data is very often beneficial for performance.

The meanings of the configuration parameters are the following:

innodb_data_home_dir The common part of the directory path for all InnoDB data files.
innodb_data_file_path Paths to individual data files and their sizes. The full directory path to each data file is acquired by concatenating innodb_data_home_dir to the paths specified here. The file sizes are specified in megabytes, hence the 'M' after the size specification above. Do not set a file size bigger than 4000M, and on most operating systems not bigger than 2000M. InnoDB also understands the abbreviation 'G', 1G meaning 1024M. The sum of the sizes of the files must be at least 10 MB.
innodb_mirrored_log_groups Number of identical copies of log groups we keep for the database. Currently this should be set to 1.
innodb_log_group_home_dir Directory path to InnoDB log files.
innodb_log_files_in_group Number of log files in the log group. InnoDB writes to the files in a circular fashion. Value 3 is recommended here.
innodb_log_file_size Size of each log file in a log group in megabytes. Sensible values range from 1M to the size of the buffer pool specified below. The bigger the value, the less checkpoint flush activity is needed in the buffer pool, saving disk i/o. But bigger log files also mean that recovery will be slower in case of a crash. File size restriction as for a data file.
innodb_log_buffer_size The size of the buffer which InnoDB uses to write log to the log files on disk. Sensible values range from 1M to half the combined size of log files. A big log buffer allows large transactions to run without a need to write the log to disk until the transaction commit. Thus, if you have big transactions, making the log buffer big will save disk i/o.
innodb_flush_log_at_trx_commit Normally this is set to 1, meaning that at a transaction commit the log is flushed to disk, and the modifications made by the transaction become permanent, and survive a database crash. If you are willing to compromise this safety, and you are running small transactions, you may set this to 0 to reduce disk i/o to the logs.
innodb_log_arch_dir The directory where fully written log files would be archived if we used log archiving. The value of this parameter should currently be set the same as innodb_log_group_home_dir.
innodb_log_archive This value should currently be set to 0. As recovery from a backup is done by MySQL using its own log files, there is currently no need to archive InnoDB log files.
innodb_buffer_pool_size The size of the memory buffer InnoDB uses to cache data and indexes of its tables. The bigger you set this the less disk i/o is needed to access data in tables. On a dedicated database server you may set this parameter up to 90 % of the machine physical memory size. Do not set it too large, though, because competition of the physical memory may cause paging in the operating system.
innodb_additional_mem_pool_size Size of a memory pool InnoDB uses to store data dictionary information and other internal data structures. A sensible value for this might be 2M, but the more tables you have in your application the more you will need to allocate here. If InnoDB runs out of memory in this pool, it will start to allocate memory from the operating system, and write warning messages to the MySQL error log.
innodb_file_io_threads Number of file i/o threads in InnoDB. Normally, this should be 4, but on Windows NT disk i/o may benefit from a larger number.
innodb_lock_wait_timeout Timeout in seconds an InnoDB transaction may wait for a lock before being rolled back. InnoDB automatically detects transaction deadlocks in its own lock table and rolls back the transaction. If you use LOCK TABLES command, or other transaction-safe table handlers than InnoDB in the same transaction, then a deadlock may arise which InnoDB cannot notice. In cases like this the timeout is useful to resolve the situation.
innodb_flush_method (Available from 3.23.40 up.) The default value for this is fdatasync. Another option is O_DSYNC.

7.6.3 Creating InnoDB table space

Suppose you have installed MySQL and have edited `my.cnf' so that it contains the necessary InnoDB configuration parameters. Before starting MySQL you should check that the directories you have specified for InnoDB data files and log files exist and that you have access rights to those directories. InnoDB cannot create directories, only files. Check also you have enough disk space for the data and log files.

When you now start MySQL, InnoDB will start creating your data files and log files. InnoDB will print something like the following: 

~/mysqlm/sql > mysqld
InnoDB: The first specified data file /home/heikki/data/ibdata1 did not exist:
InnoDB: a new database to be created!
InnoDB: Setting file /home/heikki/data/ibdata1 size to 134217728
InnoDB: Database physically writes the file full: wait...
InnoDB: Data file /home/heikki/data/ibdata2 did not exist: new to be created
InnoDB: Setting file /home/heikki/data/ibdata2 size to 262144000
InnoDB: Database physically writes the file full: wait...
InnoDB: Log file /home/heikki/data/logs/ib_logfile0 did not exist: new to be c
reated
InnoDB: Setting log file /home/heikki/data/logs/ib_logfile0 size to 5242880
InnoDB: Log file /home/heikki/data/logs/ib_logfile1 did not exist: new to be c
reated
InnoDB: Setting log file /home/heikki/data/logs/ib_logfile1 size to 5242880
InnoDB: Log file /home/heikki/data/logs/ib_logfile2 did not exist: new to be c
reated
InnoDB: Setting log file /home/heikki/data/logs/ib_logfile2 size to 5242880
InnoDB: Started
mysqld: ready for connections

A new InnoDB database has now been created. You can connect to the MySQL server with the usual MySQL client programs like mysql. When you shut down the MySQL server with `mysqladmin shutdown', InnoDB output will be like the following: 

010321 18:33:34  mysqld: Normal shutdown
010321 18:33:34  mysqld: Shutdown Complete
InnoDB: Starting shutdown...
InnoDB: Shutdown completed

You can now look at the data files and logs directories and you will see the files created. The log directory will also contain a small file named `ib_arch_log_0000000000'. That file resulted from the database creation, after which InnoDB switched off log archiving. When MySQL is again started, the output will be like the following: 

~/mysqlm/sql > mysqld
InnoDB: Started
mysqld: ready for connections

7.6.3.1 If something goes wrong in database creation

If something goes wrong in an InnoDB database creation, you should delete all files created by InnoDB. This means all data files, all log files, the small archived log file, and in the case you already did create some InnoDB tables, delete also the corresponding `.frm' files for these tables from the MySQL database directories. Then you can try the InnoDB database creation again.

7.6.4 Creating InnoDB tables

Suppose you have started the MySQL client with the command mysql test. To create a table in the InnoDB format you must specify TYPE = InnoDB in the table creation SQL command: 

CREATE TABLE CUSTOMER (A INT, B CHAR (20), INDEX (A)) TYPE = InnoDB;

This SQL command will create a table and an index on column A into the InnoDB tablespace consisting of the data files you specified in `my.cnf'. In addition MySQL will create a file `CUSTOMER.frm' to the MySQL database directory `test'. Internally, InnoDB will add to its own data dictionary an entry for table 'test/CUSTOMER'. Thus you can create a table of the same name CUSTOMER in another database of MySQL, and the table names will not collide inside InnoDB.

You can query the amount of free space in the InnoDB tablespace by issuing the table status command of MySQL for any table you have created with TYPE = InnoDB. Then the amount of free space in the tablespace appears in the table comment section in the output of SHOW. An example: 

SHOW TABLE STATUS FROM test LIKE 'CUSTOMER'

Note that the statistics SHOW gives about InnoDB tables are only approximate: they are used in SQL optimization. Table and index reserved sizes in bytes are accurate, though.

NOTE: DROP DATABASE does not currently work for InnoDB tables! You must drop the tables individually. Also take care not to delete or add `.frm' files to your InnoDB database manually: use CREATE TABLE and DROP TABLE commands. InnoDB has its own internal data dictionary, and you will get problems if the MySQL `.frm' files are out of 'sync' with the InnoDB internal data dictionary.

7.6.4.1 Converting MyISAM tables to InnoDB

InnoDB does not have a special optimization for separate index creation. Therefore it does not pay to export and import the table and create indexes afterwards. The fastest way to alter a table to InnoDB is to do the inserts directly to an InnoDB table, that is, use ALTER TABLE ... TYPE=INNODB, or create an empty InnoDB table with identical definitions and insert the rows with INSERT INTO ... SELECT * FROM ....

To get better control over the insertion process, it may be good to insert big tables in pieces: 

INSERT INTO newtable SELECT * FROM oldtable WHERE yourkey > something
                                             AND yourkey <= somethingelse;

After all data has been inserted you can rename the tables.

During the conversion of big tables you should set the InnoDB buffer pool size big to reduce disk i/o. Not bigger than 80 % of the physical memory, though. You should set InnoDB log files big, and also the log buffer large.

Make sure you do not run out of tablespace: InnoDB tables take a lot more space than MyISAM tables. If an ALTER TABLE runs out of space, it will start a rollback, and that can take hours if it is disk-bound. In inserts InnoDB uses the insert buffer to merge secondary index records to indexes in batches. That saves a lot of disk i/o. In rollback no such mechanism is used, and the rollback can take 30 times longer than the insertion.

In the case of a runaway rollback, if you do not have valuable data in your database, it is better that you kill the database process and delete all InnoDB data and log files and all InnoDB table `.frm' files, and start your job again, rather than wait for millions of disk i/os to complete.

7.6.5 Adding and removing InnoDB data and log files

You cannot increase the size of an InnoDB data file. To add more into your tablespace you have to add a new data file. To do this you have to shut down your MySQL database, edit the `my.cnf' file, adding a new file to innodb_data_file_path, and then start MySQL again.

Currently you cannot remove a data file from InnoDB. To decrease the size of your database you have to use mysqldump to dump all your tables, create a new database, and import your tables to the new database.

If you want to change the number or the size of your InnoDB log files, you have to shut down MySQL and make sure that it shuts down without errors. Then copy the old log files into a safe place just in case something went wrong in the shutdown and you will need them to recover the database. Delete then the old log files from the log file directory, edit `my.cnf', and start MySQL again. InnoDB will tell you at the startup that it is creating new log files.

7.6.6 Backing up and recovering an InnoDB database

The key to safe database management is taking regular backups. To take a 'binary' backup of your database you have to do the following:

There is currently no on-line or incremental backup tool available for InnoDB, though they are in the TODO list.

In addition to taking the binary backups described above, you should also regularly take dumps of your tables with `mysqldump'. The reason to this is that a binary file may be corrupted without you noticing it. Dumped tables are stored into text files which are human-readable and much simpler than database binary files. Seeing table corruption from dumped files is easier, and since their format is simpler, the chance for serious data corruption in them is smaller.

A good idea is to take the dumps at the same time you take a binary backup of your database. You have to shut out all clients from your database to get a consistent snapshot of all your tables into your dumps. Then you can take the binary backup, and you will then have a consistent snapshot of your database in two formats.

To be able to recover your InnoDB database to the present from the binary backup described above, you have to run your MySQL database with the general logging and log archiving of MySQL switched on. Here by the general logging we mean the logging mechanism of the MySQL server which is independent of InnoDB logs.

To recover from a crash of your MySQL server process, the only thing you have to do is to restart it. InnoDB will automatically check the logs and perform a roll-forward of the database to the present. InnoDB will automatically roll back uncommitted transactions which were present at the time of the crash. During recovery, InnoDB will print out something like the following: 

~/mysqlm/sql > mysqld
InnoDB: Database was not shut down normally.
InnoDB: Starting recovery from log files...
InnoDB: Starting log scan based on checkpoint at
InnoDB: log sequence number 0 13674004
InnoDB: Doing recovery: scanned up to log sequence number 0 13739520
InnoDB: Doing recovery: scanned up to log sequence number 0 13805056
InnoDB: Doing recovery: scanned up to log sequence number 0 13870592
InnoDB: Doing recovery: scanned up to log sequence number 0 13936128
...
InnoDB: Doing recovery: scanned up to log sequence number 0 20555264
InnoDB: Doing recovery: scanned up to log sequence number 0 20620800
InnoDB: Doing recovery: scanned up to log sequence number 0 20664692
InnoDB: 1 uncommitted transaction(s) which must be rolled back
InnoDB: Starting rollback of uncommitted transactions
InnoDB: Rolling back trx no 16745
InnoDB: Rolling back of trx no 16745 completed
InnoDB: Rollback of uncommitted transactions completed
InnoDB: Starting an apply batch of log records to the database...
InnoDB: Apply batch completed
InnoDB: Started
mysqld: ready for connections

If your database gets corrupted or your disk fails, you have to do the recovery from a backup. In the case of corruption, you should first find a backup which is not corrupted. From a backup do the recovery from the general log files of MySQL according to instructions in the MySQL manual.

7.6.6.1 Checkpoints

InnoDB implements a checkpoint mechanism called a fuzzy checkpoint. InnoDB will flush modified database pages from the buffer pool in small batches, there is no need to flush the buffer pool in one single batch, which would in practice stop processing of user SQL statements for a while.

In crash recovery InnoDB looks for a checkpoint label written to the log files. It knows that all modifications to the database before the label are already present on the disk image of the database. Then InnoDB scans the log files forward from the place of the checkpoint applying the logged modifications to the database.

InnoDB writes to the log files in a circular fashion. All committed modifications which make the database pages in the buffer pool different from the images on disk must be available in the log files in case InnoDB has to do a recovery. This means that when InnoDB starts to reuse a log file in the circular fashion, it has to make sure that the database page images on disk already contain the modifications logged in the log file InnoDB is going to reuse. In other words, InnoDB has to make a checkpoint and often this involves flushing of modified database pages to disk.

The above explains why making your log files very big may save disk i/o in checkpointing. It can make sense to set the total size of the log files as big as the buffer pool or even bigger. The drawback in big log files is that crash recovery can last longer because there will be more log to apply to the database.

7.6.7 Moving an InnoDB database to another machine

InnoDB data and log files are binary-compatible on all platforms if the floating point number format on the machines is the same. You can move an InnoDB database simply by copying all the relevant files, which we already listed in the previous section on backing up a database. If the floating point formats on the machines are different but you have not used FLOAT or DOUBLE data types in your tables then the procedure is the same: just copy the relevant files. If the formats are different and your tables contain floating point data, you have to use `mysqldump' and `mysqlimport' to move those tables.

A performance tip is to switch off the auto commit when you import data into your database, assuming your tablespace has enough space for the big rollback segment the big import transaction will generate. Do the commit only after importing a whole table or a segment of a table.

7.6.8 InnoDB transaction model

In the InnoDB transaction model the goal has been to combine the best sides of a multiversioning database to traditional two-phase locking. InnoDB does locking on row level and runs queries by default as non-locking consistent reads, in the style of Oracle. The lock table in InnoDB is stored so space-efficiently that lock escalation is not needed: typically several users are allowed to lock every row in the database, or any random subset of the rows, without InnoDB running out of memory.

In InnoDB all user activity happens inside transactions. If the auto commit mode is used in MySQL, then each SQL statement will form a single transaction. If the auto commit mode is switched off, then we can think that a user always has a transaction open. If he issues the SQL COMMIT or ROLLBACK statement, that ends the current transaction, and a new starts. Both statements will release all InnoDB locks that were set during the current transaction. A COMMIT means that the changes made in the current transaction are made permanent and become visible to other users. A ROLLBACK on the other hand cancels all modifications made by the current transaction.

7.6.8.1 Consistent read

A consistent read means that InnoDB uses its multiversioning to present to a query a snapshot of the database at a point in time. The query will see the changes made by exactly those transactions that committed before that point of time, and no changes made by later or uncommitted transactions. The exception to this rule is that the query will see the changes made by the transaction itself which issues the query.

When a transaction issues its first consistent read, InnoDB assigns the snapshot, or the point of time, which all consistent reads in the same transaction will use. In the snapshot are all transactions that committed before assigning the snapshot. Thus the consistent reads within the same transaction will also be consistent with respect to each other. You can get a fresher snapshot for your queries by committing the current transaction and after that issuing new queries.

Consistent read is the default mode in which InnoDB processes SELECT statements. A consistent read does not set any locks on the tables it accesses, and therefore other users are free to modify those tables at the same time a consistent read is being performed on the table.

7.6.8.2 Locking reads

A consistent read is not convenient in some circumstances. Suppose you want to add a new row into your table CHILD, and make sure that the child already has a parent in table PARENT.

Suppose you use a consistent read to read the table PARENT and indeed see the parent of the child in the table. Can you now safely add the child row to table CHILD? No, because it may happen that meanwhile some other user has deleted the parent row from the table PARENT, and you are not aware of that.

The solution is to perform the SELECT in a locking mode, LOCK IN SHARE MODE. 

SELECT * FROM PARENT WHERE NAME = 'Jones' LOCK IN SHARE MODE;

Performing a read in share mode means that we read the latest available data, and set a shared mode lock on the rows we read. If the latest data belongs to a yet uncommitted transaction of another user, we will wait until that transaction commits. A shared mode lock prevents others from updating or deleting the row we have read. After we see that the above query returns the parent 'Jones', we can safely add his child to table CHILD, and commit our transaction. This example shows how to implement referential integrity in your application code.

Let us look at another example: we have an integer counter field in a table CHILD_CODES which we use to assign a unique identifier to each child we add to table CHILD. Obviously, using a consistent read or a shared mode read to read the present value of the counter is not a good idea, since then two users of the database may see the same value for the counter, and we will get a duplicate key error when we add the two children with the same identifier to the table.

In this case there are two good ways to implement the reading and incrementing of the counter: (1) update the counter first by incrementing it by 1 and only after that read it, or (2) read the counter first with a lock mode FOR UPDATE, and increment after that: 

SELECT COUNTER_FIELD FROM CHILD_CODES FOR UPDATE;
UPDATE CHILD_CODES SET COUNTER_FIELD = COUNTER_FIELD + 1;

A SELECT ... FOR UPDATE will read the latest available data setting exclusive locks on each row it reads. Thus it sets the same locks a searched SQL UPDATE would set on the rows.

7.6.8.3 Next-key locking: avoiding the phantom problem

In row level locking InnoDB uses an algorithm called next-key locking. InnoDB does the row level locking so that when it searches or scans an index of a table, it sets shared or exclusive locks on the index records in encounters. Thus the row level locks are more precisely called index record locks.

The locks InnoDB sets on index records also affect the 'gap' before that index record. If a user has a shared or exclusive lock on record R in an index, then another user cannot insert a new index record immediately before R in the index order. This locking of gaps is done to prevent the so-called phantom problem. Suppose I want to read and lock all children with identifier bigger than 100 from table CHILD, and update some field in the selected rows. 

SELECT * FROM CHILD WHERE ID > 100 FOR UPDATE;

Suppose there is an index on table CHILD on column ID. Our query will scan that index starting from the first record where ID is bigger than 100. Now, if the locks set on the index records would not lock out inserts made in the gaps, a new child might meanwhile be inserted to the table. If now I in my transaction execute 

SELECT * FROM CHILD WHERE ID > 100 FOR UPDATE;

again, I will see a new child in the result set the query returns. This is against the isolation principle of transactions: a transaction should be able to run so that the data it has read does not change during the transaction. If we regard a set of rows as a data item, then the new 'phantom' child would break this isolation principle.

When InnoDB scans an index it can also lock the gap after the last record in the index. Just that happens in the previous example: the locks set by InnoDB will prevent any insert to the table where ID would be bigger than 100.

You can use the next-key locking to implement a uniqueness check in your application: if you read your data in share mode and do not see a duplicate for a row you are going to insert, then you can safely insert your row and know that the next-key lock set on the successor of your row during the read will prevent anyone meanwhile inserting a duplicate for your row. Thus the next-key locking allows you to 'lock' the non-existence of something in your table.

7.6.8.4 Locks set by different SQL statements in InnoDB

7.6.8.5 Deadlock detection and rollback

InnoDB automatically detects a deadlock of transactions and rolls back the transaction whose lock request was the last one to build a deadlock, that is, a cycle in the waits-for graph of transactions. InnoDB cannot detect deadlocks where a lock set by a MySQL LOCK TABLES statement is involved, or if a lock set in another table handler than InnoDB is involved. You have to resolve these situations using innodb_lock_wait_timeout set in `my.cnf'.

When InnoDB performs a complete rollback of a transaction, all the locks of the transaction are released. However, if just a single SQL statement is rolled back as a result of an error, some of the locks set by the SQL statement may be preserved. This is because InnoDB stores row locks in a format where it cannot afterwards know which was set by which SQL statement.

7.6.8.6 An example of how the consistent read works in InnoDB

When you issue a consistent read, that is, an ordinary SELECT statement, InnoDB will give your transaction a timepoint according to which your query sees the database. Thus, if transaction B deletes a row and commits after your timepoint was assigned, then you will not see the row deleted. Similarly with inserts and updates.

You can advance your timepoint by committing your transaction and then doing another SELECT.

This is called multiversioned concurrency control. 

                  User A                 User B

              set autocommit=0;      set autocommit=0;
time
|             SELECT * FROM t;
|             empty set
|                                    INSERT INTO t VALUES (1, 2);
|
v             SELECT * FROM t;
              empty set
                                     COMMIT;

              SELECT * FROM t;
              empty set;

              COMMIT;

              SELECT * FROM t;
              ----------------------
              |     1    |    2   |
              ----------------------

Thus user A sees the row inserted by B only when B has committed the insert, and A has committed his own transaction so that the timepoint is advanced past the the commit of B.

If you want to see the 'freshest' state of the database, you should use a locking read: 

SELECT * FROM t LOCK IN SHARE MODE;

7.6.9 Performance tuning tips

1. If the Unix `top' or the Windows `Task Manager' shows that the CPU usage percentage with your workload is less than 70 %, your workload is probably disk-bound. Maybe you are making too many transaction commits, or the buffer pool is too small. Making the buffer pool bigger can help, but do not set it bigger than 80 % of physical memory.

2. Wrap several modifications into one transaction. InnoDB must flush the log to disk at each transaction commit, if that transaction made modifications to the database. Since the rotation speed of a disk is typically at most 167 revolutions/second, that constrains the number of commits to the same 167/second if the disk does not fool the operating system.

3. If you can afford the loss of some latest committed transactions, you can set the `my.cnf' parameter innodb_flush_log_at_trx_commit to zero. InnoDB tries to flush the log anyway once in a second, though the flush is not guaranteed.

4. Make your log files big, even as big as the buffer pool. When InnoDB has written the log files full, it has to write the modified contents of the buffer pool to disk in a checkpoint. Small log files will cause many unnecessary disk writes. The drawback in big log files is that recovery time will be longer.

5. Also the log buffer should be quite big, say 8 MB.

6. (Relevant from 3.23.39 up.) In some versions of Linux and Unix, flushing files to disk with the Unix fdatasync and other similar methods is surprisingly slow. The default method InnoDB uses is the fdatasync function. If you are not satisfied with the database write performance, you may try setting innodb_flush_method in `my.cnf' to O_DSYNC, though O_DSYNC seems to be slower on most systems.

7. In importing data to InnoDB, make sure that MySQL does not have autocommit=1 on. Then every insert requires a log flush to disk. Put before your plain SQL import file line 

set autocommit=0;

and after it 

commit;

If you use the `mysqldump' option --opt, you will get dump files which are fast to import also to an InnoDB table, even without wrapping them to the above set autocommit=0; ... commit; wrappers.

8. Beware of big rollbacks of mass inserts: InnoDB uses the insert buffer to save disk i/o in inserts, but in a corresponding rollback no such mechanism is used. A disk-bound rollback can take 30 times the time of the corresponding insert. Killing the database process will not help because the rollback will start again at the database startup. The only way to get rid of a runaway rollback is to increase the buffer pool so that the rollback becomes CPU-bound and runs fast, or delete the whole InnoDB database.

9. Beware also of other big disk-bound operations. Use DROP TABLE or TRUNCATE (from MySQL-4.0 up) to empty a table, not DELETE FROM yourtable.

10. Use the multi-line INSERT to reduce communication overhead between the client and the server if you need to insert many rows: 

INSERT INTO yourtable VALUES (1, 2), (5, 5);

This tip is of course valid for inserts into any table type, not just InnoDB.

7.6.9.1 The InnoDB Monitor

Starting from version 3.23.41 InnoDB includes the InnoDB Monitor which prints information on the InnoDB internal state. When swithed on, InnoDB Monitor will make the MySQL server to print data to the standard output about once every 10 seconds. This data is useful in performance tuning.

The printed information includes data on:

You can start InnoDB Monitor through the following SQL command: 

CREATE TABLE innodb_monitor(a int) type = innodb;

and stop it by 

DROP TABLE innodb_monitor;

The CREATE TABLE syntax is just a way to pass a command to the InnoDB engine through the MySQL SQL parser: the created table is not relevant at all for InnoDB Monitor. If you shut down the database when the monitor is running, and you want to start the monitor again, you have to drop the table before you can issue a new CREATE TABLE to start the monitor. This syntax may change in a future release.

A sample output of the InnoDB Monitor: 

================================
010809 18:45:06 INNODB MONITOR OUTPUT
================================
--------------------------
LOCKS HELD BY TRANSACTIONS
--------------------------
LOCK INFO:
Number of locks in the record hash table 1294
LOCKS FOR TRANSACTION ID 0 579342744
TABLE LOCK table test/mytable trx id 0 582333343 lock_mode IX

RECORD LOCKS space id 0 page no 12758 n bits 104 table test/mytable index
PRIMARY trx id 0 582333343 lock_mode X
Record lock, heap no 2 PHYSICAL RECORD: n_fields 74; 1-byte offs FALSE;
info bits 0
 0: len 4; hex 0001a801; asc ;; 1: len 6; hex 000022b5b39f; asc ";; 2: len 7;
hex 000002001e03ec; asc ;; 3: len 4; hex 00000001;
...
-----------------------------------------------
CURRENT SEMAPHORES RESERVED AND SEMAPHORE WAITS
-----------------------------------------------
SYNC INFO:
Sorry, cannot give mutex list info in non-debug version!
Sorry, cannot give rw-lock list info in non-debug version!
-----------------------------------------------------
SYNC ARRAY INFO: reservation count 6041054, signal count 2913432
4a239430 waited for by thread 49627477 op. S-LOCK file NOT KNOWN line 0 
Mut ex 0 sp 5530989 r 62038708 sys 2155035; rws 0 8257574 8025336; rwx 0 1121090 1848344
-----------------------------------------------------
CURRENT PENDING FILE I/O'S
--------------------------
Pending normal aio reads:
Reserved slot, messages 40157658 4a4a40b8
Reserved slot, messages 40157658 4a477e28
...
Reserved slot, messages 40157658 4a4424a8
Reserved slot, messages 40157658 4a39ea38
Total of 36 reserved aio slots
Pending aio writes:
Total of 0 reserved aio slots
Pending insert buffer aio reads:
Total of 0 reserved aio slots
Pending log writes or reads:
Reserved slot, messages 40158c98 40157f98
Total of 1 reserved aio slots
Pending synchronous reads or writes:
Total of 0 reserved aio slots
-----------
BUFFER POOL
-----------
LRU list length 8034 
Free list length 0 
Flush list length 999 
Buffer pool size in pages 8192
Pending reads 39 
Pending writes: LRU 0, flush list 0, single page 0
Pages read 31383918, created 51310, written 2985115
----------------------------
END OF INNODB MONITOR OUTPUT
============================
010809 18:45:22 InnoDB starts purge
010809 18:45:22 InnoDB purged 0 pages

Some notes on the output:

7.6.10 Implementation of multiversioning

Since InnoDB is a multiversioned database, it must keep information of old versions of rows in the tablespace. This information is stored in a data structure we call a rollback segment after an analogous data structure in Oracle.

InnoDB internally adds two fields to each row stored in the database. A 6-byte field tells the transaction identifier for the last transaction which inserted or updated the row. Also a deletion is internally treated as an update where a special bit in the row is set to mark it as deleted. Each row also contains a 7-byte field called the roll pointer. The roll pointer points to an undo log record written to the rollback segment. If the row was updated, then the undo log record contains the information necessary to rebuild the content of the row before it was updated.

InnoDB uses the information in the rollback segment to perform the undo operations needed in a transaction rollback. It also uses the information to build earlier versions of a row for a consistent read.

Undo logs in the rollback segment are divided into insert and update undo logs. Insert undo logs are only needed in transaction rollback and can be discarded as soon as the transaction commits. Update undo logs are used also in consistent reads, and they can be discarded only after there is no transaction present for which InnoDB has assigned a snapshot that in a consistent read could need the information in the update undo log to build an earlier version of a database row.

You must remember to commit your transactions regularly. Otherwise InnoDB cannot discard data from the update undo logs, and the rollback segment may grow too big, filling up your tablespace.

The physical size of an undo log record in the rollback segment is typically smaller than the corresponding inserted or updated row. You can use this information to calculate the space need for your rollback segment.

In our multiversioning scheme a row is not physically removed from the database immediately when you delete it with an SQL statement. Only when InnoDB can discard the update undo log record written for the deletion, it can also physically remove the corresponding row and its index records from the database. This removal operation is called a purge, and it is quite fast, usually taking the same order of time as the SQL statement which did the deletion.

7.6.11 Table and index structures

Every InnoDB table has a special index called the clustered index where the data of the rows is stored. If you define a PRIMARY KEY on your table, then the index of the primary key will be the clustered index.

If you do not define a primary key for your table, InnoDB will internally generate a clustered index where the rows are ordered by the row id InnoDB assigns to the rows in such a table. The row id is a 6-byte field which monotonically increases as new rows are inserted. Thus the rows ordered by the row id will be physically in the insertion order.

Accessing a row through the clustered index is fast, because the row data will be on the same page where the index search leads us. In many databases the data is traditionally stored on a different page from the index record. If a table is large, the clustered index architecture often saves a disk i/o when compared to the traditional solution.

The records in non-clustered indexes (we also call them secondary indexes), in InnoDB contain the primary key value for the row. InnoDB uses this primary key value to search for the row from the clustered index. Note that if the primary key is long, the secondary indexes will use more space.

7.6.11.1 Physical structure of an index

All indexes in InnoDB are B-trees where the index records are stored in the leaf pages of the tree. The default size of an index page is 16 kB. When new records are inserted, InnoDB tries to leave 1 / 16 of the page free for future insertions and updates of the index records.

If index records are inserted in a sequential (ascending or descending) order, the resulting index pages will be about 15/16 full. If records are inserted in a random order, then the pages will be 1/2 - 15/16 full. If the fillfactor of an index page drops below 1/2, InnoDB will try to contract the index tree to free the page.

7.6.11.2 Insert buffering

It is a common situation in a database application that the primary key is a unique identifier and new rows are inserted in the ascending order of the primary key. Thus the insertions to the clustered index do not require random reads from a disk.

On the other hand, secondary indexes are usually non-unique and insertions happen in a relatively random order into secondary indexes. This would cause a lot of random disk i/o's without a special mechanism used in InnoDB.

If an index record should be inserted to a non-unique secondary index, InnoDB checks if the secondary index page is already in the buffer pool. If that is the case, InnoDB will do the insertion directly to the index page. But, if the index page is not found from the buffer pool, InnoDB inserts the record to a special insert buffer structure. The insert buffer is kept so small that it entirely fits in the buffer pool, and insertions can be made to it very fast.

The insert buffer is periodically merged to the secondary index trees in the database. Often we can merge several insertions on the same page in of the index tree, and hence save disk i/o's. It has been measured that the insert buffer can speed up insertions to a table up to 15 times.

7.6.11.3 Adaptive hash indexes

If a database fits almost entirely in main memory, then the fastest way to perform queries on it is to use hash indexes. InnoDB has an automatic mechanism which monitors index searches made to the indexes defined for a table, and if InnoDB notices that queries could benefit from building of a hash index, such an index is automatically built.

But note that the hash index is always built based on an existing B-tree index on the table. InnoDB can build a hash index on a prefix of any length of the key defined for the B-tree, depending on what search pattern InnoDB observes on the B-tree index. A hash index can be partial: it is not required that the whole B-tree index is cached in the buffer pool. InnoDB will build hash indexes on demand to those pages of the index which are often accessed.

In a sense, through the adaptive hash index mechanism InnoDB adapts itself to ample main memory, coming closer to the architecture of main memory databases.

7.6.11.4 Physical record structure

7.6.11.5 How an auto-increment column works in InnoDB

After a database startup, when a user first does an insert to a table T where an auto-increment column has been defined, and the user does not provide an explicit value for the column, then InnoDB executes SELECT MAX(auto-inc-column) FROM T, and assigns that value incremented by one to the the column and the auto-increment counter of the table. We say that the auto-increment counter for table T has been initialized.

InnoDB follows the same procedure in initializing the auto-increment counter for a freshly created table.

Note that if the user specifies in an insert the value 0 to the auto-increment column, then InnoDB treats the row like the value would not have been specified.

After the auto-increment counter has been initialized, if a user inserts a row where he explicitly specifies the column value, and the value is bigger than the current counter value, then the counter is set to the specified column value. If the user does not explicitly specify a value, then InnoDB increments the counter by one and assigns its new value to the column.

The auto-increment mechanism, when assigning values from the counter, bypasses locking and transaction handling. Therefore you may also get gaps in the number sequence if you roll back transactions which have got numbers from the counter.

The behavior of auto-increment is not defined if a user gives a negative value to the column or if the value becomes bigger than the maximum integer that can be stored in the specified integer type.

7.6.12 File space management and disk i/o

7.6.12.1 Disk i/o

In disk i/o InnoDB uses asynchronous i/o. On Windows NT it uses the native asynchronous i/o provided by the operating system. On Unix, InnoDB uses simulated asynchronous i/o built into InnoDB: InnoDB creates a number of i/o threads to take care of i/o operations, such as read-ahead. In a future version we will add support for simulated aio on Windows NT and native aio on those versions of Unix which have one.

On Windows NT InnoDB uses non-buffered i/o. That means that the disk pages InnoDB reads or writes are not buffered in the operating system file cache. This saves some memory bandwidth.

Starting from 3.23.41 InnoDB uses a novel file flush technique called doublewrite. It adds safety to crash recovery after an operating system crash or a power outage, and improves performance on most Unix flavors by reducing the need for fsync operations.

Doublewrite means that InnoDB before writing pages to a data file first writes them to a contiguous tablespace area called the doublewrite buffer. Only after the write and the flush to the doublewrite buffer has completed, InnoDB writes the pages to their proper positions in the data file. If the operating system crashes in the middle of a page write, InnoDB will in recovery find a good copy of the page from the doublewrite buffer.

Starting from 3.23.41 you can also use a raw disk partition as a data file, though this has not been tested yet. When you create a new data file you have to put the keyword newraw immediately after the data file size in innodb_data_file_path. The partition must be >= than you specify as the size. Note that 1M in InnoDB is 1024 x 1024 bytes, while in disk specifications 1 MB usually means 1000 000 bytes. 

innodb_data_file_path=hdd1:3Gnewraw;hdd2:2Gnewraw

When you start the database again you MUST change the keyword to raw. Otherwise InnoDB will write over your partition! 

innodb_data_file_path=hdd1:3Graw;hdd2:2Graw

Using a raw disk you can on some Unixes perform non-buffered i/o.

There are two read-ahead heuristics in InnoDB: sequential read-ahead and random read-ahead. In sequential read-ahead InnoDB notices that the access pattern to a segment in the tablespace is sequential. Then InnoDB will post in advance a batch of reads of database pages to the i/o system. In random read-ahead InnoDB notices that some area in a tablespace seems to be in the process of being fully read into the buffer pool. Then InnoDB posts the remaining reads to the i/o system.

7.6.12.2 File space management

The data files you define in the configuration file form the tablespace of InnoDB. The files are simply catenated to form the tablespace, there is no striping in use. Currently you cannot directly instruct where the space is allocated for your tables, except by using the following fact: from a newly created tablespace InnoDB will allocate space starting from the low end.

The tablespace consists of database pages whose default size is 16 kB. The pages are grouped into extents of 64 consecutive pages. The 'files' inside a tablespace are called segments in InnoDB. The name of the rollback segment is somewhat misleading because it actually contains many segments in the tablespace.

For each index in InnoDB we allocate two segments: one is for non-leaf nodes of the B-tree, the other is for the leaf nodes. The idea here is to achieve better sequentiality for the leaf nodes, which contain the data.

When a segment grows inside the tablespace, InnoDB allocates the first 32 pages to it individually. After that InnoDB starts to allocate whole extents to the segment. InnoDB can add to a large segment up to 4 extents at a time to ensure good sequentiality of data.

Some pages in the tablespace contain bitmaps of other pages, and therefore a few extents in an InnoDB tablespace cannot be allocated to segments as a whole, but only as individual pages.

When you issue a query SHOW TABLE STATUS FROM ... LIKE ... to ask for available free space in the tablespace, InnoDB will report you the space which is certainly usable in totally free extents of the tablespace. InnoDB always reserves some extents for clean-up and other internal purposes; these reserved extents are not included in the free space.

When you delete data from a table, InnoDB will contract the corresponding B-tree indexes. It depends on the pattern of deletes if that frees individual pages or extents to the tablespace, so that the freed space is available for other users. Dropping a table or deleting all rows from it is guaranteed to release the space to other users, but remember that deleted rows can be physically removed only in a purge operation after they are no longer needed in transaction rollback or consistent read.

7.6.12.3 Defragmenting a table

If there are random insertions or deletions in the indexes of a table, the indexes may become fragmented. By fragmentation we mean that the physical ordering of the index pages on the disk is not close to the alphabetical ordering of the records on the pages, or that there are many unused pages in the 64-page blocks which were allocated to the index.

It can speed up index scans if you periodically use mysqldump to dump the table to a text file, drop the table, and reload it from the dump. Another way to do the defragmenting is to ALTER the table type to MyISAM and back to InnoDB again. Note that a MyISAM table must fit in a single file on your operating system.

If the insertions to and index are always ascending and records are deleted only from the end, then the the file space management algorithm of InnoDB guarantees that fragmentation in the index will not occur.

7.6.13 Error handling

The error handling in InnoDB is not always the same as specified in the ANSI SQL standards. According to the ANSI standard, any error during an SQL statement should cause the rollback of that statement. InnoDB sometimes rolls back only part of the statement. The following list specifies the error handling of InnoDB.

7.6.14 Some restrictions on InnoDB tables

7.6.15 InnoDB contact information

Contact information of Innobase Oy, producer of the InnoDB engine. Website: http://www.innodb.com. Email: Heikki.Tuuri@innodb.com 

phone: 358-9-6969 3250 (office) 358-40-5617367 (mobile)
InnoDB Oy Inc.
World Trade Center Helsinki
Aleksanterinkatu 17
P.O.Box 800
00101 Helsinki
Finland

8 MySQL APIs

This chapter describes the APIs available for MySQL, where to get them, and how to use them. The C API is the most extensively covered, as it was developed by the MySQL team, and is the basis for most of the other APIs.

8.1 MySQL PHP API

PHP is a server-side, HTML-embedded scripting language that may be used to create dynamic Web pages. It contains support for accessing several databases, including MySQL. PHP may be run as a separate program or compiled as a module for use with the Apache Web server.

The distribution and documentation are available at the PHP web site.

8.1.1 Common Problems with MySQL and PHP

8.2 MySQL Perl API

This section documents the Perl DBI interface. The former interface was called mysqlperl. DBI/DBD now is the recommended Perl interface, so mysqlperl is obsolete and is not documented here.

8.2.1 DBI with DBD::mysql

DBI is a generic interface for many databases. That means that you can write a script that works with many different database engines without change. You need a DataBase Driver (DBD) defined for each database type. For MySQL, this driver is called DBD::mysql.

For more information on the Perl5 DBI, please visit the DBI Web page and read the documentation: 

http://www.symbolstone.org/technology/perl/DBI/index.html

For more information on Object Oriented Programming (OOP) as defined in Perl5, see the Perl OOP page: 

http://language.perl.com/info/documentation.html

Note that if you want to use transactions with Perl, you need to have Msql-Mysql-modules version 1.2216 or newer.

Installation instructions for MySQL Perl support are given in section M.2 Perl Installation Comments.

8.2.2 The DBI Interface

Portable DBI Methods

connect Establishes a connection to a database server.
disconnect Disconnects from the database server.
prepare Prepares a SQL statement for execution.
execute Executes prepared statements.
do Prepares and executes a SQL statement.
quote Quotes string or BLOB values to be inserted.
fetchrow_array Fetches the next row as an array of fields.
fetchrow_arrayref Fetches next row as a reference array of fields.
fetchrow_hashref Fetches next row as a reference to a hashtable.
fetchall_arrayref Fetches all data as an array of arrays.
finish Finishes a statement and lets the system free resources.
rows Returns the number of rows affected.
data_sources Returns an array of databases available on localhost.
ChopBlanks Controls whether fetchrow_* methods trim spaces.
NUM_OF_PARAMS The number of placeholders in the prepared statement.
NULLABLE Which columns can be NULL.
trace Perform tracing for debugging.

MySQL-specific Methods

insertid The latest AUTO_INCREMENT value.
is_blob Which columns are BLOB values.
is_key Which columns are keys.
is_num Which columns are numeric.
is_pri_key Which columns are primary keys.
is_not_null Which columns CANNOT be NULL. See NULLABLE.
length Maximum possible column sizes.
max_length Maximum column sizes actually present in result.
NAME Column names.
NUM_OF_FIELDS Number of fields returned.
table Table names in returned set.
type All column types.

The Perl methods are described in more detail in the following sections. Variables used for method return values have these meanings:

$dbh
Database handle
$sth
Statement handle
$rc
Return code (often a status)
$rv
Return value (often a row count)

Portable DBI Methods

connect($data_source, $username, $password)
Use the connect method to make a database connection to the data source. The $data_source value should begin with DBI:driver_name:. Example uses of connect with the DBD::mysql driver: 
$dbh = DBI->connect("DBI:mysql:$database", $user, $password);
$dbh = DBI->connect("DBI:mysql:$database:$hostname",
                    $user, $password);
$dbh = DBI->connect("DBI:mysql:$database:$hostname:$port",
                    $user, $password);
If the user name and/or password are undefined, DBI uses the values of the DBI_USER and DBI_PASS environment variables, respectively. If you don't specify a hostname, it defaults to 'localhost'. If you don't specify a port number, it defaults to the default MySQL port (3306). As of Msql-Mysql-modules Version 1.2009, the $data_source value allows certain modifiers:
mysql_read_default_file=file_name
Read `filename' as an option file. For information on option files, see section 4.1.2 my.cnf Option Files.
mysql_read_default_group=group_name
The default group when reading an option file is normally the [client] group. By specifying the mysql_read_default_group option, the default group becomes the [group_name] group.
mysql_compression=1
Use compressed communication between the client and server (MySQL Version 3.22.3 or later).
mysql_socket=/path/to/socket
Specify the pathname of the Unix socket that is used to connect to the server (MySQL Version 3.21.15 or later).
Multiple modifiers may be given; each must be preceded by a semicolon. For example, if you want to avoid hardcoding the user name and password into a DBI script, you can take them from the user's `~/.my.cnf' option file instead by writing your connect call like this: 
$dbh = DBI->connect("DBI:mysql:$database"
                . ";mysql_read_default_file=$ENV{HOME}/.my.cnf",
                $user, $password);
This call will read options defined for the [client] group in the option file. If you wanted to do the same thing but use options specified for the [perl] group as well, you could use this: 
$dbh = DBI->connect("DBI:mysql:$database"
                . ";mysql_read_default_file=$ENV{HOME}/.my.cnf"
                . ";mysql_read_default_group=perl",
                $user, $password);
disconnect
The disconnect method disconnects the database handle from the database. This is typically called right before you exit from the program. Example: 
$rc = $dbh->disconnect;
prepare($statement)
Prepares a SQL statement for execution by the database engine and returns a statement handle ($sth), which you can use to invoke the execute method. Typically you handle SELECT statements (and SELECT-like statements such as SHOW, DESCRIBE, and EXPLAIN) by means of prepare and execute. Example: 
$sth = $dbh->prepare($statement)
    or die "Can't prepare $statement: $dbh->errstr\n";
execute
The execute method executes a prepared statement. For non-SELECT statements, execute returns the number of rows affected. If no rows are affected, execute returns "0E0", which Perl treats as zero but regards as true. If an error occurs, execute returns undef. For SELECT statements, execute only starts the SQL query in the database; you need to use one of the fetch_* methods described below to retrieve the data. Example: 
$rv = $sth->execute
          or die "can't execute the query: $sth->errstr;
do($statement)
The do method prepares and executes a SQL statement and returns the number of rows affected. If no rows are affected, do returns "0E0", which Perl treats as zero but regards as true. This method is generally used for non-SELECT statements that cannot be prepared in advance (due to driver limitations) or that do not need to be executed more than once (inserts, deletes, etc.). Example: 
$rv = $dbh->do($statement)
        or die "Can't execute $statement: $dbh- >errstr\n";
Generally the 'do' statement is MUCH faster (and is preferable) than prepare/execute for statements that don't contain parameters.
quote($string)
The quote method is used to "escape" any special characters contained in the string and to add the required outer quotation marks. Example: 
$sql = $dbh->quote($string)
fetchrow_array
This method fetches the next row of data and returns it as an array of field values. Example: 
while(@row = $sth->fetchrow_array) {
        print qw($row[0]\t$row[1]\t$row[2]\n);
}
fetchrow_arrayref
This method fetches the next row of data and returns it as a reference to an array of field values. Example: 
while($row_ref = $sth->fetchrow_arrayref) {
        print qw($row_ref->[0]\t$row_ref->[1]\t$row_ref->[2]\n);
}
fetchrow_hashref
This method fetches a row of data and returns a reference to a hash table containing field name/value pairs. This method is not nearly as efficient as using array references as demonstrated above. Example: 
while($hash_ref = $sth->fetchrow_hashref) {
        print qw($hash_ref->{firstname}\t$hash_ref->{lastname}\t\
                $hash_ref- > title}\n);
}
fetchall_arrayref
This method is used to get all the data (rows) to be returned from the SQL statement. It returns a reference to an array of references to arrays for each row. You access or print the data by using a nested loop. Example: 
my $table = $sth->fetchall_arrayref
                or die "$sth->errstr\n";
my($i, $j);
for $i ( 0 .. $#{$table} ) {
        for $j ( 0 .. $#{$table->[$i]} ) {
                print "$table->[$i][$j]\t";
        }
        print "\n";
}
finish
Indicates that no more data will be fetched from this statement handle. You call this method to free up the statement handle and any system resources associated with it. Example: 
$rc = $sth->finish;
rows
Returns the number of rows changed (updated, deleted, etc.) by the last command. This is usually used after a non-SELECT execute statement. Example: 
$rv = $sth->rows;
NULLABLE
Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that this column may contain NULL values. Example: 
$null_possible = $sth->{NULLABLE};
NUM_OF_FIELDS
This attribute indicates the number of fields returned by a SELECT or SHOW FIELDS statement. You may use this for checking whether a statement returned a result: A zero value indicates a non-SELECT statement like INSERT, DELETE, or UPDATE. Example: 
$nr_of_fields = $sth->{NUM_OF_FIELDS};
data_sources($driver_name)
This method returns an array containing names of databases available to the MySQL server on the host 'localhost'. Example: 
@dbs = DBI->data_sources("mysql");
ChopBlanks
This attribute determines whether the fetchrow_* methods will chop leading and trailing blanks from the returned values. Example: 
$sth->{'ChopBlanks'} =1;
trace($trace_level)
trace($trace_level, $trace_filename)
The trace method enables or disables tracing. When invoked as a DBI class method, it affects tracing for all handles. When invoked as a database or statement handle method, it affects tracing for the given handle (and any future children of the handle). Setting $trace_level to 2 provides detailed trace information. Setting $trace_level to 0 disables tracing. Trace output goes to the standard error output by default. If $trace_filename is specified, the file is opened in append mode and output for all traced handles is written to that file. Example: 
DBI->trace(2);                # trace everything
DBI->trace(2,"/tmp/dbi.out"); # trace everything to
                              # /tmp/dbi.out
$dth->trace(2);               # trace this database handle
$sth->trace(2);               # trace this statement handle
You can also enable DBI tracing by setting the DBI_TRACE environment variable. Setting it to a numeric value is equivalent to calling DBI->(value). Setting it to a pathname is equivalent to calling DBI->(2,value).

MySQL-specific Methods

The methods shown below are MySQL-specific and not part of the DBI standard. Several of them are now deprecated: is_blob, is_key, is_num, is_pri_key, is_not_null, length, max_length, and table. Where DBI-standard alternatives exist, they are noted below:

insertid
If you use the AUTO_INCREMENT feature of MySQL, the new auto-incremented values will be stored here. Example: 
$new_id = $sth->{insertid};
As an alternative, you can use $dbh->{'mysql_insertid'}.
is_blob
Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that the respective column is a BLOB. Example: 
$keys = $sth->{is_blob};
is_key
Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that the respective column is a key. Example: 
$keys = $sth->{is_key};
is_num
Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that the respective column contains numeric values. Example: 
$nums = $sth->{is_num};
is_pri_key
Returns a reference to an array of boolean values; for each element of the array, a value of TRUE indicates that the respective column is a primary key. Example: 
$pri_keys = $sth->{is_pri_key};
is_not_null
Returns a reference to an array of boolean values; for each element of the array, a value of FALSE indicates that this column may contain NULL values. Example: 
$not_nulls = $sth->{is_not_null};
is_not_null is deprecated; it is preferable to use the NULLABLE attribute (described above), because that is a DBI standard.
length
max_length
Each of these methods returns a reference to an array of column sizes. The length array indicates the maximum possible sizes that each column may be (as declared in the table description). The max_length array indicates the maximum sizes actually present in the result table. Example: 
$lengths = $sth->{length};
$max_lengths = $sth->{max_length};
NAME
Returns a reference to an array of column names. Example: 
$names = $sth->{NAME};
table
Returns a reference to an array of table names. Example: 
$tables = $sth->{table};
type
Returns a reference to an array of column types. Example: 
$types = $sth->{type};

8.2.3 More DBI/DBD Information

You can use the perldoc command to get more information about DBI. 

perldoc DBI
perldoc DBI::FAQ
perldoc DBD::mysql

You can also use the pod2man, pod2html, etc., tools to translate to other formats.

You can find the latest DBI information at the DBI Web page: 

http://www.symbolstone.org/technology/perl/DBI/index.html

8.3 MySQL ODBC Support

MySQL provides support for ODBC by means of the MyODBC program. This chapter will teach you how to install MyODBC, and how to use it. Here, you will also find a list of common programs that are known to work with MyODBC.

8.3.1 How To Install MyODBC

MyODBC is a 32-bit ODBC (2.50) level 0 (with level 1 and level 2 features) driver for connecting an ODBC-aware application to MySQL. MyODBC works on Windows95, Windows98, NT, and on most Unix platforms.

MyODBC is in public domain, and you can find the newest version at http://www.mysql.com/downloads/api-myodbc.html.

If you have problem with MyODBC and your program also works with OLEDB, you should try the OLEDB driver that you can find in the Contrib section. See section D Contributed Programs.

Normally you only need to install MyODBC on Windows machines. You only need MyODBC for Unix if you have a program like ColdFusion that is running on the Unix machine and uses ODBC to connect to the databases.

If you want to install MyODBC on a Unix box, you will also need an ODBC manager. MyODBC is known to work with most of the Unix ODBC managers. You can find a list at these in the ODBC-related links section on the MySQL useful links page. See section 1.2.3 Useful MySQL-related Links.

To install MyODBC on Windows, you should download the appropriate MyODBC .zip file (for Windows or NT/Win2000), unpack it with WINZIP, or some similar program, and execute the SETUP.EXE file.

On Windows/NT you may get the following error when trying to install MyODBC: 

An error occurred while copying C:\WINDOWS\SYSTEM\MFC30.DLL. Restart
Windows and try installing again (before running any applications which
use ODBC)

The problem in this case is that some other program is using ODBC and because of how Windows is designed, you may not in this case be able to install a new ODBC drivers with Microsoft's ODBC setup program. In most cases you can continue by just pressing Ignore to copy the rest of the MyODBC files and the final installation should still work. If this doesn't work, the solution is to reboot your computer in ``safe mode`` (Choose this by pressing F8 just before your machine starts Windows during rebooting), install MyODBC, and reboot to normal mode.

Notice that there are other configuration options on the screen of MySQL (trace, don't prompt on connect, etc) that you can try if you run into problems.

8.3.2 How to Fill in the Various Fields in the ODBC Administrator Program

There are three possibilities for specifying the server name on Windows95:

Example of how to fill in the ODBC setup: 

Windows DSN name:   test
Description:        This is my test database
MySql Database:     test
Server:             194.216.84.21
User:               monty
Password:           my_password
Port:

The value for the Windows DSN name field is any name that is unique in your Windows ODBC setup.

You don't have to specify values for the Server, User, Password, or Port fields in the ODBC setup screen. However, if you do, the values will be used as the defaults later when you attempt to make a connection. You have the option of changing the values at that time.

If the port number is not given, the default port (3306) is used.

If you specify the option Read options from C:\my.cnf, the groups client and odbc will be read from the `C:\my.cnf' file. You can use all options that are usable by mysql_options(). See section 8.4.3.159 mysql_options().

8.3.3 Connect parameters for MyODBC

One can specify the following parameters for MyODBC on the [Servername] section of an ODBC.INI file or through the InConnectionString argument in the SQLDriverConnect() call.

Parameter Default value Comment
user ODBC (on Windows) The username used to connect to MySQL.
server localhost The hostname of the MySQL server.
database The default database
option 0 A integer by which you can specify how MyODBC should work. See below.
port 3306 The TCP/IP port to use if server is not localhost.
stmt A statement that will be executed when connection to MySQL.
password The password for the server user combination.
socket The socket or Windows pipe to connect to.

The option argument is used to tell MyODBC that the client isn't 100% ODBC compliant. On Windows, one normally sets the option flag by toggling the different options on the connection screen but one can also set this in the opton argument. The following options are listed in the same order as they appear in the MyODBC connect screen:

Bit Meaning
1 The client can't handle that MyODBC returns the real width of a column.
2 The client can't handle that MySQL returns the true value of affected rows. If this flag is set then MySQL returns 'found rows' instead. One must have MySQL 3.21.14 or newer to get this to work.
4 Make a debug log in c:\myodbc.log. This is the same as putting MYSQL_DEBUG=d:t:O,c::\myodbc.log in `AUTOEXEC.BAT'
8 Don't set any packet limit for results and parameters.
16 Don't prompt for questions even if driver would like to prompt
32 Simulate a ODBC 1.0 driver in some context.
64 Ignore use of database name in 'database.table.column'.
128 Force use of ODBC manager cursors (experimental).
256 Disable the use of extended fetch (experimental)
512 Pad CHAR fields to full column length.
1024 SQLDescribeCol() will return fully qualifed column names
2048 Use the compressed server/client protocol
4096 Tell server to ignore space after function name and before '(' (needed by PowerBuilder). This will make all function names keywords!
8192 Connect with named pipes to a mysqld server running on NT.
16384 Change LONGLONG columns to INT columns (Some applications can't handle LONGLONG).
32768 Return 'user' as Table_qualifier and Table_owner from SQLTables (experimental)
65536 Read parameters from the client and odbc groups from `my.cnf'
131072 Add some extra safety checks (should not bee needed but...)

If you want to have many options, you should add the above flags! For example setting option to 12 (4+8) gives you debugging without package limits!

The default `MYODBC.DLL' is compiled for optimal performance. If you want to to debug MyODBC (for example to enable tracing), you should instead use MYODBCD.DLL. To install this file, copy `MYODBCD.DLL' over the installed MYODBC.DLL file.

8.3.4 How to Report Problems with MyODBC

MyODBC has been tested with Access, Admndemo.exe, C++-Builder, Borland Builder 4, Centura Team Developer (formerly Gupta SQL/Windows), ColdFusion (on Solaris and NT with svc pack 5), Crystal Reports, DataJunction, Delphi, ERwin, Excel, iHTML, FileMaker Pro, FoxPro, Notes 4.5/4.6, SBSS, Perl DBD-ODBC, Paradox, Powerbuilder, Powerdesigner 32 bit, VC++, and Visual Basic.

If you know of any other applications that work with MyODBC, please send mail to myodbc@lists.mysql.com about this!

With some programs you may get an error like: Another user has modifies the record that you have modified. In most cases this can be solved by doing one of the following things:

If the above doesn't help, you should do a MyODBC trace file and try to figure out why things go wrong.

8.3.5 Programs Known to Work with MyODBC

Most programs should work with MyODBC, but for each of those listed below, we have tested it ourselves or received confirmation from some user that it works:

Program
Comment
Access
To make Access work:
ADO
When you are coding with the ADO API and MyODBC you need to put attention in some default properties that aren't supported by the MySQL server. For example, using the CursorLocation Property as adUseServer will return for the RecordCount Property a result of -1. To have the right value, you need to set this property to adUseClient, like is showing in the VB code below: 
Dim myconn As New ADODB.Connection
Dim myrs As New Recordset
Dim mySQL As String
Dim myrows As Long

myconn.Open "DSN=MyODBCsample"
mySQL = "SELECT * from user"
myrs.Source = mySQL
Set myrs.ActiveConnection = myconn
myrs.CursorLocation = adUseClient
myrs.Open
myrows = myrs.RecordCount

myrs.Close
myconn.Close
Another workaround is to use a SELECT COUNT(*) statement for a similar query to get the correct row count.
Active server pages (ASP)
You should use the option flag Return matching rows.
BDE applications
To get these to work, you should set the option flags Don't optimize column widths and Return matching rows.
Borland Builder 4
When you start a query you can use the property Active or use the method Open. Note that Active will start by automatically issuing a SELECT * FROM ... query that may not be a good thing if your tables are big!
ColdFusion (On Unix)
The following information is taken from the ColdFusion documentation: Use the following information to configure ColdFusion Server for Linux to use the unixODBC driver with MyODBC for MySQL data sources. Allaire has verified that MyODBC Version 2.50.26 works with MySQL Version 3.22.27 and ColdFusion for Linux. (Any newer version should also work.) You can download MyODBC at http://www.mysql.com/downloads/api-myodbc.html ColdFusion Version 4.5.1 allows you to us the ColdFusion Administrator to add the MySQL data source. However, the driver is not included with ColdFusion Version 4.5.1. Before the MySQL driver will appear in the ODBC datasources drop-down list, you must build and copy the MyODBC driver to `/opt/coldfusion/lib/libmyodbc.so'. The Contrib directory contains the program mydsn-xxx.zip which allows you to build and remove the DSN registry file for the MyODBC driver on Coldfusion applications.
DataJunction
You have to change it to output VARCHAR rather than ENUM, as it exports the latter in a manner that causes MySQL grief.
Excel
Works. Some tips:
Word
To retrieve data from MySQL to Word/Excel documents, you need to use the MyODBC driver and the Add-in Microsoft Query help. For example, create a db with a table containing 2 columns of text:
odbcadmin
Test program for ODBC.
Delphi
You must use BDE Version 3.2 or newer. Set the `Don't optimize column width' option field when connecting to MySQL. Also, here is some potentially useful Delphi code that sets up both an ODBC entry and a BDE entry for MyODBC (the BDE entry requires a BDE Alias Editor that is free at a Delphi Super Page near you. (Thanks to Bryan Brunton bryan@flesherfab.com for this): 
fReg:= TRegistry.Create;
  fReg.OpenKey('\Software\ODBC\ODBC.INI\DocumentsFab', True);
  fReg.WriteString('Database', 'Documents');
  fReg.WriteString('Description', ' ');
  fReg.WriteString('Driver', 'C:\WINNT\System32\myodbc.dll');
  fReg.WriteString('Flag', '1');
  fReg.WriteString('Password', '');
  fReg.WriteString('Port', ' ');
  fReg.WriteString('Server', 'xmark');
  fReg.WriteString('User', 'winuser');
  fReg.OpenKey('\Software\ODBC\ODBC.INI\ODBC Data Sources', True);
  fReg.WriteString('DocumentsFab', 'MySQL');
  fReg.CloseKey;
  fReg.Free;

  Memo1.Lines.Add('DATABASE NAME=');
  Memo1.Lines.Add('USER NAME=');
  Memo1.Lines.Add('ODBC DSN=DocumentsFab');
  Memo1.Lines.Add('OPEN MODE=READ/WRITE');
  Memo1.Lines.Add('BATCH COUNT=200');
  Memo1.Lines.Add('LANGDRIVER=');
  Memo1.Lines.Add('MAX ROWS=-1');
  Memo1.Lines.Add('SCHEMA CACHE DIR=');
  Memo1.Lines.Add('SCHEMA CACHE SIZE=8');
  Memo1.Lines.Add('SCHEMA CACHE TIME=-1');
  Memo1.Lines.Add('SQLPASSTHRU MODE=SHARED AUTOCOMMIT');
  Memo1.Lines.Add('SQLQRYMODE=');
  Memo1.Lines.Add('ENABLE SCHEMA CACHE=FALSE');
  Memo1.Lines.Add('ENABLE BCD=FALSE');
  Memo1.Lines.Add('ROWSET SIZE=20');
  Memo1.Lines.Add('BLOBS TO CACHE=64');
  Memo1.Lines.Add('BLOB SIZE=32');

  AliasEditor.Add('DocumentsFab','MySQL',Memo1.Lines);
C++ Builder
Tested with BDE Version 3.0. The only known problem is that when the table schema changes, query fields are not updated. BDE, however, does not seem to recognize primary keys, only the index PRIMARY, though this has not been a problem.
Vision
You should use the option flag Return matching rows.
Visual Basic
To be able to update a table, you must define a primary key for the table. Visual Basic with ADO can't handle big integers. This means that some queries like SHOW PROCESSLIST will not work properly. The fix is to set add the option OPTION=16834 in the ODBC connect string or set the Change BIGINT columns to INT option in the MyODBC connect screen. You may also want to set the Return matching rows option.
VisualInterDev
If you get the error [Microsoft][ODBC Driver Manager] Driver does not support this parameter the reason may be that you have a BIGINT in your result. Try setting the Change BIGINT columns to INT option in the MyODBC connect screen.
Visual Objects
You should use the option flag Don't optimize column widths.

8.3.6 How to Get the Value of an AUTO_INCREMENT Column in ODBC

A common problem is how to get the value of an automatically generated ID from an INSERT. With ODBC, you can do something like this (assuming that auto is an AUTO_INCREMENT field): 

INSERT INTO foo (auto,text) VALUES(NULL,'text');
SELECT LAST_INSERT_ID();

Or, if you are just going to insert the ID into another table, you can do this: 

INSERT INTO foo (auto,text) VALUES(NULL,'text');
INSERT INTO foo2 (id,text) VALUES(LAST_INSERT_ID(),'text');

See section 8.4.5.3 How Can I Get the Unique ID for the Last Inserted Row?.

For the benefit of some ODBC applications (at least Delphi and Access), the following query can be used to find a newly inserted row: 

SELECT * FROM tbl_name WHERE auto IS NULL;

8.3.7 Reporting Problems with MyODBC

If you encounter difficulties with MyODBC, you should start by making a log file from the ODBC manager (the log you get when requesting logs from ODBCADMIN) and a MyODBC log.

To get a MyODBC log, you need to do the following:

  1. Ensure that you are using myodbcd.dll and not myodbc.dll. The easiest way to do this is to get myodbcd.dll from the MyODBC distribution and copy it over the myodbc.dll, which is probably in your C:\windows\system32 or C:\winnt\system32 directory. Note that you probably want to restore the old myodbc.dll file when you have finished testing, as this is a lot faster than myodbcd.dll.
  2. Tag the `Trace MyODBC' option flag in the MyODBC connect/configure screen. The log will be written to file `C:\myodbc.log'. If the trace option is not remembered when you are going back to the above screen, it means that you are not using the myodbcd.dll driver (see above).
  3. Start your application and try to get it to fail.

Check the MyODBC trace file, to find out what could be wrong. You should be able to find out the issued queries by searching after the string >mysql_real_query in the `myodbc.log' file.

You should also try duplicating the queries in the mysql monitor or admndemo to find out if the error is MyODBC or MySQL.

If you find out something is wrong, please only send the relevant rows (max 40 rows) to myodbc@lists.mysql.com. Please never send the whole MyODBC or ODBC log file!

If you are unable to find out what's wrong, the last option is to make an archive (tar or zip) that contains a MyODBC trace file, the ODBC log file, and a README file that explains the problem. You can send this to ftp://support.mysql.com/pub/mysql/secret. Only we at MySQL AB will have access to the files you upload, and we will be very discrete with the data!

If you can create a program that also shows this problem, please upload this too!

If the program works with some other SQL server, you should make an ODBC log file where you do exactly the same thing in the other SQL server.

Remember that the more information you can supply to us, the more likely it is that we can fix the problem!

8.4 MySQL C API

The C API code is distributed with MySQL. It is included in the mysqlclient library and allows C programs to access a database.

Many of the clients in the MySQL source distribution are written in C. If you are looking for examples that demonstrate how to use the C API, take a look at these clients. You can find these in the clients directory in the MySQL source distribution.

Most of the other client APIs (all except Java) use the mysqlclient library to communicate with the MySQL server. This means that, for example, you can take advantage of many of the same environment variables that are used by other client programs, because they are referenced from the library. See section 4.8 MySQL Client-Side Scripts and Utilities, for a list of these variables.

The client has a maximum communication buffer size. The size of the buffer that is allocated initially (16K bytes) is automatically increased up to the maximum size (the maximum is 16M). Because buffer sizes are increased only as demand warrants, simply increasing the default maximum limit does not in itself cause more resources to be used. This size check is mostly a check for erroneous queries and communication packets.

The communication buffer must be large enough to contain a single SQL statement (for client-to-server traffic) and one row of returned data (for server-to-client traffic). Each thread's communication buffer is dynamically enlarged to handle any query or row up to the maximum limit. For example, if you have BLOB values that contain up to 16M of data, you must have a communication buffer limit of at least 16M (in both server and client). The client's default maximum is 16M, but the default maximum in the server is 1M. You can increase this by changing the value of the max_allowed_packet parameter when the server is started. See section 5.5.2 Tuning Server Parameters.

The MySQL server shrinks each communication buffer to net_buffer_length bytes after each query. For clients, the size of the buffer associated with a connection is not decreased until the connection is closed, at which time client memory is reclaimed.

For programming with threads, consult the 'how to make a thread-safe client' chapter. See section 8.4.7 How to Make a Threaded Client.

8.4.1 C API Datatypes

MYSQL
This structure represents a handle to one database connection. It is used for almost all MySQL functions.
MYSQL_RES
This structure represents the result of a query that returns rows (SELECT, SHOW, DESCRIBE, EXPLAIN). The information returned from a query is called the result set in the remainder of this section.
MYSQL_ROW
This is a type-safe representation of one row of data. It is currently implemented as an array of counted byte strings. (You cannot treat these as null-terminated strings if field values may contain binary data, because such values may contain null bytes internally.) Rows are obtained by calling mysql_fetch_row().
MYSQL_FIELD
This structure contains information about a field, such as the field's name, type, and size. Its members are described in more detail below. You may obtain the MYSQL_FIELD structures for each field by calling mysql_fetch_field() repeatedly. Field values are not part of this structure; they are contained in a MYSQL_ROW structure.
MYSQL_FIELD_OFFSET
This is a type-safe representation of an offset into a MySQL field list. (Used by mysql_field_seek().) Offsets are field numbers within a row, beginning at zero.
my_ulonglong
The type used for the number of rows and for mysql_affected_rows(), mysql_num_rows(), and mysql_insert_id(). This type provides a range of 0 to 1.84e19. On some systems, attempting to print a value of type my_ulonglong will not work. To print such a value, convert it to unsigned long and use a %lu print format. Example: 
printf (Number of rows: %lu\n", (unsigned long) mysql_num_rows(result));

The MYSQL_FIELD structure contains the members listed below:

char * name
The name of the field, as a null-terminated string.
char * table
The name of the table containing this field, if it isn't a calculated field. For calculated fields, the table value is an empty string.
char * def
The default value of this field, as a null-terminated string. This is set only if you use mysql_list_fields().
enum enum_field_types type
The type of the field. The type value may be one of the following:
Type value Type meaning
FIELD_TYPE_TINY TINYINT field
FIELD_TYPE_SHORT SMALLINT field
FIELD_TYPE_LONG INTEGER field
FIELD_TYPE_INT24 MEDIUMINT field
FIELD_TYPE_LONGLONG BIGINT field
FIELD_TYPE_DECIMAL DECIMAL or NUMERIC field
FIELD_TYPE_FLOAT FLOAT field
FIELD_TYPE_DOUBLE DOUBLE or REAL field
FIELD_TYPE_TIMESTAMP TIMESTAMP field
FIELD_TYPE_DATE DATE field
FIELD_TYPE_TIME TIME field
FIELD_TYPE_DATETIME DATETIME field
FIELD_TYPE_YEAR YEAR field
FIELD_TYPE_STRING String (CHAR or VARCHAR) field
FIELD_TYPE_BLOB BLOB or TEXT field (use max_length to determine the maximum length)
FIELD_TYPE_SET SET field
FIELD_TYPE_ENUM ENUM field
FIELD_TYPE_NULL NULL-type field
FIELD_TYPE_CHAR Deprecated; use FIELD_TYPE_TINY instead
You can use the IS_NUM() macro to test whether or not a field has a numeric type. Pass the type value to IS_NUM() and it will evaluate to TRUE if the field is numeric: 
if (IS_NUM(field->type))
    printf("Field is numeric\n");
unsigned int length
The width of the field, as specified in the table definition.
unsigned int max_length
The maximum width of the field for the result set (the length of the longest field value for the rows actually in the result set). If you use mysql_store_result() or mysql_list_fields(), this contains the maximum length for the field. If you use mysql_use_result(), the value of this variable is zero.
unsigned int flags
Different bit-flags for the field. The flags value may have zero or more of the following bits set:
Flag value Flag meaning
NOT_NULL_FLAG Field can't be NULL
PRI_KEY_FLAG Field is part of a primary key
UNIQUE_KEY_FLAG Field is part of a unique key
MULTIPLE_KEY_FLAG Field is part of a non-unique key
UNSIGNED_FLAG Field has the UNSIGNED attribute
ZEROFILL_FLAG Field has the ZEROFILL attribute
BINARY_FLAG Field has the BINARY attribute
AUTO_INCREMENT_FLAG Field has the AUTO_INCREMENT attribute
ENUM_FLAG Field is an ENUM (deprecated)
BLOB_FLAG Field is a BLOB or TEXT (deprecated)
TIMESTAMP_FLAG Field is a TIMESTAMP (deprecated)
Use of the BLOB_FLAG, ENUM_FLAG, and TIMESTAMP_FLAG flags is deprecated because they indicate the type of a field rather than an attribute of its type. It is preferable to test field->type against FIELD_TYPE_BLOB, FIELD_TYPE_ENUM, or FIELD_TYPE_TIMESTAMP instead. The example below illustrates a typical use of the flags value: 
if (field->flags & NOT_NULL_FLAG)
    printf("Field can't be null\n");
You may use the following convenience macros to determine the boolean status of the flags value:
IS_NOT_NULL(flags) True if this field is defined as NOT NULL
IS_PRI_KEY(flags) True if this field is a primary key
IS_BLOB(flags) True if this field is a BLOB or TEXT (deprecated; test field->type instead)
unsigned int decimals
The number of decimals for numeric fields.

8.4.2 C API Function Overview

The functions available in the C API are listed below and are described in greater detail in the next section. See section 8.4.3 C API Function Descriptions.

mysql_affected_rows() Returns the number of rows changed/deleted/inserted by the last UPDATE, DELETE, or INSERT query.
mysql_close() Closes a server connection.
mysql_connect() Connects to a MySQL server. This function is deprecated; use mysql_real_connect() instead.
mysql_change_user() Changes user and database on an open connection.
mysql_character_set_name() Returns the name of the default character set for the connection.
mysql_create_db() Creates a database. This function is deprecated; use the SQL command CREATE DATABASE instead.
mysql_data_seek() Seeks to an arbitrary row in a query result set.
mysql_debug() Does a DBUG_PUSH with the given string.
mysql_drop_db() Drops a database. This function is deprecated; use the SQL command DROP DATABASE instead.
mysql_dump_debug_info() Makes the server write debug information to the log.
mysql_eof() Determines whether or not the last row of a result set has been read. This function is deprecated; mysql_errno() or mysql_error() may be used instead.
mysql_errno() Returns the error number for the most recently invoked MySQL function.
mysql_error() Returns the error message for the most recently invoked MySQL function.
mysql_real_escape_string() Escapes special characters in a string for use in a SQL statement taking into account the current charset of the connection.
mysql_escape_string() Escapes special characters in a string for use in a SQL statement.
mysql_fetch_field() Returns the type of the next table field.
mysql_fetch_field_direct() Returns the type of a table field, given a field number.
mysql_fetch_fields() Returns an array of all field structures.
mysql_fetch_lengths() Returns the lengths of all columns in the current row.
mysql_fetch_row() Fetches the next row from the result set.
mysql_field_seek() Puts the column cursor on a specified column.
mysql_field_count() Returns the number of result columns for the most recent query.
mysql_field_tell() Returns the position of the field cursor used for the last mysql_fetch_field().
mysql_free_result() Frees memory used by a result set.
mysql_get_client_info() Returns client version information.
mysql_get_host_info() Returns a string describing the connection.
mysql_get_proto_info() Returns the protocol version used by the connection.
mysql_get_server_info() Returns the server version number.
mysql_info() Returns information about the most recently executed query.
mysql_init() Gets or initializes a MYSQL structure.
mysql_insert_id() Returns the ID generated for an AUTO_INCREMENT column by the previous query.
mysql_kill() Kills a given thread.
mysql_list_dbs() Returns database names matching a simple regular expression.
mysql_list_fields() Returns field names matching a simple regular expression.
mysql_list_processes() Returns a list of the current server threads.
mysql_list_tables() Returns table names matching a simple regular expression.
mysql_num_fields() Returns the number of columns in a result set.
mysql_num_rows() Returns the number of rows in a result set.
mysql_options() Sets connect options for mysql_connect().
mysql_ping() Checks whether or not the connection to the server is working, reconnecting as necessary.
mysql_query() Executes a SQL query specified as a null-terminated string.
mysql_real_connect() Connects to a MySQL server.
mysql_real_query() Executes a SQL query specified as a counted string.
mysql_reload() Tells the server to reload the grant tables.
mysql_row_seek() Seeks to a row in a result set, using value returned from mysql_row_tell().
mysql_row_tell() Returns the row cursor position.
mysql_select_db() Selects a database.
mysql_shutdown() Shuts down the database server.
mysql_stat() Returns the server status as a string.
mysql_store_result() Retrieves a complete result set to the client.
mysql_thread_id() Returns the current thread ID.
mysql_thread_save() Returns 1 if the clients are compiled as thread-safe.
mysql_use_result() Initiates a row-by-row result set retrieval.

To connect to the server, call mysql_init() to initialize a connection handler, then call mysql_real_connect() with that handler (along with other information such as the hostname, user name, and password). Upon connection, mysql_real_connect() sets the reconnect flag (part of the MYSQL structure) to a value of 1. This flag indicates, in the event that a query cannot be performed because of a lost connection, to try reconnecting to the server before giving up. When you are done with the connection, call mysql_close() to terminate it.

While a connection is active, the client may send SQL queries to the server using mysql_query() or mysql_real_query(). The difference between the two is that mysql_query() expects the query to be specified as a null-terminated string whereas mysql_real_query() expects a counted string. If the string contains binary data (which may include null bytes), you must use mysql_real_query().

For each non-SELECT query (for example, INSERT, UPDATE, DELETE), you can find out how many rows were changed (affected) by calling mysql_affected_rows().

For SELECT queries, you retrieve the selected rows as a result set. (Note that some statements are SELECT-like in that they return rows. These include SHOW, DESCRIBE, and EXPLAIN. They should be treated the same way as SELECT statements.)

There are two ways for a client to process result sets. One way is to retrieve the entire result set all at once by calling mysql_store_result(). This function acquires from the server all the rows returned by the query and stores them in the client. The second way is for the client to initiate a row-by-row result set retrieval by calling mysql_use_result(). This function initializes the retrieval, but does not actually get any rows from the server.

In both cases, you access rows by calling mysql_fetch_row(). With mysql_store_result(), mysql_fetch_row() accesses rows that have already been fetched from the server. With mysql_use_result(), mysql_fetch_row() actually retrieves the row from the server. Information about the size of the data in each row is available by calling mysql_fetch_lengths().

After you are done with a result set, call mysql_free_result() to free the memory used for it.

The two retrieval mechanisms are complementary. Client programs should choose the approach that is most appropriate for their requirements. In practice, clients tend to use mysql_store_result() more commonly.

An advantage of mysql_store_result() is that because the rows have all been fetched to the client, you not only can access rows sequentially, you can move back and forth in the result set using mysql_data_seek() or mysql_row_seek() to change the current row position within the result set. You can also find out how many rows there are by calling mysql_num_rows(). On the other hand, the memory requirements for mysql_store_result() may be very high for large result sets and you are more likely to encounter out-of-memory conditions.

An advantage of mysql_use_result() is that the client requires less memory for the result set because it maintains only one row at a time (and because there is less allocation overhead, mysql_use_result() can be faster). Disadvantages are that you must process each row quickly to avoid tying up the server, you don't have random access to rows within the result set (you can only access rows sequentially), and you don't know how many rows are in the result set until you have retrieved them all. Furthermore, you must retrieve all the rows even if you determine in mid-retrieval that you've found the information you were looking for.

The API makes it possible for clients to respond appropriately to queries (retrieving rows only as necessary) without knowing whether or not the query is a SELECT. You can do this by calling mysql_store_result() after each mysql_query() (or mysql_real_query()). If the result set call succeeds, the query was a SELECT and you can read the rows. If the result set call fails, call mysql_field_count() to determine whether or not a result was actually to be expected. If mysql_field_count() returns zero, the query returned no data (indicating that it was an INSERT, UPDATE, DELETE, etc.), and was not expected to return rows. If mysql_field_count() is non-zero, the query should have returned rows, but didn't. This indicates that the query was a SELECT that failed. See the description for mysql_field_count() for an example of how this can be done.

Both mysql_store_result() and mysql_use_result() allow you to obtain information about the fields that make up the result set (the number of fields, their names and types, etc.). You can access field information sequentially within the row by calling mysql_fetch_field() repeatedly, or by field number within the row by calling mysql_fetch_field_direct(). The current field cursor position may be changed by calling mysql_field_seek(). Setting the field cursor affects subsequent calls to mysql_fetch_field(). You can also get information for fields all at once by calling mysql_fetch_fields().

For detecting and reporting errors, MySQL provides access to error information by means of the mysql_errno() and mysql_error() functions. These return the error code or error message for the most recently invoked function that can succeed or fail, allowing you to determine when an error occurred and what it was.

8.4.3 C API Function Descriptions

In the descriptions below, a parameter or return value of NULL means NULL in the sense of the C programming language, not a MySQL NULL value.

Functions that return a value generally return a pointer or an integer. Unless specified otherwise, functions returning a pointer return a non-NULL value to indicate success or a NULL value to indicate an error, and functions returning an integer return zero to indicate success or non-zero to indicate an error. Note that ``non-zero'' means just that. Unless the function description says otherwise, do not test against a value other than zero: 

if (result)                   /* correct */
    ... error ...

if (result < 0)               /* incorrect */
    ... error ...

if (result == -1)             /* incorrect */
    ... error ...

When a function returns an error, the Errors subsection of the function description lists the possible types of errors. You can find out which of these occurred by calling mysql_errno(). A string representation of the error may be obtained by calling mysql_error().

8.4.3.1 mysql_affected_rows()

my_ulonglong mysql_affected_rows(MYSQL *mysql)

8.4.3.2 Description

Returns the number of rows changed by the last UPDATE, deleted by the last DELETE or inserted by the last INSERT statement. May be called immediately after mysql_query() for UPDATE, DELETE, or INSERT statements. For SELECT statements, mysql_affected_rows() works like mysql_num_rows().

8.4.3.3 Return Values

An integer greater than zero indicates the number of rows affected or retrieved. Zero indicates that no records where updated for an UPDATE statement, no rows matched the WHERE clause in the query or that no query has yet been executed. -1 indicates that the query returned an error or that, for a SELECT query, mysql_affected_rows() was called prior to calling mysql_store_result().

8.4.3.4 Errors

None.

8.4.3.5 Example

mysql_query(&mysql,"UPDATE products SET cost=cost*1.25 WHERE group=10");
printf("%ld products updated",(long) mysql_affected_rows(&mysql));

If one specifies the flag CLIENT_FOUND_ROWS when connecting to mysqld, mysql_affected_rows() will return the number of rows matched by the WHERE statement for UPDATE statements.

Note that when one uses a REPLACE command, mysql_affected_rows() will return 2 if the new row replaced and old row. This is because in this case one row was inserted and then the duplicate was deleted.

8.4.3.6 mysql_close()

void mysql_close(MYSQL *mysql)

8.4.3.7 Description

Closes a previously opened connection. mysql_close() also deallocates the connection handle pointed to by mysql if the handle was allocated automatically by mysql_init() or mysql_connect().

8.4.3.8 Return Values

None.

8.4.3.9 Errors

None.

8.4.3.10 mysql_connect()

MYSQL *mysql_connect(MYSQL *mysql, const char *host, const char *user, const char *passwd)

8.4.3.11 Description

This function is deprecated. It is preferable to use mysql_real_connect() instead.

mysql_connect() attempts to establish a connection to a MySQL database engine running on host. mysql_connect() must complete successfully before you can execute any of the other API functions, with the exception of mysql_get_client_info().

The meanings of the parameters are the same as for the corresponding parameters for mysql_real_connect() with the difference that the connection parameter may be NULL. In this case the C API allocates memory for the connection structure automatically and frees it when you call mysql_close(). The disadvantage of this approach is that you can't retrieve an error message if the connection fails. (To get error information from mysql_errno() or mysql_error(), you must provide a valid MYSQL pointer.)

8.4.3.12 Return Values

Same as for mysql_real_connect().

8.4.3.13 Errors

Same as for mysql_real_connect().

8.4.3.14 mysql_change_user()

my_bool mysql_change_user(MYSQL *mysql, const char *user, const char *password, const char *db)

8.4.3.15 Description

Changes the user and causes the database specified by db to become the default (current) database on the connection specified by mysql. In subsequent queries, this database is the default for table references that do not include an explicit database specifier.

This function was introduced in MySQL Version 3.23.3.

mysql_change_user() fails unless the connected user can be authenticated or if he doesn't have permission to use the database. In this case the user and database are not changed

The db parameter may be set to NULL if you don't want to have a default database.

8.4.3.16 Return values

Zero for success. Non-zero if an error occurred.

8.4.3.17 Errors

The same that you can get from mysql_real_connect().

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.
ER_UNKNOWN_COM_ERROR
The MySQL server doesn't implement this command (probably an old server)
ER_ACCESS_DENIED_ERROR
The user or password was wrong.
ER_BAD_DB_ERROR
The database didn't exist.
ER_DBACCESS_DENIED_ERROR
The user did not have access rights to the database.
ER_WRONG_DB_NAME
The database name was too long.

8.4.3.18 Example

if (mysql_change_user(&mysql, "user", "password", "new_database"))
{
   fprintf(stderr, "Failed to change user.  Error: %s\n",
           mysql_error(&mysql));
}

8.4.3.19 mysql_character_set_name()

const char *mysql_character_set_name(MYSQL *mysql)

8.4.3.20 Description

Returns the default character set for the current connection.

8.4.3.21 Return Values

The default character set

8.4.3.22 Errors

None.

8.4.3.23 mysql_create_db()

int mysql_create_db(MYSQL *mysql, const char *db)

8.4.3.24 Description

Creates the database named by the db parameter.

This function is deprecated. It is preferable to use mysql_query() to issue a SQL CREATE DATABASE statement instead.

8.4.3.25 Return Values

Zero if the database was created successfully. Non-zero if an error occurred.

8.4.3.26 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.27 Example

if(mysql_create_db(&mysql, "my_database"))
{
   fprintf(stderr, "Failed to create new database.  Error: %s\n",
           mysql_error(&mysql));
}

8.4.3.28 mysql_data_seek()

void mysql_data_seek(MYSQL_RES *result, unsigned long long offset)

8.4.3.29 Description

Seeks to an arbitrary row in a query result set. This requires that the result set structure contains the entire result of the query, so mysql_data_seek() may be used in conjunction only with mysql_store_result(), not with mysql_use_result().

The offset should be a value in the range from 0 to mysql_num_rows(result)-1.

8.4.3.30 Return Values

None.

8.4.3.31 Errors

None.

8.4.3.32 mysql_debug()

void mysql_debug(char *debug)

8.4.3.33 Description

Does a DBUG_PUSH with the given string. mysql_debug() uses the Fred Fish debug library. To use this function, you must compile the client library to support debugging. See section G.1 Debugging a MySQL server. See section G.2 Debugging a MySQL client.

8.4.3.34 Return Values

None.

8.4.3.35 Errors

None.

8.4.3.36 Example

The call shown below causes the client library to generate a trace file in `/tmp/client.trace' on the client machine: 

mysql_debug("d:t:O,/tmp/client.trace");

8.4.3.37 mysql_drop_db()

int mysql_drop_db(MYSQL *mysql, const char *db)

8.4.3.38 Description

Drops the database named by the db parameter.

This function is deprecated. It is preferable to use mysql_query() to issue a SQL DROP DATABASE statement instead.

8.4.3.39 Return Values

Zero if the database was dropped successfully. Non-zero if an error occurred.

8.4.3.40 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.41 Example

if(mysql_drop_db(&mysql, "my_database"))
  fprintf(stderr, "Failed to drop the database: Error: %s\n",
          mysql_error(&mysql));

8.4.3.42 mysql_dump_debug_info()

int mysql_dump_debug_info(MYSQL *mysql)

8.4.3.43 Description

Instructs the server to write some debug information to the log. The connected user must have the process privilege for this to work.

8.4.3.44 Return values

Zero if the command was successful. Non-zero if an error occurred.

8.4.3.45 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.46 mysql_eof()

my_bool mysql_eof(MYSQL_RES *result)

8.4.3.47 Description

This function is deprecated. mysql_errno() or mysql_error() may be used instead.

mysql_eof() determines whether or not the last row of a result set has been read.

If you acquire a result set from a successful call to mysql_store_result(), the client receives the entire set in one operation. In this case, a NULL return from mysql_fetch_row() always means the end of the result set has been reached and it is unnecessary to call mysql_eof().

On the other hand, if you use mysql_use_result() to initiate a result set retrieval, the rows of the set are obtained from the server one by one as you call mysql_fetch_row() repeatedly. Because an error may occur on the connection during this process, a NULL return value from mysql_fetch_row() does not necessarily mean the end of the result set was reached normally. In this case, you can use mysql_eof() to determine what happened. mysql_eof() returns a non-zero value if the end of the result set was reached and zero if an error occurred.

Historically, mysql_eof() predates the standard MySQL error functions mysql_errno() and mysql_error(). Because those error functions provide the same information, their use is preferred over mysql_eof(), which is now deprecated. (In fact, they provide more information, because mysql_eof() returns only a boolean value whereas the error functions indicate a reason for the error when one occurs.)

8.4.3.48 Return Values

Zero if no error occurred. Non-zero if the end of the result set has been reached.

8.4.3.49 Errors

None.

8.4.3.50 Example

The following example shows how you might use mysql_eof(): 

mysql_query(&mysql,"SELECT * FROM some_table");
result = mysql_use_result(&mysql);
while((row = mysql_fetch_row(result)))
{
    // do something with data
}
if(!mysql_eof(result))  // mysql_fetch_row() failed due to an error
{
    fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
}

However, you can achieve the same effect with the standard MySQL error functions: 

mysql_query(&mysql,"SELECT * FROM some_table");
result = mysql_use_result(&mysql);
while((row = mysql_fetch_row(result)))
{
    // do something with data
}
if(mysql_errno(&mysql))  // mysql_fetch_row() failed due to an error
{
    fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
}

8.4.3.51 mysql_errno()

unsigned int mysql_errno(MYSQL *mysql)

8.4.3.52 Description

For the connection specified by mysql, mysql_errno() returns the error code for the most recently invoked API function that can succeed or fail. A return value of zero means that no error occurred. Client error message numbers are listed in the MySQL `errmsg.h' header file. Server error message numbers are listed in `mysqld_error.h'. In the MySQL source distribution you can find a complete list of error messages and error numbers in the file `Docs/mysqld_error.txt'.

8.4.3.53 Return Values

An error code value. Zero if no error occurred.

8.4.3.54 Errors

None.

8.4.3.55 mysql_error()

char *mysql_error(MYSQL *mysql)

8.4.3.56 Description

For the connection specified by mysql, mysql_error() returns the error message for the most recently invoked API function that can succeed or fail. An empty string ("") is returned if no error occurred. This means the following two tests are equivalent: 

if(mysql_errno(&mysql))
{
    // an error occurred
}

if(mysql_error(&mysql)[0] != '\0')
{
    // an error occurred
}

The language of the client error messages may be changed by recompiling the MySQL client library. Currently you can choose error messages in several different languages. See section 4.6.2 Non-English Error Messages.

8.4.3.57 Return Values

A character string that describes the error. An empty string if no error occurred.

8.4.3.58 Errors

None.

8.4.3.59 mysql_escape_string()

You should use mysql_real_escape_string() instead!

This is identical to mysql_real_escape_string() except that it takes the connection as the first argument. mysql_real_escape_string() will escape the string according to the current character set while mysql_escape_string() does not respect the current charset setting.

8.4.3.60 mysql_fetch_field()

MYSQL_FIELD *mysql_fetch_field(MYSQL_RES *result)

8.4.3.61 Description

Returns the definition of one column of a result set as a MYSQL_FIELD structure. Call this function repeatedly to retrieve information about all columns in the result set. mysql_fetch_field() returns NULL when no more fields are left.

mysql_fetch_field() is reset to return information about the first field each time you execute a new SELECT query. The field returned by mysql_fetch_field() is also affected by calls to mysql_field_seek().

If you've called mysql_query() to perform a SELECT on a table but have not called mysql_store_result(), MySQL returns the default blob length (8K bytes) if you call mysql_fetch_field() to ask for the length of a BLOB field. (The 8K size is chosen because MySQL doesn't know the maximum length for the BLOB. This should be made configurable sometime.) Once you've retrieved the result set, field->max_length contains the length of the largest value for this column in the specific query.

8.4.3.62 Return Values

The MYSQL_FIELD structure for the current column. NULL if no columns are left.

8.4.3.63 Errors

None.

8.4.3.64 Example

MYSQL_FIELD *field;

while((field = mysql_fetch_field(result)))
{
    printf("field name %s\n", field->name);
}

8.4.3.65 mysql_fetch_fields()

MYSQL_FIELD *mysql_fetch_fields(MYSQL_RES *result)

8.4.3.66 Description

Returns an array of all MYSQL_FIELD structures for a result set. Each structure provides the field definition for one column of the result set.

8.4.3.67 Return Values

An array of MYSQL_FIELD structures for all columns of a result set.

8.4.3.68 Errors

None.

8.4.3.69 Example

unsigned int num_fields;
unsigned int i;
MYSQL_FIELD *fields;

num_fields = mysql_num_fields(result);
fields = mysql_fetch_fields(result);
for(i = 0; i < num_fields; i++)
{
   printf("Field %u is %s\n", i, fields[i].name);
}

8.4.3.70 mysql_fetch_field_direct()

MYSQL_FIELD *mysql_fetch_field_direct(MYSQL_RES *result, unsigned int fieldnr)

8.4.3.71 Description

Given a field number fieldnr for a column within a result set, returns that column's field definition as a MYSQL_FIELD structure. You may use this function to retrieve the definition for an arbitrary column. The value of fieldnr should be in the range from 0 to mysql_num_fields(result)-1.

8.4.3.72 Return Values

The MYSQL_FIELD structure for the specified column.

8.4.3.73 Errors

None.

8.4.3.74 Example

unsigned int num_fields;
unsigned int i;
MYSQL_FIELD *field;

num_fields = mysql_num_fields(result);
for(i = 0; i < num_fields; i++)
{
    field = mysql_fetch_field_direct(result, i);
    printf("Field %u is %s\n", i, field->name);
}

8.4.3.75 mysql_fetch_lengths()

unsigned long *mysql_fetch_lengths(MYSQL_RES *result)

8.4.3.76 Description

Returns the lengths of the columns of the current row within a result set. If you plan to copy field values, this length information is also useful for optimization, because you can avoid calling strlen(). In addition, if the result set contains binary data, you must use this function to determine the size of the data, because strlen() returns incorrect results for any field containing null characters.

The length for empty columns and for columns containing NULL values is zero. To see how to distinguish these two cases, see the description for mysql_fetch_row().

8.4.3.77 Return Values

An array of unsigned long integers representing the size of each column (not including any terminating null characters). NULL if an error occurred.

8.4.3.78 Errors

mysql_fetch_lengths() is valid only for the current row of the result set. It returns NULL if you call it before calling mysql_fetch_row() or after retrieving all rows in the result.

8.4.3.79 Example

MYSQL_ROW row;
unsigned long *lengths;
unsigned int num_fields;
unsigned int i;

row = mysql_fetch_row(result);
if (row)
{
    num_fields = mysql_num_fields(result);
    lengths = mysql_fetch_lengths(result);
    for(i = 0; i < num_fields; i++)
    {
         printf("Column %u is %lu bytes in length.\n", i, lengths[i]);
    }
}

8.4.3.80 mysql_fetch_row()

MYSQL_ROW mysql_fetch_row(MYSQL_RES *result)

8.4.3.81 Description

Retrieves the next row of a result set. When used after mysql_store_result(), mysql_fetch_row() returns NULL when there are no more rows to retrieve. When used after mysql_use_result(), mysql_fetch_row() returns NULL when there are no more rows to retrieve or if an error occurred.

The number of values in the row is given by mysql_num_fields(result). If row holds the return value from a call to mysql_fetch_row(), pointers to the values are accessed as row[0] to row[mysql_num_fields(result)-1]. NULL values in the row are indicated by NULL pointers.

The lengths of the field values in the row may be obtained by calling mysql_fetch_lengths(). Empty fields and fields containing NULL both have length 0; you can distinguish these by checking the pointer for the field value. If the pointer is NULL, the field is NULL; otherwise the field is empty.

8.4.3.82 Return Values

A MYSQL_ROW structure for the next row. NULL if there are no more rows to retrieve or if an error occurred.

8.4.3.83 Errors

CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.84 Example

MYSQL_ROW row;
unsigned int num_fields;
unsigned int i;

num_fields = mysql_num_fields(result);
while ((row = mysql_fetch_row(result)))
{
   unsigned long *lengths;
   lengths = mysql_fetch_lengths(result);
   for(i = 0; i < num_fields; i++)
   {
       printf("[%.*s] ", (int) lengths[i], row[i] ? row[i] : "NULL");
   }
   printf("\n");
}

8.4.3.85 mysql_field_count()

unsigned int mysql_field_count(MYSQL *mysql)

If you are using a version of MySQL earlier than Version 3.22.24, you should use unsigned int mysql_num_fields(MYSQL *mysql) instead.

8.4.3.86 Description

Returns the number of columns for the most recent query on the connection.

The normal use of this function is when mysql_store_result() returned NULL (and thus you have no result set pointer). In this case, you can call mysql_field_count() to determine whether or not mysql_store_result() should have produced a non-empty result. This allows the client program to take proper action without knowing whether or not the query was a SELECT (or SELECT-like) statement. The example shown below illustrates how this may be done.

See section 8.4.5.1 Why Is It that After mysql_query() Returns Success, mysql_store_result() Sometimes Returns NULL?.

8.4.3.87 Return Values

An unsigned integer representing the number of fields in a result set.

8.4.3.88 Errors

None.

8.4.3.89 Example

MYSQL_RES *result;
unsigned int num_fields;
unsigned int num_rows;

if (mysql_query(&mysql,query_string))
{
    // error
}
else // query succeeded, process any data returned by it
{
    result = mysql_store_result(&mysql);
    if (result)  // there are rows
    {
        num_fields = mysql_num_fields(result);
        // retrieve rows, then call mysql_free_result(result)
    }
    else  // mysql_store_result() returned nothing; should it have?
    {
        if(mysql_field_count(&mysql) == 0)
        {
            // query does not return data
            // (it was not a SELECT)
            num_rows = mysql_affected_rows(&mysql);
        }
        else // mysql_store_result() should have returned data
        {
            fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
        }
    }
}

An alternative is to replace the mysql_field_count(&mysql) call with mysql_errno(&mysql). In this case, you are checking directly for an error from mysql_store_result() rather than inferring from the value of mysql_field_count() whether or not the statement was a SELECT.

8.4.3.90 mysql_field_seek()

MYSQL_FIELD_OFFSET mysql_field_seek(MYSQL_RES *result, MYSQL_FIELD_OFFSET offset)

* Threaded clients:: How to Make a Threaded Client

8.4.3.91 Description

Sets the field cursor to the given offset. The next call to mysql_fetch_field() will retrieve the field definition of the column associated with that offset.

To seek to the beginning of a row, pass an offset value of zero.

8.4.3.92 Return Values

The previous value of the field cursor.

8.4.3.93 Errors

None.

8.4.3.94 mysql_field_tell()

MYSQL_FIELD_OFFSET mysql_field_tell(MYSQL_RES *result)

8.4.3.95 Description

Returns the position of the field cursor used for the last mysql_fetch_field(). This value can be used as an argument to mysql_field_seek().

8.4.3.96 Return Values

The current offset of the field cursor.

8.4.3.97 Errors

None.

8.4.3.98 mysql_free_result()

void mysql_free_result(MYSQL_RES *result)

8.4.3.99 Description

Frees the memory allocated for a result set by mysql_store_result(), mysql_use_result(), mysql_list_dbs(), etc. When you are done with a result set, you must free the memory it uses by calling mysql_free_result().

8.4.3.100 Return Values

None.

8.4.3.101 Errors

None.

8.4.3.102 mysql_get_client_info()

char *mysql_get_client_info(void)

8.4.3.103 Description

Returns a string that represents the client library version.

8.4.3.104 Return Values

A character string that represents the MySQL client library version.

8.4.3.105 Errors

None.

8.4.3.106 mysql_get_host_info()

char *mysql_get_host_info(MYSQL *mysql)

8.4.3.107 Description

Returns a string describing the type of connection in use, including the server host name.

8.4.3.108 Return Values

A character string representing the server host name and the connection type.

8.4.3.109 Errors

None.

8.4.3.110 mysql_get_proto_info()

unsigned int mysql_get_proto_info(MYSQL *mysql)

8.4.3.111 Description

Returns the protocol version used by current connection.

8.4.3.112 Return Values

An unsigned integer representing the protocol version used by the current connection.

8.4.3.113 Errors

None.

8.4.3.114 mysql_get_server_info()

char *mysql_get_server_info(MYSQL *mysql)

8.4.3.115 Description

Returns a string that represents the server version number.

8.4.3.116 Return Values

A character string that represents the server version number.

8.4.3.117 Errors

None.

8.4.3.118 mysql_info()

char *mysql_info(MYSQL *mysql)

8.4.3.119 Description

Retrieves a string providing information about the most recently executed query, but only for the statements listed below. For other statements, mysql_info() returns NULL. The format of the string varies depending on the type of query, as described below. The numbers are illustrative only; the string will contain values appropriate for the query.

INSERT INTO ... SELECT ...
String format: Records: 100 Duplicates: 0 Warnings: 0
INSERT INTO ... VALUES (...),(...),(...)...
String format: Records: 3 Duplicates: 0 Warnings: 0
LOAD DATA INFILE ...
String format: Records: 1 Deleted: 0 Skipped: 0 Warnings: 0
ALTER TABLE
String format: Records: 3 Duplicates: 0 Warnings: 0
UPDATE
String format: Rows matched: 40 Changed: 40 Warnings: 0

Note that mysql_info() returns a non-NULL value for the INSERT ... VALUES statement only if multiple value lists are specified in the statement.

8.4.3.120 Return Values

A character string representing additional information about the most recently executed query. NULL if no information is available for the query.

8.4.3.121 Errors

None.

8.4.3.122 mysql_init()

MYSQL *mysql_init(MYSQL *mysql)

8.4.3.123 Description

Allocates or initializes a MYSQL object suitable for mysql_real_connect(). If mysql is a NULL pointer, the function allocates, initializes, and returns a new object. Otherwise the object is initialized and the address of the object is returned. If mysql_init() allocates a new object, it will be freed when mysql_close() is called to close the connection.

8.4.3.124 Return Values

An initialized MYSQL* handle. NULL if there was insufficient memory to allocate a new object.

8.4.3.125 Errors

In case of insufficient memory, NULL is returned.

8.4.3.126 mysql_insert_id()

my_ulonglong mysql_insert_id(MYSQL *mysql)

8.4.3.127 Description

Returns the ID generated for an AUTO_INCREMENT column by the previous query. Use this function after you have performed an INSERT query into a table that contains an AUTO_INCREMENT field.

Note that mysql_insert_id() returns 0 if the previous query does not generate an AUTO_INCREMENT value. If you need to save the value for later, be sure to call mysql_insert_id() immediately after the query that generates the value.

mysql_insert_id() is updated after INSERT and UPDATE statements that generate an AUTO_INCREMENT value or that set a column value to LAST_INSERT_ID(expr). See section 6.3.5.2 Miscellaneous Functions.

Also note that the value of the SQL LAST_INSERT_ID() function always contains the most recently generated AUTO_INCREMENT value, and is not reset between queries because the value of that function is maintained in the server.

8.4.3.128 Return Values

The value of the AUTO_INCREMENT field that was updated by the previous query. Returns zero if there was no previous query on the connection or if the query did not update an AUTO_INCREMENT value.

8.4.3.129 Errors

None.

8.4.3.130 mysql_kill()

int mysql_kill(MYSQL *mysql, unsigned long pid)

8.4.3.131 Description

Asks the server to kill the thread specified by pid.

8.4.3.132 Return Values

Zero for success. Non-zero if an error occurred.

8.4.3.133 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.134 mysql_list_dbs()

MYSQL_RES *mysql_list_dbs(MYSQL *mysql, const char *wild)

8.4.3.135 Description

Returns a result set consisting of database names on the server that match the simple regular expression specified by the wild parameter. wild may contain the wild-card characters `%' or `_', or may be a NULL pointer to match all databases. Calling mysql_list_dbs() is similar to executing the query SHOW databases [LIKE wild].

You must free the result set with mysql_free_result().

8.4.3.136 Return Values

A MYSQL_RES result set for success. NULL if an error occurred.

8.4.3.137 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_OUT_OF_MEMORY
Out of memory.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.138 mysql_list_fields()

MYSQL_RES *mysql_list_fields(MYSQL *mysql, const char *table, const char *wild)

8.4.3.139 Description

Returns a result set consisting of field names in the given table that match the simple regular expression specified by the wild parameter. wild may contain the wild-card characters `%' or `_', or may be a NULL pointer to match all fields. Calling mysql_list_fields() is similar to executing the query SHOW COLUMNS FROM tbl_name [LIKE wild].

Note that it's recommended that you use SHOW COLUMNS FROM tbl_name instead of mysql_list_fields().

You must free the result set with mysql_free_result().

8.4.3.140 Return Values

A MYSQL_RES result set for success. NULL if an error occurred.

8.4.3.141 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.142 mysql_list_processes()

MYSQL_RES *mysql_list_processes(MYSQL *mysql)

8.4.3.143 Description

Returns a result set describing the current server threads. This is the same kind of information as that reported by mysqladmin processlist or a SHOW PROCESSLIST query.

You must free the result set with mysql_free_result().

8.4.3.144 Return Values

A MYSQL_RES result set for success. NULL if an error occurred.

8.4.3.145 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.146 mysql_list_tables()

MYSQL_RES *mysql_list_tables(MYSQL *mysql, const char *wild)

8.4.3.147 Description

Returns a result set consisting of table names in the current database that match the simple regular expression specified by the wild parameter. wild may contain the wild-card characters `%' or `_', or may be a NULL pointer to match all tables. Calling mysql_list_tables() is similar to executing the query SHOW tables [LIKE wild].

You must free the result set with mysql_free_result().

8.4.3.148 Return Values

A MYSQL_RES result set for success. NULL if an error occurred.

8.4.3.149 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.150 mysql_num_fields()

unsigned int mysql_num_fields(MYSQL_RES *result)

or

unsigned int mysql_num_fields(MYSQL *mysql)

The second form doesn't work on MySQL Version 3.22.24 or newer. To pass a MYSQL* argument, you must use unsigned int mysql_field_count(MYSQL *mysql) instead.

8.4.3.151 Description

Returns the number of columns in a result set.

Note that you can get the number of columns either from a pointer to a result set or to a connection handle. You would use the connection handle if mysql_store_result() or mysql_use_result() returned NULL (and thus you have no result set pointer). In this case, you can call mysql_field_count() to determine whether or not mysql_store_result() should have produced a non-empty result. This allows the client program to take proper action without knowing whether or not the query was a SELECT (or SELECT-like) statement. The example shown below illustrates how this may be done.

See section 8.4.5.1 Why Is It that After mysql_query() Returns Success, mysql_store_result() Sometimes Returns NULL?.

8.4.3.152 Return Values

An unsigned integer representing the number of fields in a result set.

8.4.3.153 Errors

None.

8.4.3.154 Example

MYSQL_RES *result;
unsigned int num_fields;
unsigned int num_rows;

if (mysql_query(&mysql,query_string))
{
    // error
}
else // query succeeded, process any data returned by it
{
    result = mysql_store_result(&mysql);
    if (result)  // there are rows
    {
        num_fields = mysql_num_fields(result);
        // retrieve rows, then call mysql_free_result(result)
    }
    else  // mysql_store_result() returned nothing; should it have?
    {
        if (mysql_errno(&mysql))
	{
           fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
	}
        else if (mysql_field_count(&mysql) == 0)
        {
            // query does not return data
            // (it was not a SELECT)
            num_rows = mysql_affected_rows(&mysql);
        }
    }
}

An alternative (if you KNOW that your query should have returned a result set) is to replace the mysql_errno(&mysql) call with a check if mysql_field_count(&mysql) is = 0. This will only happen if something went wrong.

8.4.3.155 mysql_num_rows()

my_ulonglong mysql_num_rows(MYSQL_RES *result)

8.4.3.156 Description

Returns the number of rows in the result set.

The use of mysql_num_rows() depends on whether you use mysql_store_result() or mysql_use_result() to return the result set. If you use mysql_store_result(), mysql_num_rows() may be called immediately. If you use mysql_use_result(), mysql_num_rows() will not return the correct value until all the rows in the result set have been retrieved.

8.4.3.157 Return Values

The number of rows in the result set.

8.4.3.158 Errors

None.

8.4.3.159 mysql_options()

int mysql_options(MYSQL *mysql, enum mysql_option option, const char *arg)

8.4.3.160 Description

Can be used to set extra connect options and affect behavior for a connection. This function may be called multiple times to set several options.

mysql_options() should be called after mysql_init() and before mysql_connect() or mysql_real_connect().

The option argument is the option that you want to set; the arg argument is the value for the option. If the option is an integer, then arg should point to the value of the integer.

Possible options values:

Option Argument type Function
MYSQL_OPT_CONNECT_TIMEOUT unsigned int * Connect timeout in seconds.
MYSQL_OPT_COMPRESS Not used Use the compressed client/server protocol.
MYSQL_OPT_NAMED_PIPE Not used Use named pipes to connect to a MySQL server on NT.
MYSQL_INIT_COMMAND char * Command to execute when connecting to the MySQL server. Will automatically be re-executed when reconnecting.
MYSQL_READ_DEFAULT_FILE char * Read options from the named option file instead of from `my.cnf'.
MYSQL_READ_DEFAULT_GROUP char * Read options from the named group from `my.cnf' or the file specified with MYSQL_READ_DEFAULT_FILE.

Note that the group client is always read if you use MYSQL_READ_DEFAULT_FILE or MYSQL_READ_DEFAULT_GROUP.

The specified group in the option file may contain the following options:

connect_timeout Connect timeout in seconds. On Linux this timeout is also used for waiting for the first answer from the server.
compress Use the compressed client/server protocol.
database Connect to this database if no database was specified in the connect command.
debug Debug options.
host Default host name.
init-command Command to execute when connecting to MySQL server. Will automatically be re-executed when reconnecting.
interactive-timeout Same as specifying CLIENT_INTERACTIVE to mysql_real_connect(). See section 8.4.3.171 mysql_real_connect().
password Default password.
pipe Use named pipes to connect to a MySQL server on NT.
port Default port number.
return-found-rows Tell mysql_info() to return found rows instead of updated rows when using UPDATE.
socket Default socket number.
user Default user.

Note that timeout has been replaced by connect_timeout, but timeout will still work for a while.

For more information about option files, see section 4.1.2 my.cnf Option Files.

8.4.3.161 Return Values

Zero for success. Non-zero if you used an unknown option.

8.4.3.162 Example

MYSQL mysql;

mysql_init(&mysql);
mysql_options(&mysql,MYSQL_OPT_COMPRESS,0);
mysql_options(&mysql,MYSQL_READ_DEFAULT_GROUP,"odbc");
if (!mysql_real_connect(&mysql,"host","user","passwd","database",0,NULL,0))
{
    fprintf(stderr, "Failed to connect to database: Error: %s\n",
          mysql_error(&mysql));
}

The above requests the client to use the compressed client/server protocol and read the additional options from the odbc section in the my.cnf file.

8.4.3.163 mysql_ping()

int mysql_ping(MYSQL *mysql)

8.4.3.164 Description

Checks whether or not the connection to the server is working. If it has gone down, an automatic reconnection is attempted.

This function can be used by clients that remain idle for a long while, to check whether or not the server has closed the connection and reconnect if necessary.

8.4.3.165 Return Values

Zero if the server is alive. Non-zero if an error occurred.

8.4.3.166 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.167 mysql_query()

int mysql_query(MYSQL *mysql, const char *query)

8.4.3.168 Description

Executes the SQL query pointed to by the null-terminated string query. The query must consist of a single SQL statement. You should not add a terminating semicolon (`;') or \g to the statement.

mysql_query() cannot be used for queries that contain binary data; you should use mysql_real_query() instead. (Binary data may contain the `\0' character, which mysql_query() interprets as the end of the query string.)

If you want to know if the query should return a result set or not, you can use mysql_field_count() to check for this. See section 8.4.3.85 mysql_field_count().

8.4.3.169 Return Values

Zero if the query was successful. Non-zero if an error occurred.

8.4.3.170 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.171 mysql_real_connect()

MYSQL *mysql_real_connect(MYSQL *mysql, const char *host, const char *user, const char *passwd, const char *db, unsigned int port, const char *unix_socket, unsigned int client_flag)

8.4.3.172 Description

mysql_real_connect() attempts to establish a connection to a MySQL database engine running on host. mysql_real_connect() must complete successfully before you can execute any of the other API functions, with the exception of mysql_get_client_info().

The parameters are specified as follows:

8.4.3.173 Return Values

A MYSQL* connection handle if the connection was successful, NULL if the connection was unsuccessful. For a successful connection, the return value is the same as the value of the first parameter.

8.4.3.174 Errors

CR_CONN_HOST_ERROR
Failed to connect to the MySQL server.
CR_CONNECTION_ERROR
Failed to connect to the local MySQL server.
CR_IPSOCK_ERROR
Failed to create an IP socket.
CR_OUT_OF_MEMORY
Out of memory.
CR_SOCKET_CREATE_ERROR
Failed to create a Unix socket.
CR_UNKNOWN_HOST
Failed to find the IP address for the hostname.
CR_VERSION_ERROR
A protocol mismatch resulted from attempting to connect to a server with a client library that uses a different protocol version. This can happen if you use a very old client library to connect to a new server that wasn't started with the --old-protocol option.
CR_NAMEDPIPEOPEN_ERROR
Failed to create a named pipe on Windows.
CR_NAMEDPIPEWAIT_ERROR
Failed to wait for a named pipe on Windows.
CR_NAMEDPIPESETSTATE_ERROR
Failed to get a pipe handler on Windows.
CR_SERVER_LOST
If connect_timeout > 0 and it took longer then connect_timeout seconds to connect to the server or if the server died while executing the init-command.

8.4.3.175 Example

MYSQL mysql;

mysql_init(&mysql);
mysql_options(&mysql,MYSQL_READ_DEFAULT_GROUP,"your_prog_name");
if (!mysql_real_connect(&mysql,"host","user","passwd","database",0,NULL,0))
{
    fprintf(stderr, "Failed to connect to database: Error: %s\n",
          mysql_error(&mysql));
}

By using mysql_options() the MySQL library will read the [client] and your_prog_name sections in the my.cnf file which will ensure that your program will work, even if someone has set up MySQL in some non-standard way.

Note that upon connection, mysql_real_connect() sets the reconnect flag (part of the MYSQL structure) to a value of 1. This flag indicates, in the event that a query cannot be performed because of a lost connection, to try reconnecting to the server before giving up.

8.4.3.176 mysql_real_escape_string()

unsigned int mysql_real_escape_string(MYSQL *mysql, char *to, const char *from, unsigned int length)

8.4.3.177 Description

This function is used to create a legal SQL string that you can use in a SQL statement. See section 6.1.1.1 Strings.

The string in from is encoded to an escaped SQL string, taking into account the current character set of the connection. The result is placed in to and a terminating null byte is appended. Characters encoded are NUL (ASCII 0), `\n', `\r', `\', `'', `"', and Control-Z (see section 6.1.1 Literals: How to Write Strings and Numbers).

The string pointed to by from must be length bytes long. You must allocate the to buffer to be at least length*2+1 bytes long. (In the worse case, each character may need to be encoded as using two bytes, and you need room for the terminating null byte.) When mysql_escape_string() returns, the contents of to will be a null-terminated string. The return value is the length of the encoded string, not including the terminating null character.

8.4.3.178 Example

char query[1000],*end;

end = strmov(query,"INSERT INTO test_table values(");
*end++ = '\'';
end += mysql_real_escape_string(&mysql, end,"What's this",11);
*end++ = '\'';
*end++ = ',';
*end++ = '\'';
end += mysql_real_escape_string(&mysql, end,"binary data: \0\r\n",16);
*end++ = '\'';
*end++ = ')';

if (mysql_real_query(&mysql,query,(unsigned int) (end - query)))
{
   fprintf(stderr, "Failed to insert row, Error: %s\n",
           mysql_error(&mysql));
}

The strmov() function used in the example is included in the mysqlclient library and works like strcpy() but returns a pointer to the terminating null of the first parameter.

8.4.3.179 Return Values

The length of the value placed into to, not including the terminating null character.

8.4.3.180 Errors

None.

8.4.3.181 mysql_real_query()

int mysql_real_query(MYSQL *mysql, const char *query, unsigned int length)

8.4.3.182 Description

Executes the SQL query pointed to by query, which should be a string length bytes long. The query must consist of a single SQL statement. You should not add a terminating semicolon (`;') or \g to the statement.

You must use mysql_real_query() rather than mysql_query() for queries that contain binary data, because binary data may contain the `\0' character. In addition, mysql_real_query() is faster than mysql_query() because it does not call strlen() on the query string.

If you want to know if the query should return a result set or not, you can use mysql_field_count() to check for this. See section 8.4.3.85 mysql_field_count().

8.4.3.183 Return Values

Zero if the query was successful. Non-zero if an error occurred.

8.4.3.184 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.185 mysql_reload()

int mysql_reload(MYSQL *mysql)

8.4.3.186 Description

Asks the MySQL server to reload the grant tables. The connected user must have the reload privilege.

This function is deprecated. It is preferable to use mysql_query() to issue a SQL FLUSH PRIVILEGES statement instead.

8.4.3.187 Return Values

Zero for success. Non-zero if an error occurred.

8.4.3.188 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.189 mysql_row_seek()

MYSQL_ROW_OFFSET mysql_row_seek(MYSQL_RES *result, MYSQL_ROW_OFFSET offset)

8.4.3.190 Description

Sets the row cursor to an arbitrary row in a query result set. This requires that the result set structure contains the entire result of the query, so mysql_row_seek() may be used in conjunction only with mysql_store_result(), not with mysql_use_result().

The offset should be a value returned from a call to mysql_row_tell() or to mysql_row_seek(). This value is not simply a row number; if you want to seek to a row within a result set using a row number, use mysql_data_seek() instead.

8.4.3.191 Return Values

The previous value of the row cursor. This value may be passed to a subsequent call to mysql_row_seek().

8.4.3.192 Errors

None.

8.4.3.193 mysql_row_tell()

MYSQL_ROW_OFFSET mysql_row_tell(MYSQL_RES *result)

8.4.3.194 Description

Returns the current position of the row cursor for the last mysql_fetch_row(). This value can be used as an argument to mysql_row_seek().

You should use mysql_row_tell() only after mysql_store_result(), not after mysql_use_result().

8.4.3.195 Return Values

The current offset of the row cursor.

8.4.3.196 Errors

None.

8.4.3.197 mysql_select_db()

int mysql_select_db(MYSQL *mysql, const char *db)

8.4.3.198 Description

Causes the database specified by db to become the default (current) database on the connection specified by mysql. In subsequent queries, this database is the default for table references that do not include an explicit database specifier.

mysql_select_db() fails unless the connected user can be authenticated as having permission to use the database.

8.4.3.199 Return Values

Zero for success. Non-zero if an error occurred.

8.4.3.200 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.201 mysql_shutdown()

int mysql_shutdown(MYSQL *mysql)

8.4.3.202 Description

Asks the database server to shut down. The connected user must have shutdown privileges.

8.4.3.203 Return Values

Zero for success. Non-zero if an error occurred.

8.4.3.204 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.205 mysql_stat()

char *mysql_stat(MYSQL *mysql)

8.4.3.206 Description

Returns a character string containing information similar to that provided by the mysqladmin status command. This includes uptime in seconds and the number of running threads, questions, reloads, and open tables.

8.4.3.207 Return Values

A character string describing the server status. NULL if an error occurred.

8.4.3.208 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.209 mysql_store_result()

MYSQL_RES *mysql_store_result(MYSQL *mysql)

8.4.3.210 Description

You must call mysql_store_result() or mysql_use_result() for every query that successfully retrieves data (SELECT, SHOW, DESCRIBE, EXPLAIN).

You don't have to call mysql_store_result() or mysql_use_result() for other queries, but it will not do any harm or cause any notable performance if you call mysql_store_result() in all cases. You can detect if the query didn't have a result set by checking if mysql_store_result() returns 0 (more about this later one).

If you want to know if the query should return a result set or not, you can use mysql_field_count() to check for this. See section 8.4.3.85 mysql_field_count().

mysql_store_result() reads the entire result of a query to the client, allocates a MYSQL_RES structure, and places the result into this structure.

mysql_store_results() returns a null pointer if the query didn't return a result set (if the query was, for example, an INSERT statement).

mysql_store_results() also returns a null pointer if reading of the result set failed. You can check if you got an error by checking if mysql_error() doesn't return a null pointer, if mysql_errno() returns <> 0, or if mysql_field_count() returns <> 0.

An empty result set is returned if there are no rows returned. (An empty result set differs from a null pointer as a return value.)

Once you have called mysql_store_result() and got a result back that isn't a null pointer, you may call mysql_num_rows() to find out how many rows are in the result set.

You can call mysql_fetch_row() to fetch rows from the result set, or mysql_row_seek() and mysql_row_tell() to obtain or set the current row position within the result set.

You must call mysql_free_result() once you are done with the result set.

See section 8.4.5.1 Why Is It that After mysql_query() Returns Success, mysql_store_result() Sometimes Returns NULL?.

8.4.3.211 Return Values

A MYSQL_RES result structure with the results. NULL if an error occurred.

8.4.3.212 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_OUT_OF_MEMORY
Out of memory.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.3.213 mysql_thread_id()

unsigned long mysql_thread_id(MYSQL *mysql)

8.4.3.214 Description

Returns the thread ID of the current connection. This value can be used as an argument to mysql_kill() to kill the thread.

If the connection is lost and you reconnect with mysql_ping(), the thread ID will change. This means you should not get the thread ID and store it for later. You should get it when you need it.

8.4.3.215 Return Values

The thread ID of the current connection.

8.4.3.216 Errors

None.

8.4.3.217 mysql_use_result()

MYSQL_RES *mysql_use_result(MYSQL *mysql)

8.4.3.218 Description

You must call mysql_store_result() or mysql_use_result() for every query that successfully retrieves data (SELECT, SHOW, DESCRIBE, EXPLAIN).

mysql_use_result() initiates a result set retrieval but does not actually read the result set into the client like mysql_store_result() does. Instead, each row must be retrieved individually by making calls to mysql_fetch_row(). This reads the result of a query directly from the server without storing it in a temporary table or local buffer, which is somewhat faster and uses much less memory than mysql_store_result(). The client will only allocate memory for the current row and a communication buffer that may grow up to max_allowed_packet bytes.

On the other hand, you shouldn't use mysql_use_result() if you are doing a lot of processing for each row on the client side, or if the output is sent to a screen on which the user may type a ^S (stop scroll). This will tie up the server and prevent other threads from updating any tables from which the data is being fetched.

When using mysql_use_result(), you must execute mysql_fetch_row() until a NULL value is returned, otherwise the unfetched rows will be returned as part of the result set for your next query. The C API will give the error Commands out of sync; You can't run this command now if you forget to do this!

You may not use mysql_data_seek(), mysql_row_seek(), mysql_row_tell(), mysql_num_rows(), or mysql_affected_rows() with a result returned from mysql_use_result(), nor may you issue other queries until the mysql_use_result() has finished. (However, after you have fetched all the rows, mysql_num_rows() will accurately return the number of rows fetched.)

You must call mysql_free_result() once you are done with the result set.

8.4.3.219 Return Values

A MYSQL_RES result structure. NULL if an error occurred.

8.4.3.220 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_OUT_OF_MEMORY
Out of memory.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

8.4.4 C Threaded Function Descriptions

You need to use the following functions when you want to create a threaded client. See section 8.4.7 How to Make a Threaded Client.

8.4.4.1 my_init()

8.4.4.2 Description

This function needs to be called once in the program before calling any MySQL function. This initializes some global variables that MySQL needs. If you are using a thread safe client library, this will also call my_thread_init() for this thread.

This is automatically called by mysql_init() and mysql_connect().

8.4.4.3 Return Values

none.

8.4.4.4 my_thread_init()

8.4.4.5 Description

This function needs to be called for each created thread to initialize thread specific variables.

This is automatically called by my_init() and mysql_connect().

8.4.4.6 Return Values

none.

8.4.4.7 my_thread_end()

8.4.4.8 Description

This function needs to be called before calling pthread_exit() to freed memory allocated by my_thread_init().

Note that this function is NOT invoked automatically by the client library!

8.4.4.9 Return Values

none.

8.4.5 Common questions and problems when using the C API

8.4.5.1 Why Is It that After mysql_query() Returns Success, mysql_store_result() Sometimes Returns NULL?

It is possible for mysql_store_result() to return NULL following a successful call to mysql_query(). When this happens, it means one of the following conditions occurred:

You can always check whether or not the statement should have produced a non-empty result by calling mysql_field_count(). If mysql_field_count() returns zero, the result is empty and the last query was a statement that does not return values (for example, an INSERT or a DELETE). If mysql_field_count() returns a non-zero value, the statement should have produced a non-empty result. See the description of the mysql_field_count() function for an example.

You can test for an error by calling mysql_error() or mysql_errno().

8.4.5.2 What Results Can I Get From a Query?

In addition to the result set returned by a query, you can also get the following information:

8.4.5.3 How Can I Get the Unique ID for the Last Inserted Row?

If you insert a record in a table containing a column that has the AUTO_INCREMENT attribute, you can get the most recently generated ID by calling the mysql_insert_id() function.

You can also retrieve the ID by using the LAST_INSERT_ID() function in a query string that you pass to mysql_query().

You can check if an AUTO_INCREMENT index is used by executing the following code. This also checks if the query was an INSERT with an AUTO_INCREMENT index: 

if (mysql_error(&mysql)[0] == 0 &&
    mysql_num_fields(result) == 0 &&
    mysql_insert_id(&mysql) != 0)
{
    used_id = mysql_insert_id(&mysql);
}

The most recently generated ID is maintained in the server on a per-connection basis. It will not be changed by another client. It will not even be changed if you update another AUTO_INCREMENT column with a non-magic value (that is, a value that is not NULL and not 0).

If you want to use the ID that was generated for one table and insert it into a second table, you can use SQL statements like this: 

INSERT INTO foo (auto,text)
    VALUES(NULL,'text');              # generate ID by inserting NULL
INSERT INTO foo2 (id,text)
    VALUES(LAST_INSERT_ID(),'text');  # use ID in second table

8.4.5.4 Problems Linking with the C API

When linking with the C API, the following errors may occur on some systems: 

gcc -g -o client test.o -L/usr/local/lib/mysql -lmysqlclient -lsocket -lnsl

Undefined        first referenced
 symbol          in file
floor            /usr/local/lib/mysql/libmysqlclient.a(password.o)
ld: fatal: Symbol referencing errors. No output written to client

If this happens on your system, you must include the math library by adding -lm to the end of the compile/link line.

8.4.6 Building Client Programs

If you compile MySQL clients that you've written yourself or that you obtain from a third party, they must be linked using the -lmysqlclient -lz option on the link command. You may also need to specify a -L option to tell the linker where to find the library. For example, if the library is installed in `/usr/local/mysql/lib', use -L/usr/local/mysql/lib -lmysqlclient -lz on the link command.

For clients that use MySQL header files, you may need to specify a -I option when you compile them (for example, -I/usr/local/mysql/include), so the compiler can find the header files.

8.4.7 How to Make a Threaded Client

The client library is almost thread safe. The biggest problem is that the subroutines in `net.c' that read from sockets are not interrupt safe. This was done with the thought that you might want to have your own alarm that can break a long read to a server. If you install interrupt handlers for the SIGPIPE interrupt, the socket handling should be thread safe.

In the older binaries we distribute on our Web site, the client libraries are not normally compiled with the thread-safe option (the Windows binaries are by default compiled to be thread safe). Newer binary distributions should have both a normal and a thread-safe client library.

To get a threaded client where you can interrupt the client from other threads and set timeouts when talking with the MySQL server, you should use the -lmysys, -lstring, and -ldbug libraries and the net_serv.o code that the server uses.

If you don't need interrupts or timeouts, you can just compile a thread safe client library (mysqlclient_r) and use this. See section 8.4 MySQL C API. In this case you don't have to worry about the net_serv.o object file or the other MySQL libraries.

When using a threaded client and you want to use timeouts and interrupts, you can make great use of the routines in the `thr_alarm.c' file. If you are using routines from the mysys library, the only thing you must remember is to call my_init() first! See section 8.4.4 C Threaded Function Descriptions.

All functions except mysql_real_connect() are by default thread safe. The following notes describe how to compile a thread safe client library and use it in a thread-safe manner. (The notes below for mysql_real_connect() actually apply to mysql_connect() as well, but because mysql_connect() is deprecated, you should be using mysql_real_connect() anyway.)

To make mysql_real_connect() thread safe, you must recompile the client library with this command: 

shell> ./configure --enable-thread-safe-client

This will create a thread-safe client library libmysqlclient_r. --enable-thread-safe-client. This library is thread safe per connection. You can let two threads share the same connection as long as you do the following:

You need to know the following if you have a thread that is calling MySQL functions, but that thread has not created the connection to the MySQL database:

When you call mysql_init() or mysql_connect(), MySQL will create a thread specific variable for the thread that is used by the debug library (among other things).

If you have in a thread call a MySQL function, before a thread has called mysql_init() or mysql_connect(), the thread will not have the necessary thread specific variables in place and you are likely to end up with a core dump sooner or later. The get things to work smoothly you have to do the following:

  1. Call my_init() at the start of your program if it calls any other MySQL function before calling mysql_real_connect().
  2. Call my_thread_init() in the thread handler before calling any MySQL function.
  3. In the thread, call my_thread_end() before calling pthread_exit(). This will free the memory used by MySQL thread specific variables.

You may get some errors because of undefined symbols when linking your client with mysqlclient_r. In most cases this is because you haven't included the thread libraries on the link/compile line.

8.5 MySQL C++ APIs

Two APIs are available in the MySQL Contrib directory.

8.5.1 Borland C++

You can compile the MySQL Windows source with Borland C++ 5.02. (The Windows source includes only projects for Microsoft VC++, for Borland C++ you have to do the project files yourself).

One known problem with Borland C++ is that it uses a different structure alignment than VC++. This means that you will run into problems if you try to use the default libmysql.dll libraries (that was compiled with VC++) with Borland C++. You can do one of the following to avoid this problem.

8.6 MySQL Java Connectivity (JDBC)

There are 2 supported JDBC drivers for MySQL (the mm driver and the Reisin JDBC driver). You can find a copy of the mm driver at http://mmmysql.sourceforge.net/ or http://www.mysql.com/Downloads/Contrib/ and the Reisin driver at http://www.caucho.com/projects/jdbc-mysql/index.xtp For documentation consult any JDBC documentation and the driver's own documentation for MySQL-specific features.

8.7 MySQL Python APIs

The MySQL Contrib directory contains a Python interface written by Joseph Skinner.

You can also use the Python interface to iODBC to access a MySQL server. mxODBC

8.8 MySQL Tcl APIs

Tcl at binevolve. The Contrib directory contains a Tcl interface that is based on msqltcl 1.50.

8.9 MySQL Eiffel wrapper

The MySQL Contrib directory contains an Eiffel wrapper written by Michael Ravits.

You can also find this at: http://www.netpedia.net/hosting/newplayer/

9 Extending MySQL

9.1 Adding New Functions to MySQL

There are two ways to add new functions to MySQL:

Each method has advantages and disadvantages:

Whichever method you use to add new functions, they may be used just like native functions such as ABS() or SOUNDEX().

9.1.1 CREATE FUNCTION/DROP FUNCTION Syntax

CREATE [AGGREGATE] FUNCTION function_name RETURNS {STRING|REAL|INTEGER}
       SONAME shared_library_name

DROP FUNCTION function_name

A user-definable function (UDF) is a way to extend MySQL with a new function that works like native (built in) MySQL functions such as ABS() and CONCAT().

AGGREGATE is a new option for MySQL Version 3.23. An AGGREGATE function works exactly like a native MySQL GROUP function like SUM or COUNT().

CREATE FUNCTION saves the function's name, type, and shared library name in the mysql.func system table. You must have the insert and delete privileges for the mysql database to create and drop functions.

All active functions are reloaded each time the server starts, unless you start mysqld with the --skip-grant-tables option. In this case, UDF initialization is skipped and UDFs are unavailable. (An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION.)

For instructions on writing user-definable functions, see section 9.1 Adding New Functions to MySQL. For the UDF mechanism to work, functions must be written in C or C++, your operating system must support dynamic loading and you must have compiled mysqld dynamically (not statically).

9.1.2 Adding a New User-definable Function

For the UDF mechanism to work, functions must be written in C or C++ and your operating system must support dynamic loading. The MySQL source distribution includes a file `sql/udf_example.cc' that defines 5 new functions. Consult this file to see how UDF calling conventions work.

For mysqld to be able to use UDF functions, you should configure MySQL with --with-mysqld-ldflags=-rdynamic The reason is that to on many platforms (including Linux) you can load a dynamic library (with dlopen()) from a static linked program, which you would get if you are using --with-mysqld-ldflags=-all-static If you want to use an UDF that needs to access symbols from mysqld (like the methaphone example in `sql/udf_example.cc' that uses default_charset_info), you must link the program with -rdynamic. (see man dlopen).

For each function that you want to use in SQL statements, you should define corresponding C (or C++) functions. In the discussion below, the name ``xxx'' is used for an example function name. To distinquish between SQL and C/C++ usage, XXX() (uppercase) indicates a SQL function call, and xxx() (lowercase) indicates a C/C++ function call.

The C/C++ functions that you write to implement the interface for XXX() are:

xxx() (required)
The main function. This is where the function result is computed. The correspondence between the SQL type and return type of your C/C++ function is shown below:
SQL type C/C++ type
STRING char *
INTEGER long long
REAL double
xxx_init() (optional)
The initialization function for xxx(). It can be used to:
xxx_deinit() (optional)
The deinitialization function for xxx(). It should deallocate any memory allocated by the initialization function.

When a SQL statement invokes XXX(), MySQL calls the initialization function xxx_init() to let it perform any required setup, such as argument checking or memory allocation. If xxx_init() returns an error, the SQL statement is aborted with an error message and the main and deinitialization functions are not called. Otherwise, the main function xxx() is called once for each row. After all rows have been processed, the deinitialization function xxx_deinit() is called so it can perform any required cleanup.

All functions must be thread safe (not just the main function, but the initialization and deinitialization functions as well). This means that you are not allowed to allocate any global or static variables that change! If you need memory, you should allocate it in xxx_init() and free it in xxx_deinit().

9.1.2.1 UDF Calling Sequences

The main function should be declared as shown below. Note that the return type and parameters differ, depending on whether you will declare the SQL function XXX() to return STRING, INTEGER, or REAL in the CREATE FUNCTION statement:

For STRING functions: 

char *xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *result, unsigned long *length,
              char *is_null, char *error);

For INTEGER functions: 

long long xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);

For REAL functions: 

double xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);

The initialization and deinitialization functions are declared like this: 

my_bool xxx_init(UDF_INIT *initid, UDF_ARGS *args, char *message);

void xxx_deinit(UDF_INIT *initid);

The initid parameter is passed to all three functions. It points to a UDF_INIT structure that is used to communicate information between functions. The UDF_INIT structure members are listed below. The initialization function should fill in any members that it wishes to change. (To use the default for a member, leave it unchanged.):

my_bool maybe_null
xxx_init() should set maybe_null to 1 if xxx() can return NULL. The default value is 1 if any of the arguments are declared maybe_null.
unsigned int decimals
Number of decimals. The default value is the maximum number of decimals in the arguments passed to the main function. (For example, if the function is passed 1.34, 1.345, and 1.3, the default would be 3, because 1.345 has 3 decimals.
unsigned int max_length
The maximum length of the string result. The default value differs depending on the result type of the function. For string functions, the default is the length of the longest argument. For integer functions, the default is 21 digits. For real functions, the default is 13 plus the number of decimals indicated by initid->decimals. (For numeric functions, the length includes any sign or decimal point characters.)
char *ptr
A pointer that the function can use for its own purposes. For example, functions can use initid->ptr to communicate allocated memory between functions. In xxx_init(), allocate the memory and assign it to this pointer: 
initid->ptr = allocated_memory;
In xxx() and xxx_deinit(), refer to initid->ptr to use or deallocate the memory.

9.1.2.2 Argument Processing

The args parameter points to a UDF_ARGS structure that thas the members listed below:

unsigned int arg_count
The number of arguments. Check this value in the initialization function if you want your function to be called with a particular number of arguments. For example: 
if (args->arg_count != 2)
{
    strcpy(message,"XXX() requires two arguments");
    return 1;
}
enum Item_result *arg_type
The types for each argument. The possible type values are STRING_RESULT, INT_RESULT, and REAL_RESULT. To make sure that arguments are of a given type and return an error if they are not, check the arg_type array in the initialization function. For example: 
if (args->arg_type[0] != STRING_RESULT ||
    args->arg_type[1] != INT_RESULT)
{
    strcpy(message,"XXX() requires a string and an integer");
    return 1;
}
As an alternative to requiring your function's arguments to be of particular types, you can use the initialization function to set the arg_type elements to the types you want. This causes MySQL to coerce arguments to those types for each call to xxx(). For example, to specify coercion of the first two arguments to string and integer, do this in xxx_init(): 
args->arg_type[0] = STRING_RESULT;
args->arg_type[1] = INT_RESULT;
char **args
args->args communicates information to the initialization function about the general nature of the arguments your function was called with. For a constant argument i, args->args[i] points to the argument value. (See below for instructions on how to access the value properly.) For a non-constant argument, args->args[i] is 0. A constant argument is an expression that uses only constants, such as 3 or 4*7-2 or SIN(3.14). A non-constant argument is an expression that refers to values that may change from row to row, such as column names or functions that are called with non-constant arguments. For each invocation of the main function, args->args contains the actual arguments that are passed for the row currently being processed. Functions can refer to an argument i as follows:
unsigned long *lengths
For the initialization function, the lengths array indicates the maximum string length for each argument. For each invocation of the main function, lengths contains the actual lengths of any string arguments that are passed for the row currently being processed. For arguments of types INT_RESULT or REAL_RESULT, lengths still contains the maximum length of the argument (as for the initialization function).

9.1.2.3 Return Values and Error Handling

The initialization function should return 0 if no error occurred and 1 otherwise. If an error occurs, xxx_init() should store a null-terminated error message in the message parameter. The message will be returned to the client. The message buffer is MYSQL_ERRMSG_SIZE characters long, but you should try to keep the message to less than 80 characters so that it fits the width of a standard terminal screen.

The return value of the main function xxx() is the function value, for long long and double functions. A string functions should return a pointer to the result and store the length of the string in the length arguments. result is a buffer at least 255 bytes long. Set these to the contents and length of the return value. For example: 

memcpy(result, "result string", 13);
*length = 13;

If your string functions that needs to return a string longer than 255 bytes, you must allocate the space for it with malloc() in your xxx_init() function or your xxx() function and free it in your xxx_deinit() function. You can store the allocated memory in the ptr slot in the UDF_INIT structure for reuse by future xxx() calls. See section 9.1.2.1 UDF Calling Sequences.

To indicate a return value of NULL in the main function, set is_null to 1: 

*is_null = 1;

To indicate an error return in the main function, set the error parameter to 1: 

*error = 1;

If xxx() sets *error to 1 for any row, the function value is NULL for the current row and for any subsequent rows processed by the statement in which XXX() was invoked. (xxx() will not even be called for subsequent rows.) NOTE: In MySQL versions prior to 3.22.10, you should set both *error and *is_null: 

*error = 1;
*is_null = 1;

9.1.2.4 Compiling and Installing User-definable Functions

Files implementing UDFs must be compiled and installed on the host where the server runs. This process is described below for the example UDF file `udf_example.cc' that is included in the MySQL source distribution. This file contains the following functions:

A dynamically loadable file should be compiled as a sharable object file, using a command something like this: 

shell> gcc -shared -o udf_example.so myfunc.cc

You can easily find out the correct compiler options for your system by running this command in the `sql' directory of your MySQL source tree: 

shell> make udf_example.o

You should run a compile command similar to the one that make displays, except that you should remove the -c option near the end of the line and add -o udf_example.so to the end of the line. (On some systems, you may need to leave the -c on the command.)

Once you compile a shared object containing UDFs, you must install it and tell MySQL about it. Compiling a shared object from `udf_example.cc' produces a file named something like `udf_example.so' (the exact name may vary from platform to platform). Copy this file to some directory searched by ld, such as `/usr/lib'. On many systems, you can set the LD_LIBRARY or LD_LIBRARY_PATH environment variable to point at the directory where you have your UDF function files. The dlopen manual page tells you which variable you should use on your system. You should set this in mysql.server or safe_mysqld and restart mysqld.

After the library is installed, notify mysqld about the new functions with these commands: 

mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "udf_example.so";
mysql> CREATE FUNCTION myfunc_double RETURNS REAL SONAME "udf_example.so";
mysql> CREATE FUNCTION myfunc_int RETURNS INTEGER SONAME "udf_example.so";
mysql> CREATE FUNCTION lookup RETURNS STRING SONAME "udf_example.so";
mysql> CREATE FUNCTION reverse_lookup RETURNS STRING SONAME "udf_example.so";

Functions can be deleted using DROP FUNCTION: 

mysql> DROP FUNCTION metaphon;
mysql> DROP FUNCTION myfunc_double;
mysql> DROP FUNCTION myfunc_int;
mysql> DROP FUNCTION lookup;
mysql> DROP FUNCTION reverse_lookup;

The CREATE FUNCTION and DROP FUNCTION statements update the system table func in the mysql database. The function's name, type and shared library name are saved in the table. You must have the insert and delete privileges for the mysql database to create and drop functions.

You should not use CREATE FUNCTION to add a function that has already been created. If you need to reinstall a function, you should remove it with DROP FUNCTION and then reinstall it with CREATE FUNCTION. You would need to do this, for example, if you recompile a new version of your function, so that mysqld gets the new version. Otherwise the server will continue to use the old version.

Active functions are reloaded each time the server starts, unless you start mysqld with the --skip-grant-tables option. In this case, UDF initialization is skipped and UDFs are unavailable. (An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION.)

9.1.3 Adding a New Native Function

The procedure for adding a new native function is described below. Note that you cannot add native functions to a binary distribution because the procedure involves modifying MySQL source code. You must compile MySQL yourself from a source distribution. Also note that if you migrate to another version of MySQL (for example, when a new version is released), you will need to repeat the procedure with the new version.

To add a new native MySQL function, follow these steps:

  1. Add one line to `lex.h' that defines the function name in the sql_functions[] array.
  2. If the function prototype is simple (just takes zero, one, two or three arguments), you should in lex.h specify SYM(FUNC_ARG#) (where # is the number of arguments) as the second argument in the sql_functions[] array and add a function that creates a function object in `item_create.cc'. Take a look at "ABS" and create_funcs_abs() for an example of this. If the function prototype is complicated (for example takes a variable number of arguments), you should add two lines to `sql_yacc.yy'. One indicates the preprocessor symbol that yacc should define (this should be added at the beginning of the file). Then define the function parameters and add an ``item'' with these parameters to the simple_expr parsing rule. For an example, check all occurrences of ATAN in `sql_yacc.yy' to see how this is done.
  3. In `item_func.h', declare a class inheriting from Item_num_func or Item_str_func, depending on whether your function returns a number or a string.
  4. In `item_func.cc', add one of the following declarations, depending on whether you are defining a numeric or string function: 
    double   Item_func_newname::val()
longlong Item_func_newname::val_int()
String  *Item_func_newname::Str(String *str)
    If you inherit your object from any of the standard items (like Item_num_func you probably only have to define one of the above functions and let the parent object take care of the other functions. For example, the Item_str_func class defines a val() function that executes atof() on the value returned by ::str().
  5. You should probably also define the following object function: 
    void Item_func_newname::fix_length_and_dec()
    This function should at least calculate max_length based on the given arguments. max_length is the maximum number of characters the function may return. This function should also set maybe_null = 0 if the main function can't return a NULL value. The function can check if any of the function arguments can return NULL by checking the arguments maybe_null variable. You can take a look at Item_func_mod::fix_length_and_dec for a typical example of how to do this.

All functions must be thread safe (In other words, don't use any global or static variables in the functions without protecting them with mutexes).

If you want to return NULL, from ::val(), ::val_int() or ::str() you should set null_value to 1 and return 0.

For ::str() object functions, there are some additional considerations to be aware of:

9.2 Adding New Procedures to MySQL

In MySQL, you can define a procedure in C++ that can access and modify the data in a query before it is sent to the client. The modification can be done on row-by-row or GROUP BY level.

We have created an example procedure in MySQL Version 3.23 to show you what can be done.

Additionally we recommend you to take a look at 'mylua', which you can find in the Contrib directory. See section D Contributed Programs. Which this you can use the LUA language to load a procedure at runtime into mysqld.

9.2.1 Procedure Analyse

analyse([max elements,[max memory]])

This procedure is defined in the `sql/sql_analyse.cc'. This examines the result from your query and returns an analysis of the results: 

SELECT ... FROM ... WHERE ... PROCEDURE ANALYSE([max elements,[max memory]])

9.2.2 Writing a Procedure

For the moment, the only documentation for this is the source.

You can find all information about procedures by examining the following files:

9.3 MySQL Internals

This chapter describes a lot of things that you need to know when working on the MySQL code. If you plan to contribute to MySQL development, want to have access to the bleeding-edge in-between versions code, or just want to keep track of development, follow the instructions in See section 2.3.4 Installing from the Development Source Tree. If you are interested in MySQL internals, you should also subscribe to internals@lists.mysql.com. This is a relatively low traffic list, in comparison with mysql@lists.mysql.com.

9.3.1 MySQL Threads

The MySQL server creates the following threads:

mysqladmin processlist only shows the connection, INSERT DELAYED, and replication threads.

9.3.2 MySQL Test Suite

Until recently, our main full-coverage test suite was based on proprietary customer data and for that reason has not been publicly available. The only publicly available part of our testing process consisted of the crash-me test, a Perl DBI/DBD benchmark found in the sql-bench directory, and miscellaneous tests located in tests directory. The lack of a standardized publicly available test suite has made it difficult for our users, as well developers, to do regression tests on the MySQL code. To address this problem, we have created a new test system that is included in the source and binary distributions starting in Version 3.23.29.

The current set of test cases doesn't test everything in MySQL, but it should catch most obvious bugs in the SQL processing code, OS/library issues, and is quite thorough in testing replication. Our eventual goal is to have the tests cover 100% of the code. We welcome contributions to our test suite. You may especially want to contribute tests that examine the functionality critical to your system, as this will ensure that all future MySQL releases will work well with your applications.

9.3.2.1 Running the MySQL Test Suite

The test system consist of a test language interpreter (mysqltest), a shell script to run all tests(mysql-test-run), the actual test cases written in a special test language, and their expected results. To run the test suite on your system after a build, type make test or mysql-test/mysql-test-run from the source root. If you have installed a binary distribution, cd to the install root (eg. /usr/local/mysql), and do scripts/mysql-test-run. All tests should succeed. If not, you should try to find out why and report the problem if this is a bug in MySQL. See section 9.3.2.3 Reporting Bugs in the MySQL Test Suite.

If you have a copy of mysqld running on the machine where you want to run the test suite you do not have to stop it, as long as it is not using ports 9306 and 9307. If one of those ports is taken, you should edit mysql-test-run and change the values of the master and/or slave port to one that is available.

You can run one individual test case with mysql-test/mysql-test-run test_name.

If one test fails, you should test running mysql-test-run with the --force option to check if any other tests fails.

9.3.2.2 Extending the MySQL Test Suite

You can use the mysqltest language to write your own test cases. Unfortunately, we have not yet written full documentation for it - we plan to do this shortly. You can, however, look at our current test cases and use them as an example. The following points should help you get started:

9.3.2.3 Reporting Bugs in the MySQL Test Suite

If your MySQL version doesn't pass the test suite you should do the following:

A Problems and Common Errors

This chapter lists some common problems and error messages that users have run into. You will learn how to figure out what the problem is, and what to do to solve it. You will also find proper solutions to some common problems.

A.1 How to Determine What Is Causing Problems

When you run into problems, the first thing you should do is to find out which program / piece of equipment is causing problems:

If after you have examined all other possibilities and you have concluded that it's the MySQL server or a MySQL client that is causing the problem, it's time to do a bug report for our mailing list or our support team. In the bug report, try to give a very detailed description of how the system is behaving and what you think is happening. You should also state why you think it's MySQL that is causing the problems. Take into consideration all the situations in this chapter. State any problems exactly how they appear when you examine your system. Use the 'cut and paste' method for any output and/or error messages from programs and/or log files!

Try to describe in detail which program is not working and all symptoms you see! We have in the past received many bug reports that just state "the system doesn't work". This doesn't provide us with any information about what could be the problem.

If a program fails, it's always useful to know:

When sending a bug report, you should of follow the outlines described in this manual. See section 1.2.22.2 Asking Questions or Reporting Bugs.

A.2 Some Common Errors When Using MySQL

This section lists some errors that users frequently get. You will find descriptions of the errors, and how to solve the problem here.

A.2.1 Access denied Error

See section 4.2.5 How the Privilege System Works, and especially. See section 4.2.10 Causes of Access denied Errors.

A.2.2 MySQL server has gone away Error

This section also covers the related Lost connection to server during query error.

The most common reason for the MySQL server has gone away error is that the server timed out and closed the connection. By default, the server closes the connection after 8 hours if nothing has happened. You can change the time limit by setting the wait_timeout variable when you start mysqld.

Another common reason to receive the MySQL server has gone away error is because you have issued a ``close'' on your MySQL connection and then tried to run a query on the closed connection.

You can check that the MySQL hasn't died by executing mysqladmin version and examining the uptime.

If you have a script, you just have to issue the query again for the client to do an automatic reconnection.

You normally can get the following error codes in this case (which one you get is OS-dependent):

CR_SERVER_GONE_ERROR The client couldn't send a question to the server.
CR_SERVER_LOST The client didn't get an error when writing to the server, but it didn't get a full answer (or any answer) to the question.

You can also get these errors if you send a query to the server that is incorrect or too large. If mysqld gets a packet that is too large or out of order, it assumes that something has gone wrong with the client and closes the connection. If you need big queries (for example, if you are working with big BLOB columns), you can increase the query limit by starting mysqld with the -O max_allowed_packet=# option (default 1M). The extra memory is allocated on demand, so mysqld will use more memory only when you issue a big query or when mysqld must return a big result row!

A.2.3 Can't connect to [local] MySQL server error

A MySQL client on Unix can connect to the mysqld server in two different ways: Unix sockets, which connect through a file in the file system (default `/tmp/mysqld.sock') or TCP/IP, which connects through a port number. Unix sockets are faster than TCP/IP but can only be used when connecting to a server on the same computer. Unix sockets are used if you don't specify a hostname or if you specify the special hostname localhost.

On Windows you can connect only with TCP/IP if the mysqld server is running on Win95/Win98. If it's running on NT, you can also connect with named pipes. The name of the named pipe is MySQL. If you don't give a hostname when connecting to mysqld, a MySQL client will first try to connect to the named pipe, and if this doesn't work it will connect to the TCP/IP port. You can force the use of named pipes on Windows by using . as the hostname.

The error (2002) Can't connect to ... normally means that there isn't a MySQL server running on the system or that you are using a wrong socket file or TCP/IP port when trying to connect to the mysqld server.

Start by checking (using ps or the task manager on Windows) that there is a process running named mysqld on your server! If there isn't any mysqld process, you should start one. See section 2.4.2 Problems Starting the MySQL Server.

If a mysqld process is running, you can check the server by trying these different connections (the port number and socket pathname might be different in your setup, of course): 

shell> mysqladmin version
shell> mysqladmin variables
shell> mysqladmin -h `hostname` version variables
shell> mysqladmin -h `hostname` --port=3306 version
shell> mysqladmin -h 'ip for your host' version
shell> mysqladmin --socket=/tmp/mysql.sock version

Note the use of backquotes rather than forward quotes with the hostname command; these cause the output of hostname (that is, the current hostname) to be substituted into the mysqladmin command.

Here are some reasons the Can't connect to local MySQL server error might occur:

If you get the error message Can't connect to MySQL server on some_hostname, you can try the following things to find out what the problem is :

A.2.4 Host '...' is blocked Error

If you get an error like this: 

Host 'hostname' is blocked because of many connection errors.
Unblock with 'mysqladmin flush-hosts'

this means that mysqld has gotten a lot (max_connect_errors) of connect requests from the host 'hostname' that have been interrupted in the middle. After max_connect_errors failed requests, mysqld assumes that something is wrong (like an attack from a cracker), and blocks the site from further connections until someone executes the command mysqladmin flush-hosts.

By default, mysqld blocks a host after 10 connection errors. You can easily adjust this by starting the server like this: 

shell> safe_mysqld -O max_connect_errors=10000 &

Note that if you get this error message for a given host, you should first check that there isn't anything wrong with TCP/IP connections from that host. If your TCP/IP connections aren't working, it won't do you any good to increase the value of the max_connect_errors variable!

A.2.5 Too many connections Error

If you get the error Too many connections when you try to connect to MySQL, this means that there is already max_connections clients connected to the mysqld server.

If you need more connections than the default (100), then you should restart mysqld with a bigger value for the max_connections variable.

Note that mysqld actually allows (max_connections+1) clients to connect. The last connection is reserved for a user with the process privilege. By not giving this privilege to normal users (they shouldn't need this), an administrator with this privilege can log in and use SHOW PROCESSLIST to find out what could be wrong. See section 4.5.5 SHOW Syntax.

The maximum number of connects MySQL is depending on how good the thread library is on a given platform. Linux or Solaris should be able to support 500-1000 simultaneous connections, depending on how much RAM you have and what your clients are doing.

A.2.6 Some non-transactional changed tables couldn't be rolled back Error

If you get the error/warning: Warning: Some non-transactional changed tables couldn't be rolled back when trying to do a ROLLBACK, this means that some of the tables you used in the transaction didn't support transactions. These non-transactional tables will not be affected by the ROLLBACK statement.

The most typical case when this happens is when you have tried to create a table of a type that is not supported by your mysqld binary. If mysqld doesn't support a table type (or if the table type is disabled by a startup option) , it will instead create the table type with the table type that is most resembles to the one you requested, probably MyISAM.

You can check the table type for a table by doing:

SHOW TABLE STATUS LIKE 'table_name'. See section 4.5.5.2 SHOW TABLE STATUS.

You can check the extensions your mysqld binary supports by doing:

show variables like 'have_%'. See section 4.5.5.4 SHOW VARIABLES.

A.2.7 Out of memory Error

If you issue a query and get something like the following error: 

mysql: Out of memory at line 42, 'malloc.c'
mysql: needed 8136 byte (8k), memory in use: 12481367 bytes (12189k)
ERROR 2008: MySQL client ran out of memory

note that the error refers to the MySQL client mysql. The reason for this error is simply that the client does not have enough memory to store the whole result.

To remedy the problem, first check that your query is correct. Is it reasonable that it should return so many rows? If so, you can use mysql --quick, which uses mysql_use_result() to retrieve the result set. This places less of a load on the client (but more on the server).

A.2.8 Packet too large Error

When a MySQL client or the mysqld server gets a packet bigger than max_allowed_packet bytes, it issues a Packet too large error and closes the connection.

If you are using the mysql client, you may specify a bigger buffer by starting the client with mysql --set-variable=max_allowed_packet=8M.

If you are using other clients that do not allow you to specify the maximum packet size (such as DBI), you need to set the packet size when you start the server. You cau use a command-line option to mysqld to set max_allowed_packet to a larger size. For example, if you are expecting to store the full length of a BLOB into a table, you'll need to start the server with the --set-variable=max_allowed_packet=16M option.

You can also get strange problems with large packets if you are using big blobs, but you haven't given mysqld access to enough memory to handle the query. If you suspect this is the case, try adding ulimit -d 256000 to the beginning of the safe_mysqld script and restart mysqld.

A.2.9 Communication Errors / Aborted Connection

Starting with MySQL 3.23.40 you only get the Aborted connection error of you start mysqld with --warnings.

If you find errors like the following in your error log. 

010301 14:38:23  Aborted connection 854 to db: 'users' user: 'josh'

See section 4.9.1 The Error Log.

This means that something of the following has happened:

When the above happens, the server variable Aborted_clients is incremented.

The server variable Aborted_connects is incremented when:

Note that the above could indicate that someone is trying to break into your database!

See section 4.5.5.4 SHOW VARIABLES.

Other reasons for problems with Aborted clients / Aborted connections.

A.2.10 The table is full Error

This error occurs in older MySQL versions when an in-memory temporary table becomes larger than tmp_table_size bytes. To avoid this problem, you can use the -O tmp_table_size=# option to mysqld to increase the temporary table size or use the SQL option SQL_BIG_TABLES before you issue the problematic query. See section 5.5.6 SET Syntax.

You can also start mysqld with the --big-tables option. This is exactly the same as using SQL_BIG_TABLES for all queries.

In MySQL Version 3.23, in-memory temporary tables will automatically be converted to a disk-based MyISAM table after the table size gets bigger than tmp_table_size.

A.2.11 Can't create/write to file Error

If you get an error for some queries of type: 

Can't create/write to file '\\sqla3fe_0.ism'.

this means that MySQL can't create a temporary file for the result set in the given temporary directory. (The above error is a typical error message on Windows, and the Unix error message is similar.) The fix is to start mysqld with --tmpdir=path or to add to your option file: 

[mysqld]
tmpdir=C:/temp

assuming that the `c:\\temp' directory exists. See section 4.1.2 my.cnf Option Files.

Check also the error code that you get with perror. One reason may also be a disk full error; 

shell> perror 28
Error code  28:  No space left on device

A.2.12 Commands out of sync Error in Client

If you get Commands out of sync; You can't run this command now in your client code, you are calling client functions in the wrong order!

This can happen, for example, if you are using mysql_use_result() and try to execute a new query before you have called mysql_free_result(). It can also happen if you try to execute two queries that return data without a mysql_use_result() or mysql_store_result() in between.

A.2.13 Ignoring user Error

If you get the following error:

Found wrong password for user: 'some_user@some_host'; Ignoring user

this means that when mysqld was started or when it reloaded the permissions tables, it found an entry in the user table with an invalid password. As a result, the entry is simply ignored by the permission system.

Possible causes of and fixes for this problem:

A.2.14 Table 'xxx' doesn't exist Error

If you get the error Table 'xxx' doesn't exist or Can't find file: 'xxx' (errno: 2), this means that no table exists in the current database with the name xxx.

Note that as MySQL uses directories and files to store databases and tables, the database and table names are case sensitive! (On Windows the databases and tables names are not case sensitive, but all references to a given table within a query must use the same case!)

You can check which tables you have in the current database with SHOW TABLES. See section 4.5.5 SHOW Syntax.

A.2.15 Can't initialize character set xxx error.

If you get an error like: 

MySQL Connection Failed: Can't initialize character set xxx

This means one of the following things:

A.2.16 File Not Found

If you get ERROR '...' not found (errno: 23), Can't open file: ... (errno: 24), or any other error with errno 23 or errno 24 from MySQL, it means that you haven't allocated enough file descriptors for MySQL. You can use the perror utility to get a description of what the error number means: 

shell> perror 23
File table overflow
shell> perror 24
Too many open files
shell> perror 11
Resource temporarily unavailable

The problem here is that mysqld is trying to keep open too many files simultaneously. You can either tell mysqld not to open so many files at once or increase the number of file descriptors available to mysqld.

To tell mysqld to keep open fewer files at a time, you can make the table cache smaller by using the -O table_cache=32 option to safe_mysqld (the default value is 64). Reducing the value of max_connections will also reduce the number of open files (the default value is 90).

To change the number of file descriptors available to mysqld, you can use the option --open-files-limit=# to safe_mysqld or -O open-files-limit=# to mysqld. See section 4.5.5.4 SHOW VARIABLES. The easiest way to do that is to add the option to your option file. See section 4.1.2 my.cnf Option Files. If you have an old mysqld version that doesn't support this, you can edit the safe_mysqld script. There is a commented-out line ulimit -n 256 in the script. You can remove the '#' character to uncomment this line, and change the number 256 to affect the number of file descriptors available to mysqld.

ulimit (and open-files-limit) can increase the number of file descriptors, but only up to the limit imposed by the operating system. There is also a 'hard' limit that can only be overrided if you start safe_mysqld or mysqld as root (Just remember that you need to also use the --user=.. option in this case). If you need to increase the OS limit on the number of file descriptors available to each process, consult the documentation for your operating system.

Note that if you run the tcsh shell, ulimit will not work! tcsh will also report incorrect values when you ask for the current limits! In this case you should start safe_mysqld with sh!

A.3 Installation Related Issues

A.3.1 Problems When Linking with the MySQL Client Library

If you are linking your program and you get errors for unreferenced symbols that start with mysql_, like the following: 

/tmp/ccFKsdPa.o: In function `main':
/tmp/ccFKsdPa.o(.text+0xb): undefined reference to `mysql_init'
/tmp/ccFKsdPa.o(.text+0x31): undefined reference to `mysql_real_connect'
/tmp/ccFKsdPa.o(.text+0x57): undefined reference to `mysql_real_connect'
/tmp/ccFKsdPa.o(.text+0x69): undefined reference to `mysql_error'
/tmp/ccFKsdPa.o(.text+0x9a): undefined reference to `mysql_close'

you should be able to solve this by adding -Lpath-to-the-mysql-library -lmysqlclient LAST on your link line.

If you get undefined reference errors for the uncompress or compress function, add -lz LAST on your link line and try again!

If you get undefined reference errors for functions that should exist on your system, like connect, check the man page for the function in question, for which libraries you should add to the link line!

If you get undefined reference errors for functions that don't exist on your system, like the following: 

mf_format.o(.text+0x201): undefined reference to `__lxstat'

it usually means that your library is compiled on a system that is not 100 % compatible with yours. In this case you should download the latest MySQL source distribution and compile this yourself. See section 2.3 Installing a MySQL Source Distribution.

If you are trying to run a program and you then get errors for unreferenced symbols that start with mysql_ or that the mysqlclient library can't be found, this means that your system can't find the share libmysqlclient.so library.

The fix for this is to tell your system to search after shared libraries where the library is located by one of the following methods:

Another way to solve this problem is to link your program statically, with -static, or by removing the dynamic MySQL libraries before linking your code. In the second case you should be sure that no other programs are using the dynamic libraries!

A.3.2 How to Run MySQL As a Normal User

The MySQL server mysqld can be started and run by any user. In order to change mysqld to run as a Unix user user_name, you must do the following:

  1. Stop the server if it's running (use mysqladmin shutdown).
  2. Change the database directories and files so that user_name has privileges to read and write files in them (you may need to do this as the Unix root user): 
    shell> chown -R user_name /path/to/mysql/datadir
    If directories or files within the MySQL data directory are symlinks, you'll also need to follow those links and change the directories and files they point to. chown -R may not follow symlinks for you.
  3. Start the server as user user_name, or, if you are using MySQL Version 3.22 or later, start mysqld as the Unix root user and use the --user=user_name option. mysqld will switch to run as the Unix user user_name before accepting any connections.
  4. To start the server as the given user name automatically at system startup time, add a user line that specifies the user name to the [mysqld] group of the `/etc/my.cnf' option file or the `my.cnf' option file in the server's data directory. For example: 
    [mysqld]
user=user_name

At this point, your mysqld process should be running fine and dandy as the Unix user user_name. One thing hasn't changed, though: the contents of the permissions tables. By default (right after running the permissions table install script mysql_install_db), the MySQL user root is the only user with permission to access the mysql database or to create or drop databases. Unless you have changed those permissions, they still hold. This shouldn't stop you from accessing MySQL as the MySQL root user when you're logged in as a Unix user other than root; just specify the -u root option to the client program.

Note that accessing MySQL as root, by supplying -u root on the command line, has nothing to do with MySQL running as the Unix root user, or, indeed, as another Unix user. The access permissions and user names of MySQL are completely separate from Unix user names. The only connection with Unix user names is that if you don't provide a -u option when you invoke a client program, the client will try to connect using your Unix login name as your MySQL user name.

If your Unix box itself isn't secured, you should probably at least put a password on the MySQL root users in the access tables. Otherwise, any user with an account on that machine can run mysql -u root db_name and do whatever he likes.

A.3.3 Problems with File Permissions

If you have problems with file permissions, for example, if mysql issues the following error message when you create a table: 

ERROR: Can't find file: 'path/with/filename.frm' (Errcode: 13)

then the environment variable UMASK might be set incorrectly when mysqld starts up. The default umask value is 0660. You can change this behavior by starting safe_mysqld as follows: 

shell> UMASK=384  # = 600 in octal
shell> export UMASK
shell> /path/to/safe_mysqld &

By default MySQL will create database and RAID directories with permission type 0700. You can modify this behavior by setting the UMASK_DIR variable. If you set this, new directories are created with the combined UMASK and UMASK_DIR. For example, if you want to give group access to all new directories, you can do: 

shell> UMASK_DIR=504  # = 770 in octal
shell> export UMASK_DIR
shell> /path/to/safe_mysqld &

In MySQL Version 3.23.25 and above, MySQL assumes that the value for UMASK and UMASK_DIR is in octal if it starts with a zero.

See section H Environment Variables.

A.4 Administration Related Issues

A.4.1 What To Do If MySQL Keeps Crashing

All MySQL versions are tested on many platforms before they are released. This doesn't mean that there aren't any bugs in MySQL, but it means if there are bugs, they are very few and can be hard to find. If you have a problem, it will always help if you try to find out exactly what crashes your system, as you will have a much better chance of getting this fixed quickly.

First, you should try to find out whether the problem is that the mysqld daemon dies or whether your problem has to do with your client. You can check how long your mysqld server has been up by executing mysqladmin version. If mysqld has died, you may find the reason for this in the file `mysql-data-directory/`hostname`.err'. See section 4.9.1 The Error Log.

Many crashes of MySQL are caused by corrupted index / data files. MySQL will update the data on disk, with the write() system call, after every SQL statement and before the client is notified about the result. (This is not true if you are running with delayed_key_writes, in which case only the data is written.) This means that the data is safe even if mysqld crashes, as the OS will ensure that the not flushed data is written to disk. You can force MySQL to sync everything to disk after every SQL command by starting mysqld with --flush.

The above means that normally you shouldn't get corrupted tables unless:

Because it is very difficult to know why something is crashing, first try to check whether or not things that work for others crash for you. Please try the following things:

A.4.2 How to Reset a Forgotten Password

If you have forgotten the root user password for MySQL, you can restore it with the following procedure:

  1. Take down the mysqld server by sending a kill (not kill -9) to the mysqld server. The pid is stored in a .pid file, which is normally in the MySQL database directory: 
    kill `cat /mysql-data-directory/hostname.pid`
    You must be either the Unix root user or the same user the server runs as to do this.
  2. Restart mysqld with the --skip-grant-tables option.
  3. Connect to the mysqld server with mysql -h hostname mysql and change the password with a GRANT command. See section 4.3.1 GRANT and REVOKE Syntax. You can also do this with mysqladmin -h hostname -u user password 'new password'
  4. Load the privilege tables with: mysqladmin -h hostname flush-privileges or with the SQL command FLUSH PRIVILEGES.

Note that after you started mysqld with --skip-grant-tables, any usage of GRANT commands will give you an Unknown command error until you have executed FLUSH PRIVILEGES.

A.4.3 How MySQL Handles a Full Disk

When a disk-full condition occurs, MySQL does the following:

To alleviate the problem, you can take the following actions:

Exceptions to the above behaveour is when you use REPAIR or OPTIMIZE or when the indexes are created in a batch after an LOAD DATA INFILE or after an ALTER TABLE statement.

All of the above commands may use big temporary files that left to themself would cause big problems for the rest of the system. If MySQL gets disk full while doing any of the above operations, it will remove the big temporary files and mark the table as crashed (except for ALTER TABLE, in which the old table will be left unchanged).

A.4.4 Where MySQL Stores Temporary Files

MySQL uses the value of the TMPDIR environment variable as the pathname of the directory in which to store temporary files. If you don't have TMPDIR set, MySQL uses the system default, which is normally `/tmp' or `/usr/tmp'. If the file system containing your temporary file directory is too small, you should edit safe_mysqld to set TMPDIR to point to a directory in a file system where you have enough space! You can also set the temporary directory using the --tmpdir option to mysqld.

MySQL creates all temporary files as hidden files. This ensures that the temporary files will be removed if mysqld is terminated. The disadvantage of using hidden files is that you will not see a big temporary file that fills up the file system in which the temporary file directory is located.

When sorting (ORDER BY or GROUP BY), MySQL normally uses one or two temporary files. The maximum disk-space needed is: 

(length of what is sorted + sizeof(database pointer))
* number of matched rows
* 2

sizeof(database pointer) is usually 4, but may grow in the future for really big tables.

For some SELECT queries, MySQL also creates temporary SQL tables. These are not hidden and have names of the form `SQL_*'.

ALTER TABLE creates a temporary table in the same directory as the original table.

A.4.5 How to Protect or change the MySQL socket file `/tmp/mysql.sock'

If you have problems with the fact that anyone can delete the MySQL communication socket `/tmp/mysql.sock', you can, on most versions of Unix, protect your `/tmp' file system by setting the sticky bit on it. Log in as root and do the following: 

shell> chmod +t /tmp

This will protect your `/tmp' file system so that files can be deleted only by their owners or the superuser (root).

You can check if the sticky bit is set by executing ls -ld /tmp. If the last permission bit is t, the bit is set.

You can change the place where MySQL uses / puts the socket file the following ways:

You can test that the socket works with this command: 

shell> mysqladmin --socket=/path/to/socket version

A.4.6 Time Zone Problems

If you have a problem with SELECT NOW() returning values in GMT and not your local time, you have to set the TZ environment variable to your current time zone. This should be done for the environment in which the server runs, for example, in safe_mysqld or mysql.server. See section H Environment Variables.

A.5 Query Related Issues

A.5.1 Case Sensitivity in Searches

By default, MySQL searches are case-insensitive (although there are some character sets that are never case insensitive, such as czech). That means that if you search with col_name LIKE 'a%', you will get all column values that start with A or a. If you want to make this search case-sensitive, use something like INSTR(col_name, "A")=1 to check a prefix. Or use STRCMP(col_name, "A") = 0 if the column value must be exactly "A".

Simple comparison operations (>=, >, = , < , <=, sorting and grouping) are based on each character's ``sort value''. Characters with the same sort value (like E, e and é) are treated as the same character!

In older MySQL versions LIKE comparisons where done on the uppercase value of each character (E == e but E <> é). In newer MySQL versions LIKE works just like the other comparison operators.

If you want a column always to be treated in case-sensitive fashion, declare it as BINARY. See section 6.5.3 CREATE TABLE Syntax.

If you are using Chinese data in the so-called big5 encoding, you want to make all character columns BINARY. This works because the sorting order of big5 encoding characters is based on the order of ASCII codes.

A.5.2 Problems Using DATE Columns

The format of a DATE value is 'YYYY-MM-DD'. According to ANSI SQL, no other format is allowed. You should use this format in UPDATE expressions and in the WHERE clause of SELECT statements. For example: 

mysql> SELECT * FROM tbl_name WHERE date >= '1997-05-05';

As a convenience, MySQL automatically converts a date to a number if the date is used in a numeric context (and vice versa). It is also smart enough to allow a ``relaxed'' string form when updating and in a WHERE clause that compares a date to a TIMESTAMP, DATE, or a DATETIME column. (Relaxed form means that any punctuation character may be used as the separator between parts. For example, '1998-08-15' and '1998#08#15' are equivalent.) MySQL can also convert a string containing no separators (such as '19980815'), provided it makes sense as a date.

The special date '0000-00-00' can be stored and retrieved as '0000-00-00'. When using a '0000-00-00' date through MyODBC, it will automatically be converted to NULL in MyODBC Version 2.50.12 and above, because ODBC can't handle this kind of date.

Because MySQL performs the conversions described above, the following statements work: 

mysql> INSERT INTO tbl_name (idate) VALUES (19970505);
mysql> INSERT INTO tbl_name (idate) VALUES ('19970505');
mysql> INSERT INTO tbl_name (idate) VALUES ('97-05-05');
mysql> INSERT INTO tbl_name (idate) VALUES ('1997.05.05');
mysql> INSERT INTO tbl_name (idate) VALUES ('1997 05 05');
mysql> INSERT INTO tbl_name (idate) VALUES ('0000-00-00');

mysql> SELECT idate FROM tbl_name WHERE idate >= '1997-05-05';
mysql> SELECT idate FROM tbl_name WHERE idate >= 19970505;
mysql> SELECT mod(idate,100) FROM tbl_name WHERE idate >= 19970505;
mysql> SELECT idate FROM tbl_name WHERE idate >= '19970505';

However, the following will not work: 

mysql> SELECT idate FROM tbl_name WHERE STRCMP(idate,'19970505')=0;

STRCMP() is a string function, so it converts idate to a string and performs a string comparison. It does not convert '19970505' to a date and perform a date comparison.

Note that MySQL does no checking whether or not the date is correct. If you store an incorrect date, such as '1998-2-31', the wrong date will be stored. If the date cannot be converted to any reasonable value, a 0 is stored in the DATE field. This is mainly a speed issue and we think it is up to the application to check the dates, and not the server.

A.5.3 Problems with NULL Values

The concept of the NULL value is a common source of confusion for newcomers to SQL, who often think that NULL is the same thing as an empty string ''. This is not the case! For example, the following statements are completely different: 

mysql> INSERT INTO my_table (phone) VALUES (NULL);
mysql> INSERT INTO my_table (phone) VALUES ("");

Both statements insert a value into the phone column, but the first inserts a NULL value and the second inserts an empty string. The meaning of the first can be regarded as ``phone number is not known'' and the meaning of the second can be regarded as ``she has no phone''.

In SQL, the NULL value is always false in comparison to any other value, even NULL. An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for the operators and functions involved in the expression. All columns in the following example return NULL: 

mysql> SELECT NULL,1+NULL,CONCAT('Invisible',NULL);

If you want to search for column values that are NULL, you cannot use the =NULL test. The following statement returns no rows, because expr = NULL is FALSE, for any expression: 

mysql> SELECT * FROM my_table WHERE phone = NULL;

To look for NULL values, you must use the IS NULL test. The following shows how to find the NULL phone number and the empty phone number: 

mysql> SELECT * FROM my_table WHERE phone IS NULL;
mysql> SELECT * FROM my_table WHERE phone = "";

In MySQL, as in many other SQL servers, you can't index columns that can have NULL values. You must declare such columns NOT NULL. Conversely, you cannot insert NULL into an indexed column.

When reading data with LOAD DATA INFILE, empty columns are updated with ''. If you want a NULL value in a column, you should use \N in the text file. The literal word 'NULL' may also be used under some circumstances. See section 6.4.8 LOAD DATA INFILE Syntax.

When using ORDER BY, NULL values are presented first. If you sort in descending order using DESC, NULL values are presented last. When using GROUP BY, all NULL values are regarded as equal.

To help with NULL handling, you can use the IS NULL and IS NOT NULL operators and the IFNULL() function.

For some column types, NULL values are handled specially. If you insert NULL into the first TIMESTAMP column of a table, the current date and time is inserted. If you insert NULL into an AUTO_INCREMENT column, the next number in the sequence is inserted.

A.5.4 Problems with alias

You can use an alias to refer to a column in the GROUP BY, ORDER BY, or in the HAVING part. Aliases can also be used to give columns better names: 

SELECT SQRT(a*b) as rt FROM table_name GROUP BY rt HAVING rt > 0;
SELECT id,COUNT(*) AS cnt FROM table_name GROUP BY id HAVING cnt > 0;
SELECT id AS "Customer identity" FROM table_name;

Note that ANSI SQL doesn't allow you to refer to an alias in a WHERE clause. This is because when the WHERE code is executed the column value may not yet be determined. For example, the following query is illegal: 

SELECT id,COUNT(*) AS cnt FROM table_name WHERE cnt > 0 GROUP BY id;

The WHERE statement is executed to determine which rows should be included in the GROUP BY part while HAVING is used to decide which rows from the result set should be used.

A.5.5 Deleting Rows from Related Tables

As MySQL doesn't support sub-selects or use of more than one table in the DELETE statement, you should use the following approach to delete rows from 2 related tables:

  1. SELECT the rows based on some WHERE condition in the main table.
  2. DELETE the rows in the main table based on the same condition.
  3. DELETE FROM related_table WHERE related_column IN (selected_rows).

If the total number of characters in the query with related_column is more than 1,048,576 (the default value of max_allowed_packet, you should split it into smaller parts and execute multiple DELETE statements. You will probably get the fastest DELETE by only deleting 100-1000 related_column id's per query if the related_column is an index. If the related_column isn't an index, the speed is independent of the number of arguments in the IN clause.

A.5.6 Solving Problems with No Matching Rows

If you have a complicated query that has many tables and that doesn't return any rows, you should use the following procedure to find out what is wrong with your query:

  1. Test the query with EXPLAIN and check if you can find something that is obviously wrong. See section 5.2.1 EXPLAIN Syntax (Get Information About a SELECT).
  2. Select only those fields that are used in the WHERE clause.
  3. Remove one table at a time from the query until it returns some rows. If the tables are big, it's a good idea to use LIMIT 10 with the query.
  4. Do a SELECT for the column that should have matched a row against the table that was last removed from the query.
  5. If you are comparing FLOAT or DOUBLE columns with numbers that have decimals, you can't use =! This problem is common in most computer languages because floating-point values are not exact values: 
    mysql> SELECT * FROM table_name WHERE float_column=3.5;
   ->
mysql> SELECT * FROM table_name WHERE float_column between 3.45 and 3.55;
    In most cases, changing the FLOAT to a DOUBLE will fix this!
  6. If you still can't figure out what's wrong, create a minimal test that can be run with mysql test < query.sql that shows your problems. You can create a test file with mysqldump --quick database tables > query.sql. Open the file in an editor, remove some insert lines (if there are too many of these), and add your select statement at the end of the file. Test that you still have your problem by doing: 
    shell> mysqladmin create test2
shell> mysql test2 < query.sql
    Post the test file using mysqlbug to mysql@lists.mysql.com.

A.6 Table Definition Related Issues

A.6.1 Problems with ALTER TABLE.

ALTER TABLE changes a table to the current character set. If you during ALTER TABLE get a duplicate key error, then the cause is either that the new character sets maps to keys to the same value or that the table is corrupted, in which case you should run REPAIR TABLE on the table.

If ALTER TABLE dies with an error like this: 

Error on rename of './database/name.frm' to './database/B-a.frm' (Errcode: 17)

the problem may be that MySQL has crashed in a previous ALTER TABLE and there is an old table named `A-something' or `B-something' lying around. In this case, go to the MySQL data directory and delete all files that have names starting with A- or B-. (You may want to move them elsewhere instead of deleting them.)

ALTER TABLE works the following way:

If something goes wrong with the renaming operation, MySQL tries to undo the changes. If something goes seriously wrong (this shouldn't happen, of course), MySQL may leave the old table as `B-xxx', but a simple rename on the system level should get your data back.

A.6.2 How To Change the Order of Columns in a Table

The whole point of SQL is to abstract the application from the data storage format. You should always specify the order in which you wish to retrieve your data. For example: 

SELECT col_name1, col_name2, col_name3 FROM tbl_name;

will return columns in the order col_name1, col_name2, col_name3, whereas: 

SELECT col_name1, col_name3, col_name2 FROM tbl_name;

will return columns in the order col_name1, col_name3, col_name2.

You should NEVER, in an application, use SELECT * and retrieve the columns based on their position, because the order in which columns are returned CANNOT be guaranteed over time. A simple change to your database may cause your application to fail rather dramatically.

If you want to change the order of columns anyway, you can do it as follows:

  1. Create a new table with the columns in the right order.
  2. Execute INSERT INTO new_table SELECT fields-in-new_table-order FROM old_table.
  3. Drop or rename old_table.
  4. ALTER TABLE new_table RENAME old_table.

A.6.3 TEMPORARY TABLE problems

The following are a list of the limitations with TEMPORARY TABLES.

B Some MySQL Users

This appendix lists some users of MySQL that have given us permission to list them in our documentation. It is by far not a complete list, but should give you a general idea of who uses MySQL and what it can be used for.

B.1 General News Sites

B.2 Some Web Search Engines

B.3 Some Information Search Engines Concentrated on Some Area

B.4 Online Magazines

B.5 Web Sites that Use MySQL as a Backend

B.6 Some Domain/Internet/Web and Related Services

B.7 Web Sites that Use PHP and MySQL

B.8 Some MySQL Consultants

B.9 Programming

B.10 Uncategorized Pages

Send any additions to this list to webmaster@mysql.com.

C MySQL customer usage

The section 'Some MySQL Users' contains a lot of different links to MySQL users but doesn't provide that much information about how they are using MySQL. See section B Some MySQL Users. This manual section is to give you an idea of how other MySQL users are using MySQL to solve their problems.

This manual section is very new and we plan to add more stories here shortly. If you are interested in contributing of how you use MySQL in a unique environment or have success store about how you use MySQL, you can write to docs@mysql.com with subject Success:. Note that as we are very busy it may take some time before you get some feedback for your story.

D Contributed Programs

Many users of MySQL have contributed very useful support tools and add-ons.

A list of what is available at http://www.mysql.com/Downloads/Contrib/ (or any mirror) is shown below. If you want to build MySQL support for the Perl DBI/DBD interface, you should fetch the Data-Dumper, DBI, and Msql-Mysql-modules files and install them. See section M.2 Perl Installation Comments.

D.1 APIs

D.2 Clients

D.3 Web Tools

D.4 Performance Benchmarking Tools

D.5 Authentication Tools

D.6 Converters

D.7 Using MySQL with Other Products

D.8 Useful Tools

D.9 RPMs for Common Tools (Most Are for RedHat 6.1)

D.10 Useful Functions

D.11 Windows programs

D.12 Uncategorized

E Credits

This appendix lists the developers, contributors, and supporters that have helped to make MySQL what it is today.

E.1 Developers at MySQL AB

These are the developers that are or have been employed by MySQL AB to work on MySQL, roughly in the order they started to work with us. Following each developer is a small list of the tasks that the developer is responsible for, or the accomplishments they have made.

Michael (Monty) Widenius
Has written the following parts of MySQL:
David Axmark
Jani Tolonen
Sinisa Milivojevic
Tonu Samuel
Sasha Pachev
Matt Wagner
Miguel Solorzano
Timothy Smith
Sergei Golubchik
Jeremy Cole
John Dean
Indrek Siitan

The following non-developers are also working in/with MySQL AB:

Hans Kierkegaard - Responsible for MySQL license handling.
Antti Halonen - Sales manager.
Jonas Norrman - Handles licensing questions sent to info@mysql.com.
Erik Granberg - Handles MySQL partners (and a lot of other stuff).
Allan Larsson (The BOSS for TCX DataKonsult AB).

E.2 Contributors to MySQL

Contributors to the MySQL distribution are listed here, in somewhat random order:

Paul DuBois
Help with making the Reference Manual correct and understandable. That includes rewriting Monty's and David's attempts at English into English as other people know it.
Gianmassimo Vigazzola qwerg@mbox.vol.it or qwerg@tin.it
The initial port to Win32/NT.
Kim Aldale
Helped to rewrite Monty's and David's early attempts at English into English.
Per Eric Olsson
For more or less constructive criticism and real testing of the dynamic record format.
Irena Pancirov irena@mail.yacc.it
Win32 port with Borland compiler. mysqlshutdown.exe and mysqlwatch.exe
David J. Hughes
For the effort to make a shareware SQL database. We at TcX started with mSQL, but found that it couldn't satisfy our purposes so instead we wrote a SQL interface to our application builder Unireg. mysqladmin and mysql are programs that were largely influenced by their mSQL counterparts. We have put a lot of effort into making the MySQL syntax a superset of mSQL. Many of the API's ideas are borrowed from mSQL to make it easy to port free mSQL programs to MySQL. MySQL doesn't contain any code from mSQL. Two files in the distribution (`client/insert_test.c' and `client/select_test.c') are based on the corresponding (non-copyrighted) files in the mSQL distribution, but are modified as examples showing the changes necessary to convert code from mSQL to MySQL. (mSQL is copyrighted David J. Hughes.)
Fred Fish
For his excellent C debugging and trace library. Monty has made a number of smaller improvements to the library (speed and additional options).
Richard A. O'Keefe
For his public domain string library.
Henry Spencer
For his regex library, used in WHERE column REGEXP regexp.
Free Software Foundation
From whom we got an excellent compiler (gcc), the libc library (from which we have borrowed `strto.c' to get some code working in Linux), and the readline library (for the mysql client).
Free Software Foundation & The XEmacs development team
For a really great editor/environment used by almost everybody at TcX/MySQL AB/detron.
Patrick Lynch
For helping us acquire http://www.mysql.com/.
Fred Lindberg
For setting up qmail to handle the MySQL mailing list and for the incredible help we got in managing the MySQL mailing lists.
Igor Romanenko igor@frog.kiev.ua
mysqldump (previously msqldump, but ported and enhanced by Monty).
Yuri Dario
For keeping up and extending the MySQL OS/2 port.
Tim Bunce, Alligator Descartes
For the DBD (Perl) interface.
Tim Bunce
Author of mysqlhotcopy.
Andreas Koenig a.koenig@mind.de
For the Perl interface to MySQL.
Eugene Chan eugene@acenet.com.sg
For porting PHP to MySQL.
Michael J. Miller Jr. mke@terrapin.turbolift.com
For the first MySQL manual. And a lot of spelling/language fixes for the FAQ (that turned into the MySQL manual a long time ago).
Yan Cailin
First translator of the MySQL Reference Manual into simplified chinese in early 2000 on which the Big5 and HK coded ( mysql.hitstar.com) versions were based. Personal home page at linuxdb.yeah.net.
Giovanni Maruzzelli maruzz@matrice.it
For porting iODBC (Unix ODBC).
Chris Provenzano
Portable user level pthreads. From the copyright: This product includes software developed by Chris Provenzano, the University of California, Berkeley, and contributors. We are currently using version 1_60_beta6 patched by Monty (see `mit-pthreads/Changes-mysql').
Xavier Leroy Xavier.Leroy@inria.fr
The author of LinuxThreads (used by MySQL on Linux).
Zarko Mocnik zarko.mocnik@dem.si
Sorting for Slovenian language and the `cset.tar.gz' module that makes it easier to add other character sets.
"TAMITO" tommy@valley.ne.jp
The _MB character set macros and the ujis and sjis character sets.
Joshua Chamas joshua@chamas.com
Base for concurrent insert, extended date syntax, debugging on NT, and answering on the MySQL mailing list.
Yves Carlier Yves.Carlier@rug.ac.be
mysqlaccess, a program to show the access rights for a user.
Rhys Jones rhys@wales.com (And GWE Technologies Limited)
For the JDBC, a module to extract data from MySQL with a Java client.
Dr Xiaokun Kelvin ZHU X.Zhu@brad.ac.uk
Further development of the JDBC driver and other MySQL-related Java tools.
James Cooper pixel@organic.com
For setting up a searchable mailing list archive at his site.
Rick Mehalick Rick_Mehalick@i-o.com
For xmysql, a graphical X client for MySQL.
Doug Sisk sisk@wix.com
For providing RPM packages of MySQL for RedHat Linux.
Diemand Alexander V. axeld@vial.ethz.ch
For providing RPM packages of MySQL for RedHat Linux-Alpha.
Antoni Pamies Olive toni@readysoft.es
For providing RPM versions of a lot of MySQL clients for Intel and SPARC.
Jay Bloodworth jay@pathways.sde.state.sc.us
For providing RPM versions for MySQL Version 3.21.
Jochen Wiedmann wiedmann@neckar-alb.de
For maintaining the Perl DBD::mysql module.
Therrien Gilbert gilbert@ican.net, Jean-Marc Pouyot jmp@scalaire.fr
French error messages.
Petr snajdr, snajdr@pvt.net
Czech error messages.
Jaroslaw Lewandowski jotel@itnet.com.pl
Polish error messages.
Miguel Angel Fernandez Roiz
Spanish error messages.
Roy-Magne Mo rmo@www.hivolda.no
Norwegian error messages and testing of Version 3.21.#.
Timur I. Bakeyev root@timur.tatarstan.ru
Russian error messages.
brenno@dewinter.com && Filippo Grassilli phil@hyppo.com
Italian error messages.
Dirk Munzinger dirk@trinity.saar.de
German error messages.
Billik Stefan billik@sun.uniag.sk
Slovak error messages.
Stefan Saroiu tzoompy@cs.washington.edu
Romanian error messages.
Peter Feher
Hungarian error messages.
Roberto M. Serqueira
Portugise error messages.
Carsten H. Pedersen
Danish error messages
David Sacerdote davids@secnet.com
Ideas for secure checking of DNS hostnames.
Wei-Jou Chen jou@nematic.ieo.nctu.edu.tw
Some support for Chinese(BIG5) characters.
Wei He hewei@mail.ied.ac.cn
A lot of functionality for the Chinese(GBK) character set.
Zeev Suraski bourbon@netvision.net.il
FROM_UNIXTIME() time formatting, ENCRYPT() functions, and bison advisor. Active mailing list member.
Luuk de Boer luuk@wxs.nl
Ported (and extended) the benchmark suite to DBI/DBD. Have been of great help with crash-me and running benchmarks. Some new date functions. The mysql_setpermissions script.
Jay Flaherty fty@mediapulse.com
Big parts of the Perl DBI/DBD section in the manual.
Paul Southworth pauls@etext.org, Ray Loyzaga yar@cs.su.oz.au
Proof-reading of the Reference Manual.
Alexis Mikhailov root@medinf.chuvashia.su
User-definable functions (UDFs); CREATE FUNCTION and DROP FUNCTION.
Andreas F. Bobak bobak@relog.ch
The AGGREGATE extension to UDF functions.
Ross Wakelin R.Wakelin@march.co.uk
Help to set up InstallShield for MySQL-Win32.
Jethro Wright III jetman@li.net
The `libmysql.dll' library.
James Pereria jpereira@iafrica.com
Mysqlmanager, a Win32 GUI tool for administrating MySQL.
Curt Sampson cjs@portal.ca
Porting of MIT-pthreads to NetBSD/Alpha and NetBSD 1.3/i386.
Antony T. Curtis antony.curtis@olcs.net
Porting of MySQL to OS/2.
Martin Ramsch m.ramsch@computer.org
Examples in the MySQL Tutorial.
Steve Harvey
For making mysqlaccess more secure.
Konark IA-64 Centre of Persistent Systems Private Limited
http://www.pspl.co.in/konark/. Help with the Win64 port of the MySQL server.
Albert Chin-A-Young.
Configure updates for Tru64, large file support and better TCP wrappers support.
John Birrell
Emulation of pthread_mutex() for OS/2.

Other contributors, bugfinders, and testers: James H. Thompson, Maurizio Menghini, Wojciech Tryc, Luca Berra, Zarko Mocnik, Wim Bonis, Elmar Haneke, jehamby@lightside, psmith@BayNetworks.com, duane@connect.com.au, Ted Deppner ted@psyber.com, Mike Simons, Jaakko Hyvatti.

And lots of bug report/patches from the folks on the mailing list.

A big tribute goes to those that help us answer questions on the mysql@lists.mysql.com mailing list:

Daniel Koch dkoch@amcity.com
Irix setup.
Luuk de Boer luuk@wxs.nl
Benchmark questions.
Tim Sailer tps@users.buoy.com
DBD-mysql questions.
Boyd Lynn Gerber gerberb@zenez.com
SCO-related questions.
Richard Mehalick RM186061@shellus.com
xmysql-related questions and basic installation questions.
Zeev Suraski bourbon@netvision.net.il
Apache module configuration questions (log & auth), PHP-related questions, SQL syntax-related questions and other general questions.
Francesc Guasch frankie@citel.upc.es
General questions.
Jonathan J Smith jsmith@wtp.net
Questions pertaining to OS-specifics with Linux, SQL syntax, and other things that might need some work.
David Sklar sklar@student.net
Using MySQL from PHP and Perl.
Alistair MacDonald A.MacDonald@uel.ac.uk
Not yet specified, but is flexible and can handle Linux and maybe HP-UX. Will try to get user to use mysqlbug.
John Lyon jlyon@imag.net
Questions about installing MySQL on Linux systems, using either `.rpm' files or compiling from source.
Lorvid Ltd. lorvid@WOLFENET.com
Simple billing/license/support/copyright issues.
Patrick Sherrill patrick@coconet.com
ODBC and VisualC++ interface questions.
Randy Harmon rjharmon@uptimecomputers.com
DBD, Linux, some SQL syntax questions.

E.3 Supporters to MySQL

The following companies has helped us finance development of MySQL by either paying us for developing a new feature, developed a MySQL feature themselves or by giving us hardware for MySQL development.

VA Linux / Andover.net
Funded replication.
NuSphere
Editing of the MySQL manual.
Stork Design studio
The MySQL web site in use between 1998-2000.
Intel
Contributed to development on Windows and Linux platforms.
Compaq
Contributed to Development on Linux/Alpha.
SWSoft
Development on the embedded mysqld version.
FutureQuest
--skip-show-variables

F MySQL change history

This appendix lists the changes from version to version in the MySQL source code.

Note that we tend to update the manual at the same time we make changes to MySQL. If you find a version listed below that you can't find on the MySQL download page, this means that the version has not yet been released!

F.1 Changes in release 4.0.x (Development; Alpha)

We have now started to work on MySQL 4.0. We will update this section as we add new features, so that others can follow our development.

Our TODO section contains what we plan to have in 4.0. See section 1.6.1 Things that should be in 4.0.

F.1.1 Changes in release 4.0.0

F.2 Changes in release 3.23.x (Stable)

The 3.23 release has several major features that are not present in previous versions. We have added three new table types:

MyISAM
A new ISAM library which is tuned for SQL and supports large files.
BerkeleyDB or BDB
Uses the Berkeley DB library from Sleepycat Software to implement transaction-safe tables.
InnoDB
A transaction-safe table handler that supports row level locking, and many Oracle-like features.

Note that only MyISAM is available in the standard binary distribution.

The 3.23 release also includes support for database replication between a master and many slaves, full-text indexing, and much more.

All new features are being developed in the 4.0 version. Only bug fixes and minor enhancements to existing features will be added to 3.23.

The replication code and BerkeleyDB code is still not as tested and as the rest of the code, so we will probably need to do a couple of future releases of 3.23 with small fixes for this part of the code. As long as you don't use these features, you should be quite safe with MySQL 3.23!

Note that the above doesn't mean that replication or Berkeley DB don't work; We have done a lot of testing of all code, including replication and BDB without finding any problems. It only means that not as many users use this code as the rest of the code and because of this we are not yet 100% confident in this code.

F.2.1 Changes in release 3.23.48

F.2.2 Changes in release 3.23.47

F.2.3 Changes in release 3.23.46

F.2.4 Changes in release 3.23.45

F.2.5 Changes in release 3.23.44

The following changes are for InnoDB tables:

F.2.6 Changes in release 3.23.43

F.2.7 Changes in release 3.23.42

F.2.8 Changes in release 3.23.41

F.2.9 Changes in release 3.23.40

F.2.10 Changes in release 3.23.39

F.2.11 Changes in release 3.23.38

F.2.12 Changes in release 3.23.37

F.2.13 Changes in release 3.23.36

F.2.14 Changes in release 3.23.35

F.2.15 Changes in release 3.23.34a

F.2.16 Changes in release 3.23.34

F.2.17 Changes in release 3.23.33

F.2.18 Changes in release 3.23.32

F.2.19 Changes in release 3.23.31

F.2.20 Changes in release 3.23.30

F.2.21 Changes in release 3.23.29

F.2.22 Changes in release 3.23.28

F.2.23 Changes in release 3.23.27

F.2.24 Changes in release 3.23.26

F.2.25 Changes in release 3.23.25

F.2.26 Changes in release 3.23.24

F.2.27 Changes in release 3.23.23

F.2.28 Changes in release 3.23.22

F.2.29 Changes in release 3.23.21

F.2.30 Changes in release 3.23.20

F.2.31 Changes in release 3.23.19

F.2.32 Changes in release 3.23.18

F.2.33 Changes in release 3.23.17

F.2.34 Changes in release 3.23.16

F.2.35 Changes in release 3.23.15

F.2.36 Changes in release 3.23.14

F.2.37 Changes in release 3.23.13

F.2.38 Changes in release 3.23.12

F.2.39 Changes in release 3.23.11

F.2.40 Changes in release 3.23.10

F.2.41 Changes in release 3.23.9

F.2.42 Changes in release 3.23.8

F.2.43 Changes in release 3.23.7

F.2.44 Changes in release 3.23.6

F.2.45 Changes in release 3.23.5

F.2.46 Changes in release 3.23.4

F.2.47 Changes in release 3.23.3

F.2.48 Changes in release 3.23.2

F.2.49 Changes in release 3.23.1

F.2.50 Changes in release 3.23.0

F.3 Changes in release 3.22.x (Older; Still supported)

The 3.22 version has faster and safer connect code than version 3.21, as well as a lot of new nice enhancements. The reason for not including these changes As there aren't really any MAJOR changes, upgrading from 3.21 to 3.22 should be very easy and painless. See section 2.5.2 Upgrading from Version 3.21 to Version 3.22.

F.3.1 Changes in release 3.22.35

F.3.2 Changes in release 3.22.34

F.3.3 Changes in release 3.22.33

F.3.4 Changes in release 3.22.32

F.3.5 Changes in release 3.22.31

F.3.6 Changes in release 3.22.30

F.3.7 Changes in release 3.22.29

F.3.8 Changes in release 3.22.28

F.3.9 Changes in release 3.22.27

F.3.10 Changes in release 3.22.26

F.3.11 Changes in release 3.22.25

F.3.12 Changes in release 3.22.24

F.3.13 Changes in release 3.22.23

F.3.14 Changes in release 3.22.22

F.3.15 Changes in release 3.22.21

F.3.16 Changes in release 3.22.20

F.3.17 Changes in release 3.22.19

F.3.18 Changes in release 3.22.18

F.3.19 Changes in release 3.22.17

F.3.20 Changes in release 3.22.16

F.3.21 Changes in release 3.22.15

F.3.22 Changes in release 3.22.14

F.3.23 Changes in release 3.22.13

F.3.24 Changes in release 3.22.12

F.3.25 Changes in release 3.22.11

F.3.26 Changes in release 3.22.10

F.3.27 Changes in release 3.22.9

F.3.28 Changes in release 3.22.8

F.3.29 Changes in release 3.22.7

F.3.30 Changes in release 3.22.6

F.3.31 Changes in release 3.22.5

F.3.32 Changes in release 3.22.4

F.3.33 Changes in release 3.22.3

F.3.34 Changes in release 3.22.2

F.3.35 Changes in release 3.22.1

F.3.36 Changes in release 3.22.0

F.4 Changes in release 3.21.x

Version 3.21 is quite old now, and should be avoided if possible. This information is kept here for historical purposes only.

F.4.1 Changes in release 3.21.33

F.4.2 Changes in release 3.21.32

F.4.3 Changes in release 3.21.31

F.4.4 Changes in release 3.21.30

F.4.5 Changes in release 3.21.29

F.4.6 Changes in release 3.21.28

F.4.7 Changes in release 3.21.27

F.4.8 Changes in release 3.21.26

F.4.9 Changes in release 3.21.25

F.4.10 Changes in release 3.21.24

F.4.11 Changes in release 3.21.23

F.4.12 Changes in release 3.21.22

F.4.13 Changes in release 3.21.21a

F.4.14 Changes in release 3.21.21

F.4.15 Changes in release 3.21.20

F.4.16 Changes in release 3.21.19

F.4.17 Changes in release 3.21.18

F.4.18 Changes in release 3.21.17

F.4.19 Changes in release 3.21.16

F.4.20 Changes in release 3.21.15

F.4.21 Changes in release 3.21.14b

F.4.22 Changes in release 3.21.14a

F.4.23 Changes in release 3.21.13

F.4.24 Changes in release 3.21.12

F.4.25 Changes in release 3.21.11

F.4.26 Changes in release 3.21.10

F.4.27 Changes in release 3.21.9

F.4.28 Changes in release 3.21.8

F.4.29 Changes in release 3.21.7

F.4.30 Changes in release 3.21.6

F.4.31 Changes in release 3.21.5

F.4.32 Changes in release 3.21.4

F.4.33 Changes in release 3.21.3

F.4.34 Changes in release 3.21.2

F.4.35 Changes in release 3.21.0

F.5 Changes in release 3.20.x

Version 3.20 is quite old now, and should be avoided if possible. This information is kept here for historical purposes only.

Changes from 3.20.18 to 3.20.32b are not documented here because the 3.21 release branched here. And the relevant changes are also documented as changes to the 3.21 version.

F.5.1 Changes in release 3.20.18

F.5.2 Changes in release 3.20.17

F.5.3 Changes in release 3.20.16

F.5.4 Changes in release 3.20.15

F.5.5 Changes in release 3.20.14

F.5.6 Changes in release 3.20.13

F.5.7 Changes in release 3.20.11

F.5.8 Changes in release 3.20.10

F.5.9 Changes in release 3.20.9

F.5.10 Changes in release 3.20.8

F.5.11 Changes in release 3.20.7

F.5.12 Changes in release 3.20.6

F.5.13 Changes in release 3.20.3

F.5.14 Changes in release 3.20.0

F.6 Changes in release 3.19.x

Version 3.19 is quite old now, and should be avoided if possible. This information is kept here for historical purposes only.

F.6.1 Changes in release 3.19.5

F.6.2 Changes in release 3.19.4

F.6.3 Changes in release 3.19.3

G Comments on porting to other systems

A working Posix thread library is needed for the server. On Solaris 2.5 we use Sun PThreads (the native thread support in 2.4 and earlier versions are not good enough) and on Linux we use LinuxThreads by Xavier Leroy, Xavier.Leroy@inria.fr.

The hard part of porting to a new Unix variant without good native thread support is probably to port MIT-pthreads. See `mit-pthreads/README' and Programming POSIX Threads.

The MySQL distribution includes a patched version of Provenzano's Pthreads from MIT (see MIT Pthreads web page). This can be used for some operating systems that do not have POSIX threads.

It is also possible to use another user level thread package named FSU Pthreads (see FSU Pthreads home page). This implementation is being used for the SCO port.

See the `thr_lock.c' and `thr_alarm.c' programs in the `mysys' directory for some tests/examples of these problems.

Both the server and the client need a working C++ compiler (we use gcc and have tried SparcWorks). Another compiler that is known to work is the Irix cc.

To compile only the client use ./configure --without-server.

There is currently no support for only compiling the server, nor is it likly to be added unless someone has a good reason for it.

If you want/need to change any `Makefile' or the configure script you must get Automake and Autoconf. We have used the automake-1.2 and autoconf-2.12 distributions.

All steps needed to remake everything from the most basic files. 

/bin/rm */.deps/*.P
/bin/rm -f config.cache
aclocal
autoheader
aclocal
automake
autoconf
./configure --with-debug=full --prefix='your installation directory'

# The makefiles generated above need GNU make 3.75 or newer.
# (called gmake below)
gmake clean all install init-db

If you run into problems with a new port, you may have to do some debugging of MySQL! See section G.1 Debugging a MySQL server.

NOTE: Before you start debugging mysqld, first get the test programs mysys/thr_alarm and mysys/thr_lock to work. This will ensure that your thread installation has even a remote chance to work!

G.1 Debugging a MySQL server

If you are using some functionality that is very new in MySQL, you can try to run mysqld with the --skip-new (which will disable all new, potentially unsafe functionality) or with --safe-mode which disables a lot of optimization that may cause problems. See section A.4.1 What To Do If MySQL Keeps Crashing.

If mysqld doesn't want to start, you should check that you don't have any my.cnf files that interfere with your setup! You can check your my.cnf arguments with mysqld --print-defaults and avoid using them by starting with mysqld --no-defaults ....

If mysqld starts to eat up CPU or memory or if it ``hangs'', you can use mysqladmin processlist status to find out if someone is executing a query that takes a long time. It may be a good idea to run mysqladmin -i10 processlist status in some window if you are experiencing performance problems or problems when new clients can't connect.

The command mysqladmin debug will dump some information about locks in use, used memory and query usage to the mysql log file. This may help solve some problems. This command also provides some useful information even if you haven't compiled MySQL for debugging!

If the problem is that some tables are getting slower and slower you should try to optimize the table with OPTIMIZE TABLE or myisamchk. See section 4 MySQL Database Administration. You should also check the slow queries with EXPLAIN.

You should also read the OS-specific section in this manual for problems that may be unique to your environment. See section 2.6 Operating System Specific Notes.

G.1.1 Compiling MYSQL for debugging.

If you have some very specific problem, you can always try to debug MySQL. To do this you must configure MySQL with the --with-debug or the --with-debug=full option. You can check whether or not MySQL was compiled with debugging by doing: mysqld --help. If the --debug flag is listed with the options then you have debugging enabled. mysqladmin ver also lists the mysqld version as mysql ... --debug in this case.

If you are using gcc or egcs, the recommended configure line is: 

CC=gcc CFLAGS="-O2" CXX=gcc CXXFLAGS="-O2 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-debug --with-extra-charsets=complex

This will avoid problems with the libstdc++ library and with C++ exceptions (many compilers have problems with C++ exceptions in threaded code) and compile a MySQL version with support for all character sets.

If you suspect a memory overrun error, you can configure MySQL with --with-debug=full, which will install a memory allocation (SAFEMALLOC) checker. Running with SAFEMALLOC is however quite slow, so if you get performance problems you should start mysqld with the --skip-safemalloc option. This will disable the memory overrun checks for each call to malloc and free.

If mysqld stops crashing when you compile it with --with-debug, you have probably found a compiler bug or a timing bug within MySQL. In this case you can try to add -g to the CFLAGS and CXXFLAGS variables above and not use --with-debug. If mysqld now dies, you can at least attach to it with gdb or use gdb on the core file to find out what happened.

When you configure MySQL for debugging you automatically enable a lot of extra safety check functions that monitor the health of mysqld. If they find something ``unexpected,'' an entry will be written to stderr, which safe_mysqld directs to the error log! This also means that if you are having some unexpected problems with MySQL and are using a source distribution, the first thing you should do is to configure MySQL for debugging! (The second thing, of course, is to send mail to mysql@lists.mysql.com and ask for help. Please use the mysqlbug script for all bug reports or questions regarding the MySQL version you are using!

In the Windows MySQL distribution, mysqld.exe is by default compiled with support for trace files.

G.1.2 Creating trace files

If the mysqld server doesn't start or if you can cause the mysqld server to crash quickly, you can try to create a trace file to find the problem.

To do this you have to have a mysqld that is compiled for debugging. You can check this by executing mysqld -V. If the version number ends with -debug, it's compiled with support for trace files.

Start the mysqld server with a trace log in `/tmp/mysqld.trace' (or `C:\mysqld.trace' on Windows):

mysqld --debug

On Windows you should also use the --standalone flag to not start mysqld as a service:

In a DOS window do: 

mysqld --debug --standalone

After this you can use the mysql.exe command line tool in a second DOS window to reproduce the problem. You can take down the above mysqld server with mysqladmin shutdown.

Note that the trace file will get very BIG! If you want to have a smaller trace file, you can use something like:

mysqld --debug=d,info,error,query,general,where:O,/tmp/mysqld.trace

which only prints information with the most interesting tags in `/tmp/mysqld.trace'.

If you make a bug report about this, please only send the lines from the trace file to the appropriate mailing list where something seems to go wrong! If you can't locate the wrong place, you can ftp the trace file, together with a full bug report, to ftp://support.mysql.com/pub/mysql/secret so that a MySQL developer can take a look a this.

The trace file is made with the DBUG package by Fred Fish. See section G.3 The DBUG package..

G.1.3 Debugging mysqld under gdb

On most system you can also start mysqld from gdb to get more information if mysqld crashes.

With some older gdb versions on Linux you must use run --one-thread if you want to be able to debug mysqld threads. In this case you can only have one thread active at a time.

When running mysqld under gdb, you should disable the stack trace with --skip-stack-trace to be able to catch segfaults within gdb.

It's very hard to debug MySQL under gdb if you do a lot of new connections the whole time as gdb doesn't free the memory for old threads. You can avoid this problem by starting mysqld with -O thread_cache_size= 'max_connections +1'. In most cases just using -O thread_cache_size=5' will help a lot!

If you want to get a core dump on Linux if mysqld dies with a SIGSEGV signal, you can start mysqld with the --core-file option. This core file can be used to make a backtrace that may help you find out why mysqld died: 

shell> gdb mysqld core
gdb>   backtrace full
gdb>   exit

See section A.4.1 What To Do If MySQL Keeps Crashing.

If you are using gdb 4.17.x or above on Linux, you should install a `.gdb' file, with the following information, in your current directory: 

set print sevenbit off
handle SIGUSR1 nostop noprint
handle SIGUSR2 nostop noprint
handle SIGWAITING nostop noprint
handle SIGLWP nostop noprint
handle SIGPIPE nostop
handle SIGALRM nostop
handle SIGHUP nostop
handle SIGTERM nostop noprint

If you have problems debugging threads with gdb, you should download gdb 5.x and try this instead. The new gdb version has very improved thread handling!

Here is an example how to debug mysqld: 

shell> gdb /usr/local/libexec/mysqld
gdb> run
...
backtrace full # Do this when mysqld crashes

Include the above output in a mail generated with mysqlbug and mail this to mysql@lists.mysql.com.

If mysqld hangs you can try to use some system tools like strace or /usr/proc/bin/pstack to examine where mysqld has hung. 

strace /tmp/log libexec/mysqld

If you are using the Perl DBI interface, you can turn on debugging information by using the trace method or by setting the DBI_TRACE environment variable. See section 8.2.2 The DBI Interface.

G.1.4 Using a stack trace

On some operating systems, the error log will contain a stack trace if mysqld dies unexpectedly. You can use this to find out where (and maybe why) mysqld died. See section 4.9.1 The Error Log. To get a stack trace, you should NOT compile mysqld with the -fomit-frame-pointer option to gcc. See section G.1.1 Compiling MYSQL for debugging..

If the error file contains something like the following: 

mysqld got signal 11;
The manual section 'Debugging a MySQL server' tells you how to use a
stack trace and/or the core file to produce a readable backtrace that may
help in finding out why mysqld died
Attemping backtrace. You can use the following information to find out
where mysqld died.  If you see no messages after this, something went
terribly wrong
stack range sanity check, ok, backtrace follows
0x40077552
0x81281a0
0x8128f47
0x8127be0
0x8127995
0x8104947
0x80ff28f
0x810131b
0x80ee4bc
0x80c3c91
0x80c6b43
0x80c1fd9
0x80c1686

you can find where mysqld died by doing the following:

  1. Copy the above numbers to a file, for example `mysqld.stack'.
  2. Make a symbol file for the mysqld server: 
    nm -n libexec/mysqld > /tmp/mysqld.sym
    Note that many MySQL binary distributions comes with the above file, named mysqld.sym.gz. In this case you must unpack this by doing: 
    gunzip < bin/mysqld.sym.gz > /tmp/mysqld.sym
  3. Execute resolve_stack_dump -s /tmp/mysqld.sym -n mysqld.stack. This will print out where mysqld died. If this doesn't help you find out why mysqld died, you should make a bug report and include the output from the above commend with the bug report. Note however that in most cases it will not help us to just have a stack trace to find the reason for the problem. To be able to locate the bug or provide a workaround, we would in most cases need to know the query that killed mysqld and preferable a test case so that we can repeat the problem! See section 1.2.22.3 How to Report Bugs or Problems.

G.1.5 Using log files to find cause of errors in mysqld

Note that before starting mysqld with --log you should check all your tables with myisamchk. See section 4 MySQL Database Administration.

If mysqld dies or hangs, you should start mysqld with --log. When mysqld dies again, you can examine the end of the log file for the query that killed mysqld.

If you are using --log without a file name, the log is stored in the database directory as 'hostname'.log In most cases it's the last query in the log file that killed mysqld, but if possible you should verify this by restarting mysqld and executing the found query from the mysql command line tools. If this works, you should also test all complicated queries that didn't complete.

You can also try the command EXPLAIN on all SELECT statements that takes a long time to ensure that mysqld is using indexes properly. See section 5.2.1 EXPLAIN Syntax (Get Information About a SELECT).

You can find the queries that take a long time to execute by starting mysqld with --log-slow-queries. See section 4.9.5 The Slow Query Log.

If you find the text mysqld restarted in the error log file (normally named `hostname.err') you have probably found a query that causes mysqld to fail. If this happens you should check all your tables with myisamchk (see section 4 MySQL Database Administration), and test the queries in the MySQL log files to see if one doesn't work. If you find such a query, try first upgrading to the newest MySQL version. If this doesn't help and you can't find anything in the mysql mail archive, you should report the bug to mysql@lists.mysql.com. Links to mail archives are available online at the MySQL documentation page.

If you have started mysqld with --with-myisam-recover, MySQL will automatically check and try to repair MyISAM tables if they are marked as 'not closed properly' or 'crashed'. If this happens, MySQL will write an entry in the hostname.err file 'Warning: Checking table ...' which is followed by Warning: Repairing table if the table needs to be repaired. If you get a lot of these errors, without mysqld having died unexpectedly just before, then something is wrong and needs to be investigated further. See section 4.1.1 mysqld Command-line Options.

It's of course not a good sign if mysqld did died unexpectedly, but in this case one shouldn't investigate the Checking table... messages but instead try to find out why mysqld died.

G.1.6 Making a test case when you experience table corruption

If you get corrupted tables or if mysqld always fails after some update commands, you can test if this bug is reproducible by doing the following:

You can also use the script mysql_find_rows to just execute some of the update statements if you want to narrow down the problem.

G.2 Debugging a MySQL client

To be able to debug a MySQL client with the integrated debug package, you should configure MySQL with --with-debug. See section 2.3.3 Typical configure Options.

Before running a client, you should set the MYSQL_DEBUG environment variable: 

shell> MYSQL_DEBUG=d:t:O,/tmp/client.trace
shell> export MYSQL_DEBUG

This causes clients to generate a trace file in `/tmp/client.trace'.

If you have problems with your own client code, you should attempt to connect to the server and run your query using a client that is known to work. Do this by running mysql in debugging mode (assuming you have compiled MySQL with debugging on): 

shell> mysql --debug=d:t:O,/tmp/client.trace

This will provide useful information in case you mail a bug report. See section 1.2.22.3 How to Report Bugs or Problems.

If your client crashes at some 'legal' looking code, you should check that your `mysql.h' include file matches your mysql library file. A very common mistake is to use an old `mysql.h' file from an old MySQL installation with new MySQL library.

G.3 The DBUG package.

The MySQL server and most MySQL clients are compiled with the DBUG package originally made by Fred Fish. When one has configured MySQL for debugging, this package makes it possible to get a trace file of what the program is debugging. See section G.1.2 Creating trace files.

One uses the debug package by invoking the program with the --debug="..." or the -#... option.

Most MySQL programs has a default debug string that will be used if you don't specify an option to --debug. The default trace file is usually /tmp/programname.trace on Unix and \programname.trace on Windows.

The debug control string is a sequence of colon separated fields as follows: 

<field_1>:<field_2>:...:<field_N>

Each field consists of a mandatory flag character followed by an optional "," and comma-separated list of modifiers: 

flag[,modifier,modifier,...,modifier]

The currently recognized flag characters are:

d Enable output from DBUG_<N> macros for the current state. May be followed by a list of keywords which selects output only for the DBUG macros with that keyword. An empty list of keywords implies output for all macros.
D Delay after each debugger output line. The argument is the number of tenths of seconds to delay, subject to machine capabilities. That is, -#D,20 is delay two seconds.
f Limit debugging and/or tracing, and profiling to the list of named functions. Note that a null list will disable all functions. The appropriate "d" or "t" flags must still be given, this flag only limits their actions if they are enabled.
F Identify the source file name for each line of debug or trace output.
i Identify the process with the pid or thread id for each line of debug or trace output.
g Enable profiling. Create a file called 'dbugmon.out' containing information that can be used to profile the program. May be followed by a list of keywords that select profiling only for the functions in that list. A null list implies that all functions are considered.
L Identify the source file line number for each line of debug or trace output.
n Print the current function nesting depth for each line of debug or trace output.
N Number each line of dbug output.
o Redirect the debugger output stream to the specified file. The default output is stderr.
O As O but the file is really flushed between each write. When needed the file is closed and reopened between each write.
p Limit debugger actions to specified processes. A process must be identified with the DBUG_PROCESS macro and match one in the list for debugger actions to occur.
P Print the current process name for each line of debug or trace output.
r When pushing a new state, do not inherit the previous state's function nesting level. Useful when the output is to start at the left margin.
S Do function _sanity(_file_,_line_) at each debugged function until _sanity() returns something that differs from 0. (Mostly used with safemalloc to find memory leaks)
t Enable function call/exit trace lines. May be followed by a list (containing only one modifier) giving a numeric maximum trace level, beyond which no output will occur for either debugging or tracing macros. The default is a compile time option.

Some examples of debug control strings which might appear on a shell command line (the "-#" is typically used to introduce a control string to an application program) are: 

-#d:t
-#d:f,main,subr1:F:L:t,20
-#d,input,output,files:n
-#d:t:i:O,\\mysqld.trace

In MySQL, common tags to print (with the d option) are: enter,exit,error,warning,info and loop.

G.4 Locking methods

Currently MySQL only supports table locking for ISAM/MyISAM and HEAP tables and page level locking for BDB tables. See section 5.3.1 How MySQL Locks Tables. With MyISAM tables one can freely mix INSERT and SELECT without locks (Versioning).

Starting in version 3.23.33, you can analyze the table lock contention on your system by checking Table_locks_waited and Table_locks_immediate environment variables.

Some database users claim that MySQL cannot support near the number of concurrent users because it lacks row-level locking. This may be true for some specific applications, but is not generally true. As always this depends totally on what the application does and what is the access/update pattern of the data.

Pros for row locking:

Cons:

Table locks are superior to page level / row level locks in the following cases:

Other options than row / page level locking:

Versioning (like we use in MySQL for concurrent inserts) where you can have one writer at the same time as many readers. This means that the database/table supports different views for the data depending on when one started to access it. Other names for this are time travel, copy on write or copy on demand.

Copy on demand is in many case much better than page or row level locking; The worst case does, however, use much more memory than when using normal locks.

Instead of using row level locks one can use application level locks. (Like get_lock/release_lock in MySQL). This works of course only in well-behaved applications.

In many cases one can do an educated guess which locking type is best for the application but generally it's very hard to say that a given lock type is better than another; Everything depends on the application and different part of the application may require different lock types.

Here are some tips about locking in MySQL:

On web application most applications do lots of selects, very few deletes, updates mainly on keys and inserts in some specific tables. The base MySQL setup is VERY tuned for this.

Concurrent users is not a problem if one doesn't mix updates and selects that needs to examine many rows in the same table.

If one mixes inserts and deletes on the same table then INSERT DELAYED may be of great help.

One can also use LOCK TABLES to speed up things (many updates within a single lock is much faster than updates without locks). Splitting thing to different tables will also helps.

If you get speed problems with the table locks in MySQL, you may be able to solve these to convert some of your tables to BDB tables. See section 7.5 BDB or Berkeley_DB Tables.

The optimization section in the manual covers a lot of different aspects of how to tune ones application. See section 5.2.11 Other Optimization Tips.

G.5 Comments about RTS threads

I have tried to use the RTS thread packages with MySQL but stumbled on the following problems:

They use an old version of a lot of POSIX calls and it is very tedious to make wrappers for all functions. I am inclined to think that it would be easier to change the thread libraries to the newest POSIX specification.

Some wrappers are already written. See `mysys/my_pthread.c' for more info.

At least the following should be changed:

pthread_get_specific should use one argument. sigwait should take two arguments. A lot of functions (at least pthread_cond_wait, pthread_cond_timedwait) should return the error code on error. Now they return -1 and set errno.

Another problem is that user-level threads use the ALRM signal and this aborts a lot of functions (read, write, open...). MySQL should do a retry on interrupt on all of these but it is not that easy to verify it.

The biggest unsolved problem is the following:

To get thread-level alarms I changed `mysys/thr_alarm.c' to wait between alarms with pthread_cond_timedwait(), but this aborts with error EINTR. I tried to debug the thread library as to why this happens, but couldn't find any easy solution.

If someone wants to try MySQL with RTS threads I suggest the following:

G.6 Differences between different thread packages

MySQL is very dependent on the thread package used. So when choosing a good platform for MySQL, the thread package is very important.

There are at least three types of thread packages:

In some systems kernel threads are managed by integrating user level threads in the system libraries. In such cases, the thread switching can only be done by the thread library and the kernel isn't really ``thread aware''.

H Environment Variables

Here is a list of all the environment variables that are used directly or indirectly by MySQL. Most of these can also be found in other places in this manual.

Note that any options on the command line will take precedence over values specified in configuration files and environment variables, and values in configuration files take precedence over values in environment variables.

In many cases it's preferable to use a configure file instead of environment variables to modify the behavior of MySQL. See section 4.1.2 my.cnf Option Files.

CCX Set this to your C++ compiler when running configure.
CC Set this to your C compiler when running configure.
CFLAGS Flags for your C compiler when running configure.
CXXFLAGS Flags for your C++ compiler when running configure.
DBI_USER The default user name for Perl DBI.
DBI_TRACE Used when tracing Perl DBI.
HOME The default path for the mysql history file is `$HOME/.mysql_history'.
LD_RUN_PATH Used to specify where your libmysqlclient.so is.
MYSQL_DEBUG Debug-trace options when debugging.
MYSQL_HISTFILE The path to the mysql history file.
MYSQL_HOST Default host name used by the mysql command-line prompt.
MYSQL_PWD The default password when connecting to mysqld. Note that use of this is insecure!
MYSQL_TCP_PORT The default TCP/IP port.
MYSQL_UNIX_PORT The default socket; used for connections to localhost.
PATH Used by the shell to finds the MySQL programs.
TMPDIR The directory where temporary tables/files are created.
TZ This should be set to your local time zone. See section A.4.6 Time Zone Problems.
UMASK_DIR The user-directory creation mask when creating directories. Note that this is ANDed with UMASK!
UMASK The user-file creation mask when creating files.
USER The default user on Windows to use when connecting to mysqld.

I Description of MySQL regular expression syntax

A regular expression (regex) is a powerful way of specifying a complex search.

MySQL uses Henry Spencer's implementation of regular expressions, which is aimed at conformance with POSIX 1003.2. MySQL uses the extended version.

This is a simplistic reference that skips the details. To get more exact information, see Henry Spencer's regex(7) manual page that is included in the source distribution. See section E Credits.

A regular expression describes a set of strings. The simplest regexp is one that has no special characters in it. For example, the regexp hello matches hello and nothing else.

Non-trivial regular expressions use certain special constructs so that they can match more than one string. For example, the regexp hello|word matches either the string hello or the string word.

As a more complex example, the regexp B[an]*s matches any of the strings Bananas, Baaaaas, Bs, and any other string starting with a B, ending with an s, and containing any number of a or n characters in between.

A regular expression may use any of the following special characters/constructs:

^
Match the beginning of a string. 
mysql> select "fo\nfo" REGEXP "^fo$";           -> 0
mysql> select "fofo" REGEXP "^fo";              -> 1
$
Match the end of a string. 
mysql> select "fo\no" REGEXP "^fo\no$";         -> 1
mysql> select "fo\no" REGEXP "^fo$";            -> 0
.
Match any character (including newline). 
mysql> select "fofo" REGEXP "^f.*";             -> 1
mysql> select "fo\nfo" REGEXP "^f.*";           -> 1
a*
Match any sequence of zero or more a characters. 
mysql> select "Ban" REGEXP "^Ba*n";             -> 1
mysql> select "Baaan" REGEXP "^Ba*n";           -> 1
mysql> select "Bn" REGEXP "^Ba*n";              -> 1
a+
Match any sequence of one or more a characters. 
mysql> select "Ban" REGEXP "^Ba+n";             -> 1
mysql> select "Bn" REGEXP "^Ba+n";              -> 0
a?
Match either zero or one a character. 
mysql> select "Bn" REGEXP "^Ba?n";              -> 1
mysql> select "Ban" REGEXP "^Ba?n";             -> 1
mysql> select "Baan" REGEXP "^Ba?n";            -> 0
de|abc
Match either of the sequences de or abc. 
mysql> select "pi" REGEXP "pi|apa";             -> 1
mysql> select "axe" REGEXP "pi|apa";            -> 0
mysql> select "apa" REGEXP "pi|apa";            -> 1
mysql> select "apa" REGEXP "^(pi|apa)$";        -> 1
mysql> select "pi" REGEXP "^(pi|apa)$";         -> 1
mysql> select "pix" REGEXP "^(pi|apa)$";        -> 0
(abc)*
Match zero or more instances of the sequence abc. 
mysql> select "pi" REGEXP "^(pi)*$";            -> 1
mysql> select "pip" REGEXP "^(pi)*$";           -> 0
mysql> select "pipi" REGEXP "^(pi)*$";          -> 1
{1}
{2,3}
The is a more general way of writing regexps that match many occurrences of the previous atom.
a*
Can be written as a{0,}.
a+
Can be written as a{1,}.
a?
Can be written as a{0,1}.
To be more precise, an atom followed by a bound containing one integer i and no comma matches a sequence of exactly i matches of the atom. An atom followed by a bound containing one integer i and a comma matches a sequence of i or more matches of the atom. An atom followed by a bound containing two integers i and j matches a sequence of i through j (inclusive) matches of the atom. Both arguments must be in the range from 0 to RE_DUP_MAX (default 255), inclusive. If there are two arguments, the second must be greater than or equal to the first.
[a-dX]
[^a-dX]
Matches any character which is (or is not, if ^ is used) either a, b, c, d or X. To include a literal ] character, it must immediately follow the opening bracket [. To include a literal - character, it must be written first or last. So [0-9] matches any decimal digit. Any character that does not have a defined meaning inside a [] pair has no special meaning and matches only itself. 
mysql> select "aXbc" REGEXP "[a-dXYZ]";         -> 1
mysql> select "aXbc" REGEXP "^[a-dXYZ]$";       -> 0
mysql> select "aXbc" REGEXP "^[a-dXYZ]+$";      -> 1
mysql> select "aXbc" REGEXP "^[^a-dXYZ]+$";     -> 0
mysql> select "gheis" REGEXP "^[^a-dXYZ]+$";    -> 1
mysql> select "gheisa" REGEXP "^[^a-dXYZ]+$";   -> 0
[[.characters.]]
The sequence of characters of that collating element. The sequence is a single element of the bracket expression's list. A bracket expression containing a multi-character collating element can thus match more than one character, for example, if the collating sequence includes a ch collating element, then the regular expression [[.ch.]]*c matches the first five characters of chchcc.
[=character_class=]
An equivalence class, standing for the sequences of characters of all collating elements equivalent to that one, including itself. For example, if o and (+) are the members of an equivalence class, then [[=o=]], [[=(+)=]], and [o(+)] are all synonymous. An equivalence class may not be an endpoint of a range.
[:character_class:]
Within a bracket expression, the name of a character class enclosed in [: and :] stands for the list of all characters belonging to that class. Standard character class names are:
alnum digit punct
alpha graph space
blank lower upper
cntrl print xdigit
These stand for the character classes defined in the ctype(3) manual page. A locale may provide others. A character class may not be used as an endpoint of a range. 
mysql> select "justalnums" REGEXP "[[:alnum:]]+";       -> 1
mysql> select "!!" REGEXP "[[:alnum:]]+";               -> 0
[[:<:]]
[[:>:]]
These match the null string at the beginning and end of a word respectively. A word is defined as a sequence of word characters which is neither preceded nor followed by word characters. A word character is an alnum character (as defined by ctype(3)) or an underscore (_). 
mysql> select "a word a" REGEXP "[[:<:]]word[[:>:]]";      -> 1
mysql> select "a xword a" REGEXP "[[:<:]]word[[:>:]]";     -> 0

mysql> select "weeknights" REGEXP "^(wee|week)(knights|nights)$"; -> 1

J What is Unireg?

Unireg is our tty interface builder, but it uses a low-level connection to our ISAM (which is used by MySQL) and because of this it is very quick. It has existed since 1979 (on Unix in C since ~1986).

Unireg has the following components:

We update most of our production databases with the Unireg interface and serve web pages through MySQL (and in some extreme cases the Unireg report generator).

Unireg takes about 3M of disk space and works on at least the following platforms: SunOS 4.x, Solaris, Linux, HP-UX, ICL Unix, DNIX, SCO and MS-DOS.

Unireg is currently only available in Swedish and Finnish.

The price tag for Unireg is 10,000 Swedish kr (about $1500 US), but this includes support. Unireg is distributed as a binary. (But all the ISAM sources can be found in MySQL). Usually we compile the binary for the customer at their site.

All new development is concentrated to MySQL.

K GNU GENERAL PUBLIC LICENSE

Version 2, June 1991 

Copyright © 1989, 1991 Free Software Foundation, Inc.
59 Temple Place - Suite 330, Boston, MA  02111-1307, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

K.1 Preamble

The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.

When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.

To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.

For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.

We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.

Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations.

Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.

The precise terms and conditions for copying, distribution and modification follow.

K.2 TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  1. This License applies to any program or other work which contains a notice placed by the copyright holder saying it may be distributed under the terms of this General Public License. The ``Program'', below, refers to any such program or work, and a ``work based on the Program'' means either the Program or any derivative work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim or with modifications and/or translated into another language. (Hereinafter, translation is included without limitation in the term ``modification''.) Each licensee is addressed as ``you''. Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.
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  9. If the distribution and/or use of the Program is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Program under this License may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded. In such case, this License incorporates the limitation as if written in the body of this License.
  10. The Free Software Foundation may publish revised and/or new versions of the General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and ``any later version'', you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation.
  11. If you wish to incorporate parts of the Program into other free programs whose distribution conditions are different, write to the author to ask for permission. For software which is copyrighted by the Free Software Foundation, write to the Free Software Foundation; we sometimes make exceptions for this. Our decision will be guided by the two goals of preserving the free status of all derivatives of our free software and of promoting the sharing and reuse of software generally.

    9.4 NO WARRANTY

  12. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
  13. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

9.5 END OF TERMS AND CONDITIONS

K.3 How to Apply These Terms to Your New Programs

If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.

To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the ``copyright'' line and a pointer to where the full notice is found. 

one line to give the program's name and a brief idea of what it does.
Copyright (C) yyyy  name of author

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.

Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this when it starts in an interactive mode: 

Gnomovision version 69, Copyright (C) 19yy name of author
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.

The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than `show w' and `show c'; they could even be mouse-clicks or menu items--whatever suits your program.

You should also get your employer (if you work as a programmer) or your school, if any, to sign a ``copyright disclaimer'' for the program, if necessary. Here is a sample; alter the names: 

Yoyodyne, Inc., hereby disclaims all copyright interest in the program
`Gnomovision' (which makes passes at compilers) written by James Hacker.

signature of Ty Coon, 1 April 1989
Ty Coon, President of Vice

This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.

L GNU LESSER GENERAL PUBLIC LICENSE

Version 2.1, February 1999 

Copyright © 1991, 1999 Free Software Foundation, Inc.
59 Temple Place -- Suite 330, Boston, MA 02111-1307, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

[This is the first released version of the Lesser GPL.  It also counts
as the successor of the GNU Library Public License, version 2, hence the
version number 2.1.]

L.1 Preamble

The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public Licenses are intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users.

This license, the Lesser General Public License, applies to some specially designated software--typically libraries--of the Free Software Foundation and other authors who decide to use it. You can use it too, but we suggest you first think carefully about whether this license or the ordinary General Public License is the better strategy to use in any particular case, based on the explanations below.

When we speak of free software, we are referring to freedom of use, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish); that you receive source code or can get it if you want it; that you can change the software and use pieces of it in new free programs; and that you are informed that you can do these things.

To protect your rights, we need to make restrictions that forbid distributors to deny you these rights or to ask you to surrender these rights. These restrictions translate to certain responsibilities for you if you distribute copies of the library or if you modify it.

For example, if you distribute copies of the library, whether gratis or for a fee, you must give the recipients all the rights that we gave you. You must make sure that they, too, receive or can get the source code. If you link other code with the library, you must provide complete object files to the recipients, so that they can relink them with the library after making changes to the library and recompiling it. And you must show them these terms so they know their rights.

We protect your rights with a two-step method: (1) we copyright the library, and (2) we offer you this license, which gives you legal permission to copy, distribute and/or modify the library.

To protect each distributor, we want to make it very clear that there is no warranty for the free library. Also, if the library is modified by someone else and passed on, the recipients should know that what they have is not the original version, so that the original author's reputation will not be affected by problems that might be introduced by others.

Finally, software patents pose a constant threat to the existence of any free program. We wish to make sure that a company cannot effectively restrict the users of a free program by obtaining a restrictive license from a patent holder. Therefore, we insist that any patent license obtained for a version of the library must be consistent with the full freedom of use specified in this license.

Most GNU software, including some libraries, is covered by the ordinary GNU General Public License. This license, the GNU Lesser General Public License, applies to certain designated libraries, and is quite different from the ordinary General Public License. We use this license for certain libraries in order to permit linking those libraries into non-free programs.

When a program is linked with a library, whether statically or using a shared library, the combination of the two is legally speaking a combined work, a derivative of the original library. The ordinary General Public License therefore permits such linking only if the entire combination fits its criteria of freedom. The Lesser General Public License permits more lax criteria for linking other code with the library.

We call this license the Lesser General Public License because it does Less to protect the user's freedom than the ordinary General Public License. It also provides other free software developers Less of an advantage over competing non-free programs. These disadvantages are the reason we use the ordinary General Public License for many libraries. However, the Lesser license provides advantages in certain special circumstances.

For example, on rare occasions, there may be a special need to encourage the widest possible use of a certain library, so that it becomes a de-facto standard. To achieve this, non-free programs must be allowed to use the library. A more frequent case is that a free library does the same job as widely used non-free libraries. In this case, there is little to gain by limiting the free library to free software only, so we use the Lesser General Public License.

In other cases, permission to use a particular library in non-free programs enables a greater number of people to use a large body of free software. For example, permission to use the GNU C Library in non-free programs enables many more people to use the whole GNU operating system, as well as its variant, the GNU/Linux operating system.

Although the Lesser General Public License is Less protective of the users' freedom, it does ensure that the user of a program that is linked with the Library has the freedom and the wherewithal to run that program using a modified version of the Library.

The precise terms and conditions for copying, distribution and modification follow. Pay close attention to the difference between a ``work based on the library'' and a ``work that uses the library''. The former contains code derived from the library, whereas the latter must be combined with the library in order to run.

L.2 TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  1. This License Agreement applies to any software library or other program which contains a notice placed by the copyright holder or other authorized party saying it may be distributed under the terms of this Lesser General Public License (also called ``this License''). Each licensee is addressed as ``you''. A ``library'' means a collection of software functions and/or data prepared so as to be conveniently linked with application programs (which use some of those functions and data) to form executables. The ``Library'', below, refers to any such software library or work which has been distributed under these terms. A ``work based on the Library'' means either the Library or any derivative work under copyright law: that is to say, a work containing the Library or a portion of it, either verbatim or with modifications and/or translated straightforwardly into another language. (Hereinafter, translation is included without limitation in the term ``modification''.) ``Source code'' for a work means the preferred form of the work for making modifications to it. For a library, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the library. Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running a program using the Library is not restricted, and output from such a program is covered only if its contents constitute a work based on the Library (independent of the use of the Library in a tool for writing it). Whether that is true depends on what the Library does and what the program that uses the Library does.
  2. You may copy and distribute verbatim copies of the Library's complete source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and distribute a copy of this License along with the Library. You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.
  3. You may modify your copy or copies of the Library or any portion of it, thus forming a work based on the Library, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions:
    1. The modified work must itself be a software library.
    2. You must cause the files modified to carry prominent notices stating that you changed the files and the date of any change.
    3. You must cause the whole of the work to be licensed at no charge to all third parties under the terms of this License.
    4. If a facility in the modified Library refers to a function or a table of data to be supplied by an application program that uses the facility, other than as an argument passed when the facility is invoked, then you must make a good faith effort to ensure that, in the event an application does not supply such function or table, the facility still operates, and performs whatever part of its purpose remains meaningful. (For example, a function in a library to compute square roots has a purpose that is entirely well-defined independent of the application. Therefore, Subsection 2d requires that any application-supplied function or table used by this function must be optional: if the application does not supply it, the square root function must still compute square roots.)
    These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Library, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Library, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it. Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Library. In addition, mere aggregation of another work not based on the Library with the Library (or with a work based on the Library) on a volume of a storage or distribution medium does not bring the other work under the scope of this License.
  4. You may opt to apply the terms of the ordinary GNU General Public License instead of this License to a given copy of the Library. To do this, you must alter all the notices that refer to this License, so that they refer to the ordinary GNU General Public License, version 2, instead of to this License. (If a newer version than version 2 of the ordinary GNU General Public License has appeared, then you can specify that version instead if you wish.) Do not make any other change in these notices. Once this change is made in a given copy, it is irreversible for that copy, so the ordinary GNU General Public License applies to all subsequent copies and derivative works made from that copy. This option is useful when you wish to copy part of the code of the Library into a program that is not a library.
  5. You may copy and distribute the Library (or a portion or derivative of it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange. If distribution of object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place satisfies the requirement to distribute the source code, even though third parties are not compelled to copy the source along with the object code.
  6. A program that contains no derivative of any portion of the Library, but is designed to work with the Library by being compiled or linked with it, is called a ``work that uses the Library''. Such a work, in isolation, is not a derivative work of the Library, and therefore falls outside the scope of this License. However, linking a ``work that uses the Library'' with the Library creates an executable that is a derivative of the Library (because it contains portions of the Library), rather than a ``work that uses the library''. The executable is therefore covered by this License. Section 6 states terms for distribution of such executables. When a ``work that uses the Library'' uses material from a header file that is part of the Library, the object code for the work may be a derivative work of the Library even though the source code is not. Whether this is true is especially significant if the work can be linked without the Library, or if the work is itself a library. The threshold for this to be true is not precisely defined by law. If such an object file uses only numerical parameters, data structure layouts and accessors, and small macros and small inline functions (ten lines or less in length), then the use of the object file is unrestricted, regardless of whether it is legally a derivative work. (Executables containing this object code plus portions of the Library will still fall under Section 6.) Otherwise, if the work is a derivative of the Library, you may distribute the object code for the work under the terms of Section 6. Any executables containing that work also fall under Section 6, whether or not they are linked directly with the Library itself.
  7. As an exception to the Sections above, you may also combine or link a ``work that uses the Library'' with the Library to produce a work containing portions of the Library, and distribute that work under terms of your choice, provided that the terms permit modification of the work for the customer's own use and reverse engineering for debugging such modifications. You must give prominent notice with each copy of the work that the Library is used in it and that the Library and its use are covered by this License. You must supply a copy of this License. If the work during execution displays copyright notices, you must include the copyright notice for the Library among them, as well as a reference directing the user to the copy of this License. Also, you must do one of these things:
    1. Accompany the work with the complete corresponding machine-readable source code for the Library including whatever changes were used in the work (which must be distributed under Sections 1 and 2 above); and, if the work is an executable linked with the Library, with the complete machine-readable ``work that uses the Library'', as object code and/or source code, so that the user can modify the Library and then relink to produce a modified executable containing the modified Library. (It is understood that the user who changes the contents of definitions files in the Library will not necessarily be able to recompile the application to use the modified definitions.)
    2. Use a suitable shared library mechanism for linking with the Library. A suitable mechanism is one that (1) uses at run time a copy of the library already present on the user's computer system, rather than copying library functions into the executable, and (2) will operate properly with a modified version of the library, if the user installs one, as long as the modified version is interface-compatible with the version that the work was made with.
    3. Accompany the work with a written offer, valid for at least three years, to give the same user the materials specified in Subsection 6a, above, for a charge no more than the cost of performing this distribution.
    4. If distribution of the work is made by offering access to copy from a designated place, offer equivalent access to copy the above specified materials from the same place.
    5. Verify that the user has already received a copy of these materials or that you have already sent this user a copy.
    For an executable, the required form of the ``work that uses the Library'' must include any data and utility programs needed for reproducing the executable from it. However, as a special exception, the materials to be distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable. It may happen that this requirement contradicts the license restrictions of other proprietary libraries that do not normally accompany the operating system. Such a contradiction means you cannot use both them and the Library together in an executable that you distribute.
  8. You may place library facilities that are a work based on the Library side-by-side in a single library together with other library facilities not covered by this License, and distribute such a combined library, provided that the separate distribution of the work based on the Library and of the other library facilities is otherwise permitted, and provided that you do these two things:
    1. Accompany the combined library with a copy of the same work based on the Library, uncombined with any other library facilities. This must be distributed under the terms of the Sections above.
    2. Give prominent notice with the combined library of the fact that part of it is a work based on the Library, and explaining where to find the accompanying uncombined form of the same work.
  9. You may not copy, modify, sublicense, link with, or distribute the Library except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, link with, or distribute the Library is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
  10. You are not required to accept this License, since you have not signed it. However, nothing else grants you permission to modify or distribute the Library or its derivative works. These actions are prohibited by law if you do not accept this License. Therefore, by modifying or distributing the Library (or any work based on the Library), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Library or works based on it.
  11. Each time you redistribute the Library (or any work based on the Library), the recipient automatically receives a license from the original licensor to copy, distribute, link with or modify the Library subject to these terms and conditions. You may not impose any further restrictions on the recipients' exercise of the rights granted herein. You are not responsible for enforcing compliance by third parties with this License.
  12. If, as a consequence of a court judgment or allegation of patent infringement or for any other reason (not limited to patent issues), conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot distribute so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not distribute the Library at all. For example, if a patent license would not permit royalty-free redistribution of the Library by all those who receive copies directly or indirectly through you, then the only way you could satisfy both it and this License would be to refrain entirely from distribution of the Library. If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply, and the section as a whole is intended to apply in other circumstances. It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system which is implemented by public license practices. Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice. This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.
  13. If the distribution and/or use of the Library is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Library under this License may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded. In such case, this License incorporates the limitation as if written in the body of this License.
  14. The Free Software Foundation may publish revised and/or new versions of the Lesser General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. Each version is given a distinguishing version number. If the Library specifies a version number of this License which applies to it and ``any later version'', you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Library does not specify a license version number, you may choose any version ever published by the Free Software Foundation.
  15. If you wish to incorporate parts of the Library into other free programs whose distribution conditions are incompatible with these, write to the author to ask for permission. For software which is copyrighted by the Free Software Foundation, write to the Free Software Foundation; we sometimes make exceptions for this. Our decision will be guided by the two goals of preserving the free status of all derivatives of our free software and of promoting the sharing and reuse of software generally.

    9.6 NO WARRANTY

  16. BECAUSE THE LIBRARY IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE LIBRARY, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE LIBRARY ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE LIBRARY IS WITH YOU. SHOULD THE LIBRARY PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
  17. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE LIBRARY AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE LIBRARY (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE LIBRARY TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

9.7 END OF TERMS AND CONDITIONS

L.3 How to Apply These Terms to Your New Libraries

If you develop a new library, and you want it to be of the greatest possible use to the public, we recommend making it free software that everyone can redistribute and change. You can do so by permitting redistribution under these terms (or, alternatively, under the terms of the ordinary General Public License).

To apply these terms, attach the following notices to the library. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the ``copyright'' line and a pointer to where the full notice is found. 

one line to give the library's name and an idea of what it does.
Copyright (C) year  name of author

This library is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or (at
your option) any later version.

This library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307,
USA.

Also add information on how to contact you by electronic and paper mail.

You should also get your employer (if you work as a programmer) or your school, if any, to sign a ``copyright disclaimer'' for the library, if necessary. Here is a sample; alter the names: 

Yoyodyne, Inc., hereby disclaims all copyright interest in the library
`Frob' (a library for tweaking knobs) written by James Random Hacker.

signature of Ty Coon, 1 April 1990
Ty Coon, President of Vice

That's all there is to it!

M Pieces of the manual in transit

M.1 Installing a MySQL Binary Distribution

You need the following tools to install a MySQL binary distribution:

An alternative installation method under Linux is to use RPM (RedHat Package Manager) distributions. See section 2.1.1 Installing MySQL on Linux.

If you run into problems, PLEASE ALWAYS USE mysqlbug when posting questions to mysql@lists.mysql.com. Even if the problem isn't a bug, mysqlbug gathers system information that will help others solve your problem. By not using mysqlbug, you lessen the likelihood of getting a solution to your problem! You will find mysqlbug in the `bin' directory after you unpack the distribution. See section 1.2.22.3 How to Report Bugs or Problems.

The basic commands you must execute to install and use a MySQL binary distribution are: 

shell> groupadd mysql
shell> useradd -g mysql mysql
shell> cd /usr/local
shell> gunzip < /path/to/mysql-VERSION-OS.tar.gz | tar xvf -
shell> ln -s mysql-VERSION-OS mysql
shell> cd mysql
shell> scripts/mysql_install_db
shell> chown -R root  /usr/local/mysql
shell> chown -R mysql /usr/local/mysql/data
shell> chgrp -R mysql /usr/local/mysql
shell> chown -R root /usr/local/mysql/bin
shell> bin/safe_mysqld --user=mysql &

You can add new users using the bin/mysql_setpermission script if you install the DBI and Msql-Mysql-modules Perl modules.

A more detailed description follows.

To install a binary distribution, follow the steps below, then proceed to section 2.4 Post-installation Setup and Testing, for post-installation setup and testing:

  1. Pick the directory under which you want to unpack the distribution, and move into it. In the example below, we unpack the distribution under `/usr/local' and create a directory `/usr/local/mysql' into which MySQL is installed. (The following instructions therefore assume you have permission to create files in `/usr/local'. If that directory is protected, you will need to perform the installation as root.)
  2. Obtain a distribution file from one of the sites listed in section 2.2.1 How to Get MySQL. MySQL binary distributions are provided as compressed tar archives and have names like `mysql-VERSION-OS.tar.gz', where VERSION is a number (for example, 3.21.15), and OS indicates the type of operating system for which the distribution is intended (for example, pc-linux-gnu-i586).
  3. If you see a binary distribution marked with the -max prefix, this means that the binary has support for transaction-safe tables and other features. See section 4.7.5 mysqld-max, An extended mysqld server. Note that all binaries are built from the same MySQL source distribution.
  4. Add a user and group for mysqld to run as: 
    shell> groupadd mysql
shell> useradd -g mysql mysql
    These commands add the mysql group and the mysql user. The syntax for useradd and groupadd may differ slightly on different versions of Unix. They may also be called adduser and addgroup. You may wish to call the user and group something else instead of mysql.
  5. Change into the intended installation directory: 
    shell> cd /usr/local
  6. Unpack the distribution and create the installation directory: 
    shell> gunzip < /path/to/mysql-VERSION-OS.tar.gz | tar xvf -
shell> ln -s mysql-VERSION-OS mysql
    The first command creates a directory named `mysql-VERSION-OS'. The second command makes a symbolic link to that directory. This lets you refer more easily to the installation directory as `/usr/local/mysql'.
  7. Change into the installation directory: 
    shell> cd mysql
    You will find several files and subdirectories in the mysql directory. The most important for installation purposes are the `bin' and `scripts' subdirectories.
    `bin'
    This directory contains client programs and the server You should add the full pathname of this directory to your PATH environment variable so that your shell finds the MySQL programs properly. See section H Environment Variables.
    `scripts'
    This directory contains the mysql_install_db script used to initialize the mysql database containing the grant tables that store the server access permissions.
  8. If you would like to use mysqlaccess and have the MySQL distribution in some non-standard place, you must change the location where mysqlaccess expects to find the mysql client. Edit the `bin/mysqlaccess' script at approximately line 18. Search for a line that looks like this: 
    $MYSQL     = '/usr/local/bin/mysql';    # path to mysql executable
    Change the path to reflect the location where mysql actually is stored on your system. If you do not do this, you will get a Broken pipe error when you run mysqlaccess.
  9. Create the MySQL grant tables (necessary only if you haven't installed MySQL before): 
    shell> scripts/mysql_install_db
    Note that MySQL versions older than Version 3.22.10 started the MySQL server when you run mysql_install_db. This is no longer true!
  10. Change ownership of binaries to root and ownership of the data directory to the user that you will run mysqld as: 
    shell> chown -R root  /usr/local/mysql
shell> chown -R mysql /usr/local/mysql/data
shell> chgrp -R mysql /usr/local/mysql
    The first command changes the owner attribute of the files to the root user, the second one changes the owner attribute of the data directory to the mysql user, and the third one changes the group attribute to the mysql group.
  11. If you want to install support for the Perl DBI/DBD interface, see section M.2 Perl Installation Comments.
  12. If you would like MySQL to start automatically when you boot your machine, you can copy support-files/mysql.server to the location where your system has its startup files. More information can be found in the support-files/mysql.server script itself and in section 2.4.3 Starting and Stopping MySQL Automatically.

After everything has been unpacked and installed, you should initialize and test your distribution.

You can start the MySQL server with the following command: 

shell> bin/safe_mysqld --user=mysql &

See section 4.7.2 safe_mysqld, the wrapper around mysqld.

See section 2.4 Post-installation Setup and Testing.

M.2 Perl Installation Comments

M.2.1 Installing Perl on Unix

Perl support for MySQL is provided by means of the DBI/DBD client interface. See section 8.2 MySQL Perl API. The Perl DBD/DBI client code requires Perl Version 5.004 or later. The interface will not work if you have an older version of Perl.

MySQL Perl support also requires that you've installed MySQL client programming support. If you installed MySQL from RPM files, client programs are in the client RPM, but client programming support is in the developer RPM. Make sure you've installed the latter RPM.

As of Version 3.22.8, Perl support is distributed separately from the main MySQL distribution. If you want to install Perl support, the files you will need can be obtained from http://www.mysql.com/Downloads/Contrib/.

The Perl distributions are provided as compressed tar archives and have names like `MODULE-VERSION.tar.gz', where MODULE is the module name and VERSION is the version number. You should get the Data-Dumper, DBI, and Msql-Mysql-modules distributions and install them in that order. The installation procedure is shown below. The example shown is for the Data-Dumper module, but the procedure is the same for all three distributions:

  1. Unpack the distribution into the current directory: 
    shell> gunzip < Data-Dumper-VERSION.tar.gz | tar xvf -
    This command creates a directory named `Data-Dumper-VERSION'.
  2. Change into the top-level directory of the unpacked distribution: 
    shell> cd Data-Dumper-VERSION
  3. Build the distribution and compile everything: 
    shell> perl Makefile.PL
shell> make
shell> make test
shell> make install

The make test command is important because it verifies that the module is working. Note that when you run that command during the Msql-Mysql-modules installation to exercise the interface code, the MySQL server must be running or the test will fail.

It is a good idea to rebuild and reinstall the Msql-Mysql-modules distribution whenever you install a new release of MySQL, particularly if you notice symptoms such as all your DBI scripts dumping core after you upgrade MySQL.

If you don't have the right to install Perl modules in the system directory or if you to install local Perl modules, the following reference may help you: 

http://www.iserver.com/support/contrib/perl5/modules.html

Look under the heading Installing New Modules that Require Locally Installed Modules.

M.2.2 Installing ActiveState Perl on Windows

To install the MySQL DBD module with ActiveState Perl on Windows, you should do the following:

The above should work at least with ActiveState Perl Version 5.6.

If you can't get the above to work, you should instead install the MyODBC driver and connect to MySQL server through ODBC: 

use DBI;
$dbh= DBI->connect("DBI:ODBC:$dsn","$user","$password") ||
  die "Got error $DBI::errstr when connecting to $dsn\n";

M.2.3 Installing the MySQL Perl Distribution on Windows

The MySQL Perl distribution contains DBI, DBD:MySQL and DBD:ODBC.

M.2.4 Problems Using the Perl DBI/DBD Interface

If Perl reports that it can't find the `../mysql/mysql.so' module, then the problem is probably that Perl can't locate the shared library `libmysqlclient.so'.

You can fix this by any of the following methods:

If you get the following errors from DBD-mysql, you are probably using gcc (or using an old binary compiled with gcc): 

/usr/bin/perl: can't resolve symbol '__moddi3'
/usr/bin/perl: can't resolve symbol '__divdi3'

Add -L/usr/lib/gcc-lib/... -lgcc to the link command when the `mysql.so' library gets built (check the output from make for `mysql.so' when you compile the Perl client). The -L option should specify the pathname of the directory where `libgcc.a' is located on your system.

Another cause of this problem may be that Perl and MySQL aren't both compiled with gcc. In this case, you can solve the mismatch by compiling both with gcc.

If you get the following error from Msql-Mysql-modules when you run the tests: 

t/00base............install_driver(mysql) failed: Can't load '../blib/arch/auto/DBD/mysql/mysql.so' for module DBD::mysql: ../blib/arch/auto/DBD/mysql/mysql.so: undefined symbol: uncompress at /usr/lib/perl5/5.00503/i586-linux/DynaLoader.pm line 169.

it means that you need to include the compression library, -lz, to the link line. This can be doing the following change in the file `lib/DBD/mysql/Install.pm': 

$sysliblist .= " -lm";

to

$sysliblist .= " -lm -lz";

After this, you MUST run 'make realclean' and then proceed with the installation from the beginning.

If you want to use the Perl module on a system that doesn't support dynamic linking (like SCO) you can generate a static version of Perl that includes DBI and DBD-mysql. The way this works is that you generate a version of Perl with the DBI code linked in and install it on top of your current Perl. Then you use that to build a version of Perl that additionally has the DBD code linked in, and install that.

On SCO, you must have the following environment variables set: 

shell> LD_LIBRARY_PATH=/lib:/usr/lib:/usr/local/lib:/usr/progressive/lib
or
shell> LD_LIBRARY_PATH=/usr/lib:/lib:/usr/local/lib:/usr/ccs/lib:/usr/progressive/lib:/usr/skunk/lib
shell> LIBPATH=/usr/lib:/lib:/usr/local/lib:/usr/ccs/lib:/usr/progressive/lib:/usr/skunk/lib
shell> MANPATH=scohelp:/usr/man:/usr/local1/man:/usr/local/man:/usr/skunk/man:

First, create a Perl that includes a statically linked DBI by running these commands in the directory where your DBI distribution is located: 

shell> perl Makefile.PL -static -config
shell> make
shell> make install
shell> make perl

Then you must install the new Perl. The output of make perl will indicate the exact make command you will need to execute to perform the installation. On SCO, this is make -f Makefile.aperl inst_perl MAP_TARGET=perl.

Next, use the just-created Perl to create another Perl that also includes a statically-linked DBD::mysql by running these commands in the directory where your Msql-Mysql-modules distribution is located: 

shell> perl Makefile.PL -static -config
shell> make
shell> make install
shell> make perl

Finally, you should install this new Perl. Again, the output of make perl indicates the command to use.

M.3 Functions for Use with GROUP BY Clauses

If you use a group function in a statement containing no GROUP BY clause, it is equivalent to grouping on all rows.

COUNT(expr)
Returns a count of the number of non-NULL values in the rows retrieved by a SELECT statement: 
mysql> select student.student_name,COUNT(*)
           from student,course
           where student.student_id=course.student_id
           GROUP BY student_name;
COUNT(*) is somewhat different in that it returns a count of the number of rows retrieved, whether or not they contain NULL values. COUNT(*) is optimized to return very quickly if the SELECT retrieves from one table, no other columns are retrieved, and there is no WHERE clause. For example: 
mysql> select COUNT(*) from student;
COUNT(DISTINCT expr,[expr...])
Returns a count of the number of different non-NULL values: 
mysql> select COUNT(DISTINCT results) from student;
In MySQL you can get the number of distinct expression combinations that don't contain NULL by giving a list of expressions. In ANSI SQL you would have to do a concatenation of all expressions inside CODE(DISTINCT ..).
AVG(expr)
Returns the average value of expr: 
mysql> select student_name, AVG(test_score)
           from student
           GROUP BY student_name;
MIN(expr)
MAX(expr)
Returns the minimum or maximum value of expr. MIN() and MAX() may take a string argument; in such cases they return the minimum or maximum string value. See section 5.4.3 How MySQL Uses Indexes. 
mysql> select student_name, MIN(test_score), MAX(test_score)
           from student
           GROUP BY student_name;
SUM(expr)
Returns the sum of expr. Note that if the return set has no rows, it returns NULL!
STD(expr)
STDDEV(expr)
Returns the standard deviation of expr. This is an extension to ANSI SQL. The STDDEV() form of this function is provided for Oracle compatibility.
BIT_OR(expr)
Returns the bitwise OR of all bits in expr. The calculation is performed with 64-bit (BIGINT) precision.
BIT_AND(expr)
Returns the bitwise AND of all bits in expr. The calculation is performed with 64-bit (BIGINT) precision.

MySQL has extended the use of GROUP BY. You can use columns or calculations in the SELECT expressions that don't appear in the GROUP BY part. This stands for any possible value for this group. You can use this to get better performance by avoiding sorting and grouping on unnecessary items. For example, you don't need to group on customer.name in the following query: 

mysql> select order.custid,customer.name,max(payments)
       from order,customer
       where order.custid = customer.custid
       GROUP BY order.custid;

In ANSI SQL, you would have to add customer.name to the GROUP BY clause. In MySQL, the name is redundant if you don't run in ANSI mode.

Don't use this feature if the columns you omit from the GROUP BY part aren't unique in the group! You will get unpredictable results.

In some cases, you can use MIN() and MAX() to obtain a specific column value even if it isn't unique. The following gives the value of column from the row containing the smallest value in the sort column: 

substr(MIN(concat(rpad(sort,6,' '),column)),7)

See section 3.5.4 The Rows Holding the Group-wise Maximum of a Certain Field.

Note that if you are using MySQL Version 3.22 (or earlier) or if you are trying to follow ANSI SQL, you can't use expressions in GROUP BY or ORDER BY clauses. You can work around this limitation by using an alias for the expression: 

mysql> SELECT id,FLOOR(value/100) AS val FROM tbl_name
           GROUP BY id,val ORDER BY val;

In MySQL Version 3.23 you can do: 

mysql> SELECT id,FLOOR(value/100) FROM tbl_name ORDER BY RAND();

SQL command, type and function index

  • ! (logical NOT)
  • != (not equal)
  • "
  • % (modulo)
  • % (wild card character)
  • & (bitwise AND)
  • && (logical AND)
  • () (parentheses)
  • (Control-Z) \z
  • * (multiplication)
  • + (addition)
  • - (subtraction)
  • - (unary minus)
  • -p option
  • -password option
  • .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file, .my.cnf file
  • .mysql_history file, .mysql_history file
  • .pid (process ID) file
  • / (division)
  • /etc/passwd, /etc/passwd
  • < (less than)
  • <<
  • << (left shift)
  • <= (less than or equal)
  • <=> (Equal to)
  • <> (not equal)
  • = (equal)
  • > (greater than)
  • >= (greater than or equal)
  • >> (right shift)
  • \" (double quote)
  • \' (single quote)
  • \0 (ASCII 0)
  • \\ (escape)
  • \b (backspace)
  • \n (newline)
  • \r (carriage return)
  • \t (tab)
  • \z (Control-Z) ASCII(26)
  • _ (wild card character)
  • `

    • A

  • ABS()
  • ACOS()
  • ADDDATE()
  • addition (+)
  • alias
  • ALTER COLUMN
  • ALTER TABLE, ALTER TABLE, ALTER TABLE
  • ANALYZE TABLE
  • AND, bitwise
  • AND, logical
  • arithmetic functions
  • ASCII()
  • ASIN()
  • ATAN()
  • ATAN2()
  • AUTO_INCREMENT, using with DBI
  • AVG()

    • B

  • backspace (\b)
  • BACKUP TABLE
  • BEGIN
  • BENCHMARK()
  • BETWEEN ... AND
  • BIGINT
  • BIN()
  • BINARY
  • BIT
  • bit functions
  • BIT_AND()
  • BIT_COUNT
  • BIT_COUNT()
  • BIT_OR
  • BIT_OR()
  • BLOB, BLOB
  • BOOL

    • C

  • carriage return (\r)
  • CASE
  • casts
  • CC environment variable, CC environment variable
  • CC environment variable
  • CCX environment variable
  • CEILING()
  • CFLAGS environment variable, CFLAGS environment variable
  • CHAR, CHAR
  • CHAR VARYING
  • CHAR()
  • CHAR_LENGTH()
  • CHARACTER
  • CHARACTER VARYING
  • CHARACTER_LENGTH()
  • CHECK TABLE
  • ChopBlanks DBI method
  • COALESCE()
  • command-line options
  • Comment syntax
  • COMMIT, COMMIT
  • comparison operators
  • CONCAT()
  • CONCAT_WS()
  • configure option, --with-charset
  • configure option, --with-extra-charset
  • connect() DBI method
  • CONNECTION_ID()
  • control flow functions
  • CONV()
  • COS()
  • COT()
  • COUNT()
  • COUNT(DISTINCT)
  • CREATE DATABASE
  • CREATE FUNCTION
  • CREATE INDEX
  • CREATE TABLE
  • CROSS JOIN
  • CURDATE()
  • CURRENT_DATE
  • CURRENT_TIME
  • CURRENT_TIMESTAMP
  • CURTIME()
  • CXX environment variable
  • CXX environment variable, CXX environment variable, CXX environment variable
  • CXXFLAGS environment variable, CXXFLAGS environment variable, CXXFLAGS environment variable, CXXFLAGS environment variable

    • D

  • data_sources() DBI method
  • DATABASE()
  • DATE, DATE, DATE
  • date and time functions
  • DATE_ADD()
  • DATE_FORMAT()
  • DATE_SUB()
  • DATETIME, DATETIME
  • DAYNAME()
  • DAYOFMONTH()
  • DAYOFWEEK()
  • DAYOFYEAR()
  • DBI->connect()
  • DBI->data_sources()
  • DBI->disconnect
  • DBI->do()
  • DBI->execute
  • DBI->fetchall_arrayref
  • DBI->fetchrow_array
  • DBI->fetchrow_arrayref
  • DBI->fetchrow_hashref
  • DBI->finish
  • DBI->prepare()
  • DBI->quote
  • DBI->quote()
  • DBI->rows
  • DBI->trace, DBI->trace
  • DBI->{ChopBlanks}
  • DBI->{insertid}
  • DBI->{is_blob}
  • DBI->{is_key}
  • DBI->{is_not_null}
  • DBI->{is_num}
  • DBI->{is_pri_key}
  • DBI->{length}
  • DBI->{max_length}
  • DBI->{NAME}
  • DBI->{NULLABLE}
  • DBI->{NUM_OF_FIELDS}
  • DBI->{table}
  • DBI->{type}
  • DBI_TRACE environment variable
  • DBI_TRACE environment variable, DBI_TRACE environment variable
  • DBI_USER environment variable
  • DECIMAL
  • DECODE()
  • DEGREES()
  • DELAYED
  • DELETE
  • DESC
  • DESCRIBE, DESCRIBE
  • disconnect DBI method
  • DISTINCT, DISTINCT, DISTINCT
  • division (/)
  • DO
  • do() DBI method
  • DOUBLE
  • DOUBLE PRECISION
  • double quote (\")
  • DROP DATABASE
  • DROP FUNCTION
  • DROP INDEX, DROP INDEX
  • DROP PRIMARY KEY
  • DROP TABLE
  • DUMPFILE

    • E

  • ELT()
  • ENCODE()
  • ENCRYPT()
  • ENUM, ENUM
  • environment variable, CC
  • Environment variable, CC
  • environment variable, CC
  • Environment variable, CCX
  • environment variable, CFLAGS
  • Environment variable, CFLAGS
  • environment variable, CXX
  • Environment variable, CXX
  • environment variable, CXX
  • Environment variable, CXXFLAGS
  • environment variable, CXXFLAGS, environment variable, CXXFLAGS, environment variable, CXXFLAGS
  • Environment variable, DBI_TRACE, Environment variable, DBI_TRACE
  • environment variable, DBI_TRACE
  • Environment variable, DBI_USER
  • Environment variable, HOME
  • environment variable, HOME, environment variable, HOME
  • environment variable, LD_RUN_PATH
  • Environment variable, LD_RUN_PATH, Environment variable, LD_RUN_PATH, Environment variable, LD_RUN_PATH
  • environment variable, MYSQL_DEBUG, environment variable, MYSQL_DEBUG
  • Environment variable, MYSQL_DEBUG, Environment variable, MYSQL_DEBUG
  • environment variable, MYSQL_HISTFILE, environment variable, MYSQL_HISTFILE
  • Environment variable, MYSQL_HISTFILE
  • environment variable, MYSQL_HOST
  • Environment variable, MYSQL_HOST
  • Environment variable, MYSQL_PWD
  • environment variable, MYSQL_PWD, environment variable, MYSQL_PWD, environment variable, MYSQL_PWD
  • Environment variable, MYSQL_TCP_PORT
  • environment variable, MYSQL_TCP_PORT
  • environment variable, MYSQL_TCP_PORT, environment variable, MYSQL_TCP_PORT, environment variable, MYSQL_TCP_PORT
  • environment variable, MYSQL_UNIX_PORT, environment variable, MYSQL_UNIX_PORT, environment variable, MYSQL_UNIX_PORT
  • environment variable, MYSQL_UNIX_PORT
  • Environment variable, MYSQL_UNIX_PORT, Environment variable, MYSQL_UNIX_PORT
  • environment variable, PATH
  • Environment variable, PATH
  • Environment variable, TMPDIR, Environment variable, TMPDIR
  • Environment variable, TZ, Environment variable, TZ
  • Environment variable, UMASK, Environment variable, UMASK
  • Environment variable, UMASK_DIR, Environment variable, UMASK_DIR
  • environment variable, USER
  • Environment variable, USER
  • Environment variables, CXX
  • equal (=)
  • escape (\\)
  • execute DBI method
  • EXP()
  • EXPLAIN
  • EXPORT_SET()
  • EXTRACT(), EXTRACT()

    • F

  • fetchall_arrayref DBI method
  • fetchrow_array DBI method
  • fetchrow_arrayref DBI method
  • fetchrow_hashref DBI method
  • FIELD()
  • FILE
  • FIND_IN_SET()
  • finish DBI method
  • FLOAT, FLOAT
  • FLOAT(M,D)
  • FLOAT(precision), FLOAT(precision)
  • FLOOR()
  • FLUSH
  • FORMAT()
  • FROM_DAYS()
  • FROM_UNIXTIME(), FROM_UNIXTIME()
  • functions, arithmetic
  • functions, bit
  • functions, control flow
  • functions, date and time
  • functions, GROUP BY
  • Functions, logical
  • functions, mathematical
  • functions, miscellaneous
  • functions, string
  • functions, string comparison
  • Functions, user-defined

    • G

  • GET_LOCK()
  • GRANT
  • GRANT statement
  • greater than (>)
  • greater than or equal (>=)
  • GREATEST()
  • GROUP BY functions

    • H

  • HEX()
  • hexadecimal values
  • HOME environment variable, HOME environment variable
  • HOME environment variable
  • host.frm, problems finding
  • HOUR()

    • I

  • identifiers, quoting
  • IF()
  • IFNULL()
  • IN
  • INET_ATON()
  • INET_NTOA()
  • INNER JOIN
  • INSERT, INSERT
  • INSERT ... SELECT
  • INSERT DELAYED
  • INSERT statement, grant privileges
  • INSERT()
  • insertid DBI method
  • INSTR()
  • INT
  • INTEGER
  • INTERVAL()
  • IS NOT NULL
  • IS NULL
  • IS NULL, and indexes
  • is_blob DBI method
  • is_key DBI method
  • is_not_null DBI method
  • is_num DBI method
  • is_pri_key DBI method
  • ISNULL()
  • ISOLATION LEVEL

    • J

  • JOIN

    • K

  • KILL

    • L

  • LAST_INSERT_ID()
  • LAST_INSERT_ID([expr])
  • LCASE()
  • LD_RUN_PATH environment variable, LD_RUN_PATH environment variable, LD_RUN_PATH environment variable, LD_RUN_PATH environment variable
  • LEAST()
  • LEFT JOIN, LEFT JOIN
  • LEFT OUTER JOIN
  • LEFT()
  • length DBI method
  • LENGTH()
  • less than (<)
  • less than or equal (<=)
  • LIKE
  • LIKE, and indexes
  • LIKE, and wildcards
  • LIMIT
  • LOAD DATA INFILE, LOAD DATA INFILE
  • LOAD_FILE()
  • LOCATE(), LOCATE()
  • LOCK TABLES
  • LOG()
  • LOG10()
  • Logical functions
  • LONGBLOB
  • LONGTEXT
  • LOWER()
  • LPAD()
  • LTRIM()

    • M

  • MAKE_SET()
  • MASTER_POS_WAIT()
  • MATCH ... AGAINST()
  • mathematical functions
  • MAX()
  • max_length DBI method
  • MD5()
  • MEDIUMBLOB
  • MEDIUMINT
  • MEDIUMTEXT
  • MID()
  • MIN()
  • minus, unary (-)
  • MINUTE()
  • miscellaneous functions
  • MOD()
  • modulo (%)
  • MONTH()
  • MONTHNAME()
  • multiplication (*)
  • my_init()
  • my_thread_end()
  • my_thread_init()
  • my_ulonglong C type
  • my_ulonglong values, printing
  • MYSQL C type
  • mysql_affected_rows()
  • mysql_change_user()
  • mysql_character_set_name()
  • mysql_close()
  • mysql_connect()
  • mysql_create_db()
  • mysql_data_seek()
  • MYSQL_DEBUG environment variable, MYSQL_DEBUG environment variable
  • MYSQL_DEBUG environment variable, MYSQL_DEBUG environment variable
  • mysql_debug()
  • mysql_drop_db()
  • mysql_dump_debug_info()
  • mysql_eof()
  • mysql_errno()
  • mysql_error()
  • mysql_escape_string()
  • mysql_escape_string()
  • mysql_fetch_field()
  • mysql_fetch_field_direct()
  • mysql_fetch_fields()
  • mysql_fetch_lengths()
  • mysql_fetch_row()
  • MYSQL_FIELD C type
  • mysql_field_count(), mysql_field_count()
  • MYSQL_FIELD_OFFSET C type
  • mysql_field_seek()
  • mysql_field_tell()
  • mysql_free_result()
  • mysql_get_client_info()
  • mysql_get_host_info()
  • mysql_get_proto_info()
  • mysql_get_server_info()
  • MYSQL_HISTFILE environment variable, MYSQL_HISTFILE environment variable
  • MYSQL_HISTFILE environment variable
  • MYSQL_HOST environment variable
  • MYSQL_HOST environment variable
  • mysql_info()
  • mysql_info(), mysql_info(), mysql_info(), mysql_info()
  • mysql_init()
  • mysql_insert_id()
  • mysql_insert_id()
  • mysql_kill()
  • mysql_list_dbs()
  • mysql_list_fields()
  • mysql_list_processes()
  • mysql_list_tables()
  • mysql_num_fields()
  • mysql_num_rows()
  • mysql_options()
  • mysql_ping()
  • MYSQL_PWD environment variable
  • MYSQL_PWD environment variable, MYSQL_PWD environment variable, MYSQL_PWD environment variable
  • mysql_query(), mysql_query()
  • mysql_real_connect()
  • mysql_real_escape_string()
  • mysql_real_query()
  • mysql_reload()
  • MYSQL_RES C type
  • MYSQL_ROW C type
  • mysql_row_seek()
  • mysql_row_tell()
  • mysql_select_db()
  • mysql_shutdown()
  • mysql_stat()
  • mysql_store_result(), mysql_store_result()
  • MYSQL_TCP_PORT environment variable, MYSQL_TCP_PORT environment variable, MYSQL_TCP_PORT environment variable
  • MYSQL_TCP_PORT environment variable, MYSQL_TCP_PORT environment variable
  • mysql_thread_id()
  • MYSQL_UNIX_PORT environment variable, MYSQL_UNIX_PORT environment variable, MYSQL_UNIX_PORT environment variable
  • MYSQL_UNIX_PORT environment variable, MYSQL_UNIX_PORT environment variable, MYSQL_UNIX_PORT environment variable
  • mysql_use_result()

    • N

  • NAME DBI method
  • NATIONAL CHAR
  • NATURAL LEFT JOIN
  • NATURAL LEFT OUTER JOIN
  • NATURAL RIGHT JOIN
  • NATURAL RIGHT OUTER JOIN
  • NCHAR
  • newline (\n)
  • not equal (!=)
  • not equal (<>)
  • NOT IN
  • NOT LIKE
  • NOT REGEXP
  • NOT, logical
  • NOW()
  • NUL
  • NULL, NULL
  • NULL value
  • NULLABLE DBI method
  • NULLIF()
  • NUM_OF_FIELDS DBI method
  • NUMERIC

    • O

  • OCT()
  • OCTET_LENGTH()
  • OPTIMIZE TABLE
  • OR, bitwise
  • OR, logical
  • ORD()
  • ORDER BY

    • P

  • parentheses ( and )
  • PASSWORD(), PASSWORD(), PASSWORD(), PASSWORD()
  • PATH environment variable, PATH environment variable
  • PERIOD_ADD()
  • PERIOD_DIFF()
  • PI()
  • POSITION()
  • POW()
  • POWER()
  • prepare() DBI method
  • PRIMARY KEY, PRIMARY KEY
  • PROCESSLIST

    • Q

  • QUARTER()
  • quote() DBI method
  • quoting of identifiers

    • R

  • RADIANS()
  • RAND()
  • REAL
  • REGEXP
  • RELEASE_LOCK()
  • RENAME TABLE
  • REPAIR TABLE
  • REPEAT()
  • REPLACE
  • REPLACE ... SELECT
  • REPLACE()
  • RESTORE TABLE
  • return (\r)
  • REVERSE()
  • REVOKE
  • RIGHT JOIN
  • RIGHT OUTER JOIN
  • RIGHT()
  • RLIKE
  • ROLLBACK, ROLLBACK
  • ROUND(), ROUND()
  • rows DBI method
  • RPAD()
  • RTRIM()

    • S

  • SEC_TO_TIME()
  • SECOND()
  • SELECT
  • SELECT INTO TABLE
  • SELECT speed
  • SELECT, optimizing
  • SESSION_USER()
  • SET, SET
  • SET OPTION
  • SET PASSWORD statement
  • SHOW COLUMNS
  • SHOW CREATE TABLE
  • SHOW DATABASE INFO
  • SHOW DATABASES
  • SHOW FIELDS
  • SHOW GRANTS
  • SHOW INDEX
  • SHOW KEYS
  • SHOW MASTER LOGS
  • SHOW MASTER STATUS
  • SHOW PROCESSLIST
  • SHOW SLAVE STATUS
  • SHOW STATUS
  • SHOW TABLE STATUS
  • SHOW TABLES
  • SHOW VARIABLES
  • SIGN()
  • SIN()
  • single quote (\')
  • SMALLINT
  • SOUNDEX()
  • SPACE()
  • SQRT()
  • statements, GRANT
  • statements, INSERT
  • STD()
  • STDDEV()
  • STRAIGHT_JOIN
  • STRCMP()
  • string comparison functions
  • string functions
  • SUBDATE()
  • SUBSTRING(), SUBSTRING()
  • SUBSTRING_INDEX()
  • subtraction (-)
  • SUM()
  • SYSDATE()
  • SYSTEM_USER()

    • T

  • tab (\t)
  • table DBI method
  • table_cache
  • TAN()
  • TEXT, TEXT
  • threads
  • TIME, TIME
  • TIME_FORMAT()
  • TIME_TO_SEC()
  • TIMESTAMP, TIMESTAMP
  • TINYBLOB
  • TINYINT
  • TINYTEXT
  • TMPDIR environment variable, TMPDIR environment variable
  • TO_DAYS()
  • trace DBI method, trace DBI method
  • TRIM()
  • TRUNCATE
  • TRUNCATE()
  • type DBI method
  • Types
  • TZ environment variable, TZ environment variable

    • U

  • UCASE()
  • UDF functions
  • ulimit
  • UMASK environment variable, UMASK environment variable
  • UMASK_DIR environment variable, UMASK_DIR environment variable
  • unary minus (-)
  • UNION
  • UNIQUE
  • UNIX_TIMESTAMP()
  • UNLOCK TABLES
  • UPDATE
  • UPPER()
  • USE
  • USER environment variable
  • USER environment variable
  • USER()
  • User-defined functions

    • V

  • VARCHAR, VARCHAR
  • VERSION()

    • W

  • WEEK()
  • WEEKDAY()
  • WHERE
  • Wild card character (%)
  • Wild card character (_)
  • without-server option

    • Y

  • YEAR, YEAR
  • YEAR()

  • | (bitwise OR)
  • || (logical OR)
  • ~

    • Concept Index

    A

  • aborted clients
  • aborted connection
  • access control
  • access denied errors
  • access privileges
  • Access program
  • ACLs
  • ActiveState Perl
  • adding, character sets
  • adding, native functions
  • adding, new functions
  • adding, new user privileges
  • adding, new users
  • adding, procedures
  • adding, user-definable functions
  • administration, server
  • ADO program
  • advertising, contact information
  • age, calculating
  • alias names, case sensitivity
  • aliases, for expressions
  • aliases, for tables
  • aliases, in GROUP BY clauses
  • aliases, in ORDER BY clauses
  • aliases, names
  • aliases, on expressions
  • anonymous user, anonymous user, anonymous user, anonymous user
  • ANSI mode, running
  • ANSI SQL, differences from
  • ANSI SQL92, extensions to
  • answering questions, etiquette
  • Apache
  • APIs
  • APIs, Perl
  • applying, patches
  • argument processing
  • arithmetic expressions
  • authentication tools
  • AUTO-INCREMENT, ODBC
  • AUTO_INCREMENT, and NULL values

    • B

  • backing up, databases, backing up, databases
  • backslash, escape character
  • backups
  • backups, database
  • batch mode
  • batch, mysql option
  • BDB table type
  • BDB tables
  • benchmark suite
  • benchmarking, tools
  • benchmarks
  • Berkeley_db table type
  • Big5 Chinese character encoding
  • binary distributions
  • binary distributions, installing
  • binary distributions, on HP-UX
  • binary distributions, on Linux
  • binary log
  • Binlog_Dump
  • bit_functions, example
  • BitKeeper tree
  • BLOB columns, default values
  • BLOB columns, indexing
  • BLOB, inserting binary data
  • BLOB, size
  • books, about MySQL
  • Borland Builder 4 program
  • Borland C++ compiler
  • brackets, square
  • buffer sizes, client
  • buffer sizes, mysqld server
  • bug reports, criteria for
  • bug reports, email address
  • bugs, known
  • bugs, reporting
  • building, client programs

    • C

  • C API, datatypes
  • C API, functions
  • C API, linking problems
  • C++
  • C++ APIs
  • C++ Builder
  • C++ compiler cannot create executables
  • C++ compiler, gcc
  • caches, clearing
  • calculating, dates
  • calling sequences, UDF
  • can't create/write to file
  • case sensitivity, in access checking
  • case sensitivity, in searches
  • case sensitivity, in string comparisons
  • case sensitivity, of database names
  • case sensitivity, of table names
  • case-sensitivity, in names
  • cast operators
  • casts
  • cc1plus problems
  • ChangeLog
  • changes to privileges
  • changes, log
  • changes, version 3.19
  • changes, version 3.20
  • changes, version 3.21
  • changes, version 3.22
  • changes, version 3.23
  • changes, version 4.0
  • changing socket location, changing socket location, changing socket location
  • changing, column order
  • character sets, character sets
  • character sets, adding
  • character-sets-dir, mysql option
  • characters, multi-byte
  • check options, myisamchk
  • checking, tables for errors
  • checksum errors
  • Chinese
  • choosing types
  • choosing, a MySQL version
  • clearing, caches
  • client programs, building
  • client tools
  • clients, debugging
  • clients, threaded
  • closing, tables
  • ColdFusion program
  • collating, strings
  • column names, case sensitivity
  • columns, changing
  • columns, displaying
  • columns, indexes
  • columns, names
  • columns, other types
  • columns, selecting
  • columns, storage requirements
  • columns, types
  • command line history, command line history
  • command line options, mysql
  • command line tool
  • command syntax
  • commands out of sync
  • commands, for binary distribution
  • commands, list of
  • commands, replication
  • comments, adding
  • comments, starting
  • commercial support, types
  • communications protocols
  • comparisons, MySQL vs. others
  • compatibility, between MySQL versions, compatibility, between MySQL versions
  • compatibility, with ANSI SQL
  • compatibility, with mSQL
  • compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC, compatibility, with ODBC
  • compatibility, with Oracle, compatibility, with Oracle, compatibility, with Oracle
  • compatibility, with PostgreSQL
  • compatibility, with Sybase
  • compiler, C++ gcc
  • compiling, on Windows
  • compiling, optimizing
  • compiling, problems
  • compiling, speed
  • compiling, statically
  • compiling, user-defined functions
  • compliance, Y2K
  • compress, mysql option
  • compressed tables
  • config-file option
  • config.cache
  • config.cache file
  • configuration files
  • configuration options
  • configure option, --with-low-memory
  • configure script
  • configure, running after prior invocation
  • connect_timeout variable
  • connecting, remotely with SSH
  • connecting, to the server, connecting, to the server
  • connecting, verification
  • connection, aborted
  • constant table, constant table
  • consultants, list of
  • contact information
  • Contrib directory
  • contributed programs
  • contributing companies, list of
  • contributors, list of
  • control access
  • conventions, typographical
  • converters
  • converting, tools
  • copyrights
  • costs, licensing and support
  • counting, table rows
  • crackers, security against
  • crash
  • crash, recovery
  • crash, repeated
  • crash-me
  • crash-me program, crash-me program
  • creating, bug reports
  • creating, databases
  • creating, default startup options
  • creating, tables
  • customer support, mailing address
  • customers, of MySQL
  • cvs tree

    • D

  • data, character sets
  • data, importing
  • data, ISAM table handler
  • data, loading into tables
  • data, retrieving
  • data, size
  • database design
  • database names, case sensitivity, database names, case sensitivity
  • database, mysql option
  • databases, backups
  • databases, creating
  • databases, defined
  • databases, displaying
  • databases, dumping, databases, dumping
  • databases, information about
  • databases, MySQL vs. others
  • databases, names
  • databases, replicating
  • databases, selecting
  • databases, symbolic links, databases, symbolic links
  • databases, using
  • DataJunction
  • datatypes, C API
  • Date and Time types
  • date calculations
  • DATE columns, problems
  • date functions, Y2K compliance
  • date types
  • date types, Y2K issues
  • date values, problems
  • db table, sorting
  • DBI interface
  • DBI Perl module
  • DBI/DBD
  • dbServ
  • DBUG package
  • debug-info, mysql option
  • debug, mysql option
  • debugging, client
  • debugging, server
  • decimal point
  • default hostname
  • default installation location
  • default options
  • default values, BLOB and TEXT columns
  • default values, suppression
  • default, privileges
  • default-character-set, mysql option
  • delayed_insert_limit
  • deleting, rows
  • deletion, mysql.sock
  • Delphi
  • Delphi program
  • design, choices
  • design, issues
  • design, limitations
  • developers, list of
  • development source tree
  • digits
  • directory structure, default
  • disconnecting, from the server
  • disk full
  • disk issues
  • disks, splitting data across
  • display size
  • displaying, database information
  • displaying, information, SHOW
  • displaying, table status
  • DNS
  • downgrading
  • downloading
  • dumping, databases, dumping, databases
  • dynamic table characteristics

    • E

  • Eiffel Wrapper
  • email lists
  • email, technical support
  • employment with MySQL
  • employment, contact information
  • enable-named-commands, mysql option
  • entering, queries
  • ENUM, size
  • environment variables, environment variables, environment variables, environment variables
  • environment variables, list of
  • error mesaages, can't find file
  • error messages, displaying
  • error messages, languages
  • errors, access denied
  • errors, checking tables for
  • errors, common
  • errors, directory checksum
  • errors, handling for UDFs
  • errors, known
  • errors, linking
  • errors, list of
  • errors, reporting, errors, reporting
  • escape characters
  • estimating, query performance
  • example option
  • examples, compressed tables
  • examples, myisamchk output
  • examples, queries
  • Excel
  • execute, mysql option
  • expression aliases, expression aliases
  • expressions, extended
  • extended email support
  • extensions, to ANSI SQL
  • extracting, dates

    • F

  • fatal signal 11
  • features of MySQL
  • files, binary log
  • files, config.cache
  • files, error messages
  • files, log, files, log
  • files, not found message
  • files, permissions
  • files, query log
  • files, repairing
  • files, script
  • files, size limits
  • files, slow query log
  • files, text
  • files, tmp
  • files, update log
  • files,my.cnf
  • floating-point number
  • floats
  • flush tables
  • force, mysql option
  • foreign keys, foreign keys, foreign keys
  • foreign keys, reasons not to use
  • forums, Tek-Tips
  • free licensing
  • FreeBSD troubleshooting
  • full disk
  • full-text search
  • FULLTEXT
  • functionality, missing
  • functions for SELECT and WHERE clauses
  • functions, C API
  • functions, grouping
  • functions, native, adding
  • functions, new
  • functions, useful
  • functions, user-definable, adding
  • functions, user-defined

    • G

  • gcc
  • gdb, using
  • general information
  • General Public License
  • General Public License, MySQL
  • getting MySQL
  • global privileges
  • goals of MySQL
  • GPL, General Public License
  • GPL, GNU General Public License
  • grant tables
  • grant tables, re-creating
  • grant tables, sorting, grant tables, sorting
  • granting, privleges
  • GROUP BY, aliases in
  • GROUP BY, extensions to ANSI SQL, GROUP BY, extensions to ANSI SQL
  • grouping, expressions

    • H

  • handling, errors
  • HEAP table type
  • help option
  • help, mysql option
  • history file, history file
  • history of MySQL
  • host table
  • host table, sorting
  • host, mysql option
  • hostname caching
  • hostname, default
  • HP-UX, binary distribution
  • html, mysql option

    • I

  • ID, unique
  • ignore-space, mysql option
  • importing, data
  • increasing, performance
  • increasing, speed
  • indexes
  • indexes, and BLOB columns
  • indexes, and IS NULL
  • indexes, and LIKE
  • indexes, and NULL values
  • indexes, and TEXT columns
  • indexes, block size
  • indexes, columns
  • indexes, leftmost prefix of
  • indexes, multi-column
  • indexes, multi-part
  • indexes, names
  • indexes, use of
  • InnoDB table type
  • InnoDB tables
  • INSERT DELAYED
  • inserting, speed of
  • installation layouts
  • installation overview
  • Installing many servers
  • installing, binary distribution
  • installing, overview
  • installing, Perl
  • installing, Perl on Windows
  • installing, source distribution
  • installing, user-defined functions
  • integers
  • interface builder
  • internal compiler errors
  • internal locking
  • internals
  • Internet Service Providers
  • ISAM table handler
  • ISAM table type
  • ISP services

    • J

  • Java connectivity
  • JDBC, JDBC
  • jobs at MySQL

    • K

  • key space, MyISAM
  • keys
  • keys, foreign, keys, foreign
  • keys, multi-column
  • keys, searching on two
  • keywords
  • known errors

    • L

  • language support
  • last row, unique ID
  • layout of installation
  • leftmost prefix of indexes
  • legal names
  • LGPL, GNU Library General Public License
  • LGPL, Lesser General Public License
  • library, mysqlclient
  • licensing costs
  • licensing policy
  • licensing terms
  • licensing, contact information
  • licensing, examples
  • licensing, free
  • limitations, design
  • limits, file size
  • linking
  • linking, errors
  • linking, problems
  • linking, speed
  • links, symbolic
  • Linux, binary distribution
  • literals
  • loading, tables
  • locking
  • locking methods
  • locking, tables
  • Log files
  • log files
  • log files, maintaining
  • log files, names
  • log option
  • log, changes
  • login support

    • M

  • magazines, online
  • mailing address, for customer support
  • mailing list address
  • mailing lists
  • mailing lists, archive location
  • mailing lists, guidelines
  • main features of MySQL
  • maintaining, log files
  • maintaining, tables
  • make_binary_distribution, make_binary_distribution
  • manual, available formats
  • manual, online location
  • manual, typographical conventions
  • manuals, about MySQL
  • master-slave setup
  • matching, patterns
  • max memory used
  • max_allowed_packet
  • max_join_size
  • memory usage, myisamchk
  • memory use, memory use
  • MERGE table type
  • MERGE tables, defined
  • messages, languages
  • methods, locking
  • mirror sites
  • missing functionality
  • MIT-pthreads
  • modes, batch
  • modules, list of
  • monitor, terminal
  • mSQL compatibility
  • mSQL vs. MySQL
  • msql2mysql, msql2mysql
  • multi mysqld
  • multi-byte characters
  • multi-column indexes
  • multi-part index
  • multibyte character sets
  • multiple servers
  • My, derivation
  • my.cnf file
  • MyISAM table type
  • MyISAM, compressed tables
  • myisamchk, myisamchk, myisamchk
  • myisamchk, example output
  • myisamchk, options
  • myisampack, myisampack
  • MyODBC
  • MyODBC, reporting problems
  • mysladmn
  • mysql
  • MySQL AB, defined
  • MySQL binary distribution
  • mysql command line options
  • MySQL history
  • MySQL mailing lists
  • MySQL name
  • MySQL source distribution
  • MySQL table types
  • MySQL tools, conversion
  • MySQL usage
  • MySQL version
  • MySQL, defined
  • MySQL, introduction
  • MySQL, pronunciation
  • mysql.sock, changing location of
  • mysql.sock, protection
  • mysql_fix_privilege_tables
  • mysql_install_db, mysql_install_db
  • mysql_install_db script
  • mysqlaccess, mysqlaccess
  • mysqladmin, mysqladmin, mysqladmin, mysqladmin, mysqladmin, mysqladmin, mysqladmin
  • mysqladmin option
  • mysqlbug, mysqlbug
  • mysqlbug script
  • mysqlbug script, location
  • mysqlclient library
  • mysqld, mysqld
  • mysqld option
  • mysqld options
  • mysqld options
  • mysqld server, buffer sizes
  • mysqld, starting
  • mysqld-max
  • mysqld_multi
  • mysqldump, mysqldump, mysqldump, mysqldump
  • mysqlimport, mysqlimport, mysqlimport, mysqlimport, mysqlimport
  • mysqlshow, mysqlshow
  • mysqltest, MySQL Test Suite

    • N

  • named pipes
  • names
  • names, case-sensitivity
  • names, variables
  • naming, releases of MySQL
  • native functions, adding
  • native thread support
  • negative values
  • net etiquette, net etiquette
  • net_buffer_length
  • netmask notation, in mysql.user table
  • new procedures, adding
  • new users, adding
  • news sites
  • no matching rows
  • no-auto-rehash, mysql option
  • no-log option
  • no-named-commands, mysql option
  • no-pager, mysql option
  • no-tee, mysql option
  • non-delimited strings
  • Non-transactional tables
  • NULL value
  • NULL values, and indexes
  • NULL values, vs. empty values
  • NULL, testing for null, NULL, testing for null, NULL, testing for null, NULL, testing for null
  • NULL values, and AUTO_INCREMENT columns
  • NULL values, and TIMESTAMP columns
  • numbers
  • numeric types

    • O

  • ODBC
  • ODBC compatibility, ODBC compatibility, ODBC compatibility, ODBC compatibility, ODBC compatibility, ODBC compatibility
  • ODBC, administrator
  • odbcadmin program
  • Old Photo Album
  • OLEDB
  • one-database, mysql option
  • online location of manual
  • online magazines
  • open source, defined
  • open tables, open tables
  • opening, tables
  • opens
  • operating systems, file size limits
  • operating systems, supported
  • operating systems, Windows versus Unix
  • operations, arithmetic
  • operators, cast
  • optimization, tips
  • optimizations
  • optimizing, DISTINCT
  • optimizing, LEFT JOIN
  • optimizing, LIMIT
  • optimizing, tables
  • option files
  • options, command line, mysql
  • options, command-line
  • options, configure
  • options, myisamchk
  • options, provided by MySQL
  • options, replication
  • Oracle compatibility, Oracle compatibility, Oracle compatibility
  • ORDER BY, aliases in
  • overview

    • P

  • pack_isam
  • pager, mysql option
  • parameters, server
  • partnering with MySQL
  • password encryption, reversibility of
  • password option
  • password, root user
  • password, mysql option
  • passwords, for users
  • passwords, forgotten
  • passwords, resetting
  • passwords, security
  • passwords, setting, passwords, setting, passwords, setting
  • patches, applying
  • pattern matching
  • payment information
  • performance, benchmarks
  • performance, disk issues
  • performance, estimating
  • performance, improving, performance, improving
  • Perl API
  • Perl DBI/DBD, installation problems
  • Perl, installing
  • Perl, installing on Windows
  • Perl, modules
  • permission checks, effect on speed
  • perror
  • PhoneSweep
  • PHP API
  • PHP, web sites
  • Pluribus
  • port, mysql option
  • portability
  • portability, types
  • porting, to other systems
  • post-install, many servers
  • post-installation, setup and testing
  • PostgreSQL compatibility
  • PostgreSQL vs. MySQL, benchmarks
  • PostgreSQL vs. MySQL, overview
  • PostgreSQL/MySQL, features
  • PostgreSQL/MySQL, strategies
  • prices, licensing and support
  • privilege information, location
  • privilege system
  • privilege system, described
  • privilege, changes
  • privileges, access
  • privileges, adding
  • privileges, default
  • privileges, display
  • privileges, granting
  • privileges, revoking
  • problems, access denied errors
  • problems, common errors
  • problems, compiling
  • problems, DATE columns
  • problems, date values
  • problems, installing on IBM-AIX
  • problems, installing on Solaris
  • problems, installing Perl
  • problems, linking
  • problems, ODBC
  • problems, reporting
  • problems, starting the server
  • problems, table locking
  • problems, timezone
  • procedures, adding
  • procedures, stored
  • process support
  • processes, display
  • processing, arguments
  • products, selling
  • programs, client
  • programs, contributed
  • programs, crash-me
  • programs, list of, programs, list of
  • prompts, meanings
  • pronunciation, MySQL
  • Protocol mismatch
  • pwPage
  • Python APIs

    • Q

  • queries, C API results
  • queries, entering
  • queries, estimating performance
  • queries, examples
  • queries, speed of
  • queries, Twin Studeis project
  • query log
  • questions
  • questions, answering
  • quick, mysql option
  • quotes, in strings
  • quoting
  • quoting binary data
  • quoting strings

    • R

  • raw, mysql option
  • re-creating, grant tables
  • reconfiguring, reconfiguring
  • recovery, from crash
  • RedHat Package Manager, RedHat Package Manager
  • reducing, data size
  • references
  • regex
  • regular expression syntax, described
  • related information URLs
  • relational databases, defined
  • release numbers
  • releases, naming scheme
  • releases, testing
  • releases, updating
  • reordering, columns
  • repair options, myisamchk
  • repairing, tables
  • replace, replace
  • replication
  • replication, commands
  • replication, two-way
  • reporting, bugs
  • reporting, errors, reporting, errors
  • reporting, MyODBC problems
  • reserved words, exceptions
  • restarting, the server
  • retrieving, data
  • retrieving, data from tables
  • return values, UDFs
  • revoking, privleges
  • root password
  • root user, password resetting
  • rounding errors, rounding errors
  • rows, counting
  • rows, deleting
  • rows, locking
  • rows, matching problems
  • rows, selecting
  • rows, sorting
  • RPM file
  • RPM, defined
  • RPMs, for common tools
  • RTS-threads
  • running configure after prior invocation
  • running, a web server
  • running, ANSI mode
  • running, batch mode
  • running, multiple servers
  • running, queries

    • S

  • safe-mode command
  • safe-updates, mysql option
  • safe_mysqld
  • SCMDB
  • script files
  • scripts, scripts, scripts
  • scripts, mysql_install_db
  • scripts, mysqlbug
  • search engines, web
  • searching, and case-sensitivity
  • searching, full-text
  • searching, MySQL webpages
  • searching, two keys
  • security system
  • security, against crackers
  • select_limit
  • selecting, databases
  • selling products
  • sequence emulation
  • server administration
  • server, connecting, server, connecting
  • server, debugging
  • server, disconnecting
  • server, restart
  • server, shutdown
  • server, starting
  • server, starting and stopping
  • server, starting problems
  • servers, multiple
  • services
  • services, ISP
  • services, web
  • SET, size
  • set-variable, mysql option
  • setting, passwords
  • setup, post-installation
  • shell syntax
  • showing, database information
  • shutting down, the server
  • silent column changes
  • silent, mysql option
  • size of tables
  • sizes, display
  • skip-column-names, mysql option
  • skip-line-numbers, mysql option
  • slow queries
  • slow query log
  • socket location, changing
  • socket, mysql option
  • Solaris installation problems
  • Solaris troubleshooting
  • sorting, character sets
  • sorting, data
  • sorting, grant tables, sorting, grant tables
  • sorting, table rows
  • source distribution, installing
  • speed, compiling
  • speed, increasing
  • speed, inserting
  • speed, linking
  • speed, of queries, speed, of queries
  • SQL commands, replication
  • SQL, defined
  • sql_yacc.cc problems
  • square brackets
  • SSH
  • stability
  • standards compatibility
  • Starting many servers
  • starting, comments
  • starting, mysqld
  • starting, the server
  • starting, the server automatically
  • startup options, default
  • startup parameters
  • startup parameters, mysql
  • startup parameters, tuning
  • statically, compiling
  • status command
  • status command, results
  • status, tables
  • stopping, the server
  • storage of data
  • storage requirements, column type
  • storage space, minimizing
  • stored procedures and triggers, defined
  • storing, data
  • string collating
  • string comparisons, case sensitivity
  • string types
  • strings, defined
  • strings, escaping characters
  • strings, non-delimited
  • strings, quoting
  • striping, defined
  • sub-selects
  • superuser
  • support costs
  • support terms
  • support, BDB Tables
  • support, for operating systems
  • support, InnoDB Tables
  • support, licensing
  • support, mailing address
  • support, types
  • suppression, default values
  • Sybase compatibility
  • symbolic links, symbolic links
  • syntax, regular expression
  • system optimization
  • system table
  • system, privilege
  • system, security

    • T

  • table aliases
  • table cache
  • table is full, table is full
  • table names, case sensitivity, table names, case sensitivity
  • table types, choosing
  • table, mysql option
  • tables, BDB
  • tables, Berkeley DB
  • tables, changing column order
  • tables, checking
  • tables, closing
  • tables, compressed
  • tables, compressed format
  • tables, constant, tables, constant
  • tables, counting rows
  • tables, creating
  • tables, defragment, tables, defragment
  • tables, defragmenting
  • tables, deleting rows
  • tables, displaying
  • tables, displaying status
  • tables, dumping, tables, dumping
  • tables, dynamic
  • tables, error checking
  • tables, flush
  • tables, fragmentation
  • tables, grant
  • tables, HEAP
  • tables, host
  • tables, improving performance
  • tables, information
  • tables, information about
  • tables, ISAM
  • tables, loading data
  • tables, locking
  • tables, maintenance regimen
  • tables, maximum size
  • tables, merging
  • tables, multiple
  • tables, names
  • tables, open
  • tables, opening
  • tables, optimizing
  • tables, repairing
  • tables, retrieving data
  • tables, selecting columns
  • tables, selecting rows
  • tables, sorting rows
  • tables, system
  • tables, too many
  • tables, unique ID for last row
  • tables, updating
  • tar, problems on Solaris
  • Tcl APIs
  • tcp-ip option
  • TCP/IP
  • technical support, by email
  • technical support, licensing
  • technical support, mailing address
  • tee, mysql option
  • Tek-Tips forums
  • temporary file, write access
  • temporary tables, problems
  • terminal monitor, defined
  • testing mysqld, mysqltest
  • testing, connection to the server
  • testing, installation
  • testing, of MySQL releases
  • testing, post-installation
  • testing, the server
  • Texinfo
  • TEXT columns, default values
  • TEXT columns, indexing
  • text files, importing
  • TEXT, size
  • thread packages, differences between
  • thread support
  • thread support, non-native
  • threaded clients
  • threads, threads
  • threads, display
  • threads, RTS
  • time types
  • timeout, timeout, timeout
  • timeout, connect_timeout variable
  • TIMESTAMP, and NULL values
  • timezone problems
  • tips, optimization
  • ToDo list for MySQL
  • TODO, symlinks
  • tools, authentication
  • tools, benchmarking
  • tools, command line
  • tools, converting
  • tools, mysqld_multi
  • tools, RPMs for
  • tools, safe_mysqld
  • tools, useful
  • tools,, web
  • transaction-safe tables
  • transactions, support
  • triggers, stored
  • troubleshooting, FreeBSD
  • troubleshooting, Solaris
  • tutorial
  • Twin Studies, queries
  • type conversions
  • types, columns, types, columns
  • types, date
  • types, Date and Time
  • types, numeric
  • types, of support
  • types, of tables
  • types, portability
  • types, strings
  • types, time
  • typographical conventions

    • U

  • UDFs, compiling
  • UDFs, defined
  • UDFs, return values
  • unbuffered, mysql option
  • unique ID
  • Unireg, described
  • unloading, tables
  • update log
  • updating, releases of MySQL
  • updating, tables
  • upgrading
  • upgrading, 3.20 to 3.21
  • upgrading, 3.21 to 3.22
  • upgrading, 3.22 to 3.23
  • upgrading, different architecture
  • uptime
  • URLS for downloading MySQL
  • URLs to MySQL information
  • user names, and passwords
  • user option
  • user privileges, adding
  • user table, sorting
  • user variables
  • user-defined functions, adding, user-defined functions, adding
  • user, mysql option
  • users, adding
  • users, of MySQL
  • users, root
  • uses, of MySQL
  • using multiple disks to start data

    • V

  • valid numbers, examples
  • VARCHAR, size
  • variables, mysqld
  • variables, status
  • variables, user
  • variables, values
  • verbose, mysql option
  • version option
  • version, choosing
  • version, latest
  • version, mysql option
  • vertical, mysql option
  • views
  • virtual memory, problems while compiling
  • Visual Basic

    • W

  • wait, mysql option
  • Web clients
  • web pages, miscellaneous
  • web search engines
  • web server, running
  • web sites
  • web tools
  • Web+
  • Webmerger
  • wild cards, in mysql.columns_priv table
  • wild cards, in mysql.db table
  • wild cards, in mysql.host table
  • wild cards, in mysql.tables_priv table
  • wildcards, and LIKE
  • wildcards, in mysql.user table
  • Windows
  • Windows, compiling on
  • Windows, open issues
  • Windows, versus Unix
  • Word program
  • wrappers, Eiffel
  • write access, tmp

    • Y

  • Year 2000 compliance
  • Year 2000 issues

    • This document was generated on 7 Febuary 2002 using the texi2html translator version 1.52 (extended by davida@detron.se).