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1 General Information
The
The
The The following list describes some sections of particular interest in this manual:
Important: Reports of errors (often called bugs), as well as questions and comments, should be sent to the general MySQL mailing list. See section 1.7.1.1 The MySQL Mailing Lists. See section 1.7.1.3 How to Report Bugs or Problems.
The
For source distributions, the
If you have found a sensitive security bug in 1.1 About This Manual
This is the
Being a reference manual, it does not provide general instruction on
As the
The primary document is the Texinfo file.
The HTML version is produced automatically using a modified version of
The index can assist you in finding information in the manual. For online use, you can try the searchable version of the manual available at http://www.mysql.com/doc/. If you have any suggestions concerning additions or corrections to this manual, please send them to the documentation team at docs@mysql.com. This manual was initially written by David Axmark and Michael (Monty) Widenius. It is now maintained by the MySQL Documentation Team, consisting of Arjen Lentz, Paul DuBois and Stefan Hinz. For the many other contributors, see section B Credits.
The copyright (2004) to this manual is owned by the Swedish company
1.1.1 Conventions Used in This ManualThis manual uses certain typographical conventions:
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> type a shell command here mysql> type a mysql statement here
The ``shell'' is your command interpreter. On Unix, this is typically a
program such as Note that to enter a command or statement from an example, you do not type the prompt shown in the example.
Commands to set shell variables are shown using Bourne shell syntax. If you
are using shell> VARNAME=value some_command
For shell> setenv VARNAME value shell> some_command
Database, table, and column names must often be substituted into commands. To
indicate that such substitution is necessary, this manual uses
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 keywords are not case sensitive and may be written in uppercase or lowercase. This manual uses uppercase.
In syntax descriptions, square brackets (`[' and `]') are used
to indicate optional words or clauses. For example, in the following
statement, 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}
An ellipsis (
An ellipsis can also indicate that the preceding syntax element of a statement
may be repeated. In the following example, multiple RESET reset_option [,reset_option] ... 1.2 Overview of the MySQL Database Management System
The
The official way to pronounce 1.2.1 History of MySQL
We started out with the intention of using
The derivation of the name
The name of the MySQL Dolphin (our logo) is 1.2.2 The Main Features of MySQL
The following list describes some of the important characteristics
of the
1.2.3 MySQL StabilityThis section addresses the questions ``How stable is MySQL Server?'' and ``Can I depend on MySQL Server in this project?'' We will try to clarify these issues and answer some important questions that concern many potential users. The information in this section is based on data gathered from the mailing list, which is very active in identifying problems as well as reporting types of use.
The original code stems back to the early 1980s. It provides a stable code
base, and the
Each release of the
The
Paying customers receive high-quality support directly from MySQL AB. MySQL AB also provides the MySQL mailing list as a community resource where anyone may ask questions. Bugs are usually fixed right away with a patch. For serious bugs, there is almost always a new release. 1.2.4 How Big MySQL Tables Can Be
The The following table lists some examples of operating system file-size limits:
On Linux 2.2, you can get For a very detailed overview about LFS in Linux, have a look at Andreas Jaeger's ``Large File Support in Linux'' page at http://www.suse.de/~aj/linux_lfs.php.
By default,
If you need a
Other ways to work around file-size limits for
1.2.5 Year 2000 Compliance
The
The following simple demonstration illustrates that
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)
The final
The example also shows that the
Although
Thus, even though 1.3 Overview of MySQL AB
The developers of the
We are dedicated to developing the
The
We want the
The By the way, the ``AB'' part of the company name is the acronym for the Swedish ``aktiebolag'', or ``stock company.'' It translates to ``MySQL, Inc.'' In fact, MySQL Inc. and MySQL GmbH are examples of MySQL AB subsidiaries. They are located in the US and Germany, respectively. 1.3.1 The Business Model and Services of MySQL ABOne of the most common questions we encounter is: ``How can you make a living from something you give away for free?'' This is how:
The company has been profitable since its inception. In October 2001, we accepted venture financing from leading Scandinavian investors and a handful of business angels. This investment is used to solidify our business model and build a basis for sustainable growth. 1.3.1.1 Support
For more information and ordering support at various levels, see http://www.mysql.com/support/ or contact our sales staff at sales@mysql.com. 1.3.1.2 Training and Certification
Our training material uses the same example databases used in our
documentation and our sample applications, and is always updated
to reflect the latest
Attending our training courses will enable you to achieve your
If you are interested in our training as a potential participant or as a training partner, please visit the training section at http://www.mysql.com/training/ or contact us at: training@mysql.com.
For details about the 1.3.1.3 Consulting
Our consultants can help you design and tune your databases, construct
efficient queries, tune your platform for optimal performance, resolve
migration issues, set up replication, build robust transactional
applications, and more.
We also help customers embed
Our consultants work in close collaboration with our development team,
which ensures the technical quality of our professional services.
Consulting assignments range from 2-day power-start sessions to
projects that span weeks and months. Our expertise not only covers
If you are interested in our consulting services or want to become a consulting partner, please visit the consulting section of our Web site at http://www.mysql.com/consulting/ or contact our consulting staff at consulting@mysql.com. 1.3.1.4 Commercial Licenses
The
We also sell commercial licenses of third-party 1.3.1.5 Partnering
If you are interested in becoming a
The word
The 1.3.2 Contact Information
The For press services and inquiries not covered in our News releases (http://www.mysql.com/news/), please send email to press@mysql.com.
If you have a valid support contract with
For information about
For information on the
If you're interested in consulting, please visit the consulting
section of our Web site at http://www.mysql.com/consulting/. If you have
restricted access to the Internet, please contact the
Commercial licenses may be purchased online at
https://order.mysql.com/. There you will also find information
on how to fax your purchase order to
If you represent a business that is interested in partnering with
For more information on the
If you are interested in any of the For general discussion among our many users, please direct your attention to the appropriate mailing list. See section 1.7.1 MySQL Mailing Lists.
Reports of errors (often called bugs), as well as questions and
comments, should be sent to the general MySQL mailing list.
See section 1.7.1.1 The MySQL Mailing Lists.
If you have found a sensitive security bug in If you have benchmark results that we can publish, please contact us via email at benchmarks@mysql.com. If you have suggestions concerning additions or corrections to this manual, please send them to the manual team via email at docs@mysql.com.
For questions or comments about the workings or content of the
For all other inquires, please send an email to info@mysql.com. 1.4 MySQL Support and Licensing
This section describes 1.4.1 Support Offered by MySQL AB
Technical support from
We try to take a broad and inclusive view of technical support. Almost
any problem involving Detailed information about our various support options is given at http://www.mysql.com/support/, where support contracts can also be ordered online. If you have restricted access to the Internet, please contact our sales staff via email at sales@mysql.com.
Technical support is like life insurance. You can live happily
without it for years. However, when your hour arrives, it becomes
critically important, but it's too late to buy it.
If you use 1.4.2 Copyrights and Licenses Used by MySQL
For information about how the 1.4.3 MySQL Licenses
The
Since the
Older versions of
Please note that the use of the 1.4.3.1 Using the MySQL Software Under a Commercial License
The You need a commercial license:
If you require a license, you will need one for each installation of the
For commercial licenses, please visit our Web site at http://www.mysql.com/products/licensing/. For support contracts, see http://www.mysql.com/support/. If you have special needs or you have restricted access to the Internet, please contact our sales staff via email at sales@mysql.com. 1.4.3.2 Using the MySQL Software for Free Under GPL
You can use the
If your use of
If you use the 1.4.4 MySQL AB Logos and Trademarks
Many users of the 1.4.4.1 The Original MySQL Logo
The
The original 1.4.4.2 MySQL Logos that may be Used Without Written Permission
We have designed a set of special Conditional Use logos that may be
downloaded from our Web site at
http://www.mysql.com/press/logos.php
and used on third-party Web sites without written permission from
Contact us via email at trademark@mysql.com to inquire about special arrangements to fit your needs. 1.4.4.3 When You Need Written Permission to Use MySQL Logos
You need written permission from
Due to legal and commercial reasons we monitor the use of MySQL
trademarks on products, books, and other items. We usually require a fee for
displaying 1.4.4.4 MySQL AB Partnership Logos
1.4.4.5 Using the Word
|
| Feature | MySQL Version |
| Unions | 4.0 |
| Subqueries | 4.1 |
| R-trees | 4.1 (for MyISAM tables)
|
| Stored procedures | 5.0 |
| Views | 5.0 or 5.1 |
| Cursors | 5.0 |
| Foreign keys | 5.1 (already implemented in 3.23 for InnoDB)
|
| Triggers | 5.1 |
| Full outer join | 5.1 |
| Constraints | 5.1 |
Long awaited by our users, MySQL Server 4.0 is now available in production status.
MySQL 4.0 is available for download from http://www.mysql.com/ and from our mirrors. MySQL 4.0 has been tested by a large number of users and is in production use at many large sites.
The major new features of MySQL Server 4.0 are geared toward our existing business and community users, enhancing the MySQL database software as the solution for mission-critical, heavy-load database systems. Other new features target the users of embedded databases.
INSERT statements, searching on
packed indexes, full-text searching (using FULLTEXT indexes), and
COUNT(DISTINCT).
InnoDB storage engine as standard
InnoDB storage engine is now offered as a standard feature of the
MySQL server. This means full support for ACID transactions, foreign
keys with cascading UPDATE and DELETE, and row-level locking
are now standard features.
See section 16 The InnoDB Storage Engine.
FULLTEXT search properties of MySQL Server 4.0 enables
FULLTEXT indexing of large text masses with both binary
and natural-language searching logic. You can customize minimal word
length and define your own stop word lists in any human language,
enabling a new set of applications to be built with MySQL Server.
See section 13.6 Full-text Search Functions.
UNION statement, a long-awaited standard SQL feature.
TRUNCATE TABLE (as in Oracle).
MySQL now
supports a new character set, latin1_de, which ensures that the
German sorting order sorts words with umlauts in the same order
as do German telephone books.
mysqld parameters (startup options) can now be set without taking
down the server. This is a convenient feature for database administrators
(DBAs).
See section 14.5.3.1 SET Syntax.
DELETE and UPDATE statements have been added..
MyISAM storage engine now supports symbolic
linking at the table level (and not just the database level as before).
SQL_CALC_FOUND_ROWS and FOUND_ROWS() are new functions that make it
possible to find out the number of rows a SELECT query that includes a
LIMIT clause would have returned without that clause.
The news section of this manual includes a more in-depth list of features. See section C.3 Changes in release 4.0.x (Production).
The libmysqld embedded server library makes MySQL Server suitable for
a vastly expanded realm of applications. By using this library, developers can
embed MySQL Server into various applications and electronics devices, where
the end user has no knowledge of there actually being an underlying
database. Embedded MySQL Server is ideal for use behind
the scenes in Internet appliances, public kiosks, turnkey
hardware/software combination units, high performance Internet
servers, self-contained databases distributed on CD-ROM, and so on.
Many users of libmysqld will benefit from the MySQL
Dual Licensing. For those not wishing to be bound by the GPL,
the software is also made available under a commercial license.
The embedded MySQL library uses the same interface as the normal
client library, so it is convenient and easy to use.
See section 20.2.15 libmysqld, the Embedded MySQL Server Library.
MySQL Server 4.0 laid the foundation for new features implemented in MySQL 4.1, such as subqueries and Unicode support, and for the work on stored procedures being done in version 5.0. These features come at the top of the wish list of many of our customers.
With these additions, critics of the MySQL Database Server have to be more imaginative than ever in pointing out deficiencies in the MySQL database management system. Already well-known for its stability, speed, and ease of use, MySQL Server will be able to fulfill the requirement checklists of very demanding buyers.
The features listed in this section are implemented in MySQL 4.1. A few other features are still planned for MySQL 4.1. See section 1.6.1 New Features Planned for 4.1.
Most new features being coded are or will be available in MySQL 5.0. See section 1.6.2 New Features Planned for 5.0.
SELECT statement nested within another statement.
A derived table (an unnamed view) is a subquery in the FROM clause
of another statement.
See section 14.1.8 Subquery Syntax.
BTREE indexing is now supported for HEAP tables,
significantly improving response time for non-exact searches.
CREATE TABLE tbl_name2 LIKE tbl_name1 allows you to create, with
a single statement, a new table with a structure exactly like that of an
existing table.
MyISAM storage engine now supports
OpenGIS spatial types for storing geographical data.
See section 18 Spatial Extensions in MySQL.
SHOW WARNINGS shows warnings for the last command.
See section 14.5.3.20 SHOW WARNINGS Syntax.
utf8 and ucs2 character sets.
HELP command that can be used
to get help information for SQL statements.
The advantage of having this information on the server side is that the
information is always applicable to the particular server version that you
actually are using.
Because this information is available by issuing a SQL statement, any client
can be written to access it.
For example, the help command of the mysql command-line client
has been modified to have this capability.
INSERT ... ON DUPLICATE KEY UPDATE ... syntax has been
implemented. This allows you to UPDATE an existing row if the
INSERT would have caused a duplicate in a PRIMARY or
UNIQUE key (index).
See section 14.1.4 INSERT Syntax.
GROUP_CONCAT()
adds the extremely useful capability of concatenating column values from
grouped rows into a single result string.
See section 13.9 Functions and Modifiers for Use with GROUP BY Clauses.
The news section of this manual includes a more in-depth list of features. See section C.2 Changes in release 4.1.x (Alpha).
New features are being added to MySQL 4.1. The alpha version is already available for download. See section 1.5.2.3 Ready for Immediate Development Use.
The set of features that are being added to version 4.1 is mostly fixed. Additional development is already ongoing for version 5.0. MySQL 4.1 will go through the steps of Alpha (during which time new features might still be added/changed), Beta (when we have feature freeze and only bug corrections will be done), and Gamma (indicating that a production release is just weeks ahead). At the end of this process, MySQL 4.1 will become the new production release.
MySQL 4.1 is currently in the alpha stage, and binaries are available for download at http://www.mysql.com/downloads/mysql-4.1.php. All binary releases pass our extensive test suite without any errors on the platforms on which we test. See section C.2 Changes in release 4.1.x (Alpha).
For those wishing to use the most recent development source for MySQL 4.1, we make our 4.1 BitKeeper repository publicly available. See section 2.3.3 Installing from the Development Source Tree.
New development for MySQL is focused on the 5.0 release, featuring Stored Procedures and other new features. See section 1.6.2 New Features Planned for 5.0.
For those wishing to take a look at the bleeding edge of MySQL development, we make our BitKeeper repository for MySQL version 5.0 publicly available. See section 2.3.3 Installing from the Development Source Tree. As of December 2003, binary builds of version 5.0 are also available.
This section summarizes the features that we plan to implement in
MySQL Server. The items are ordered by release series. Within a list,
items are shown in approximately the order they will be done.
Note: If you are an enterprise level user with an urgent need for a particular feature, please contact sales@mysql.com to discuss sponsoring options. Targeted financing by sponsor companies allows us to allocate additional resources for specific purposes. One example of a feature sponsored in the past is replication.
The following features are not yet implemented in MySQL 4.1, but are planned for implementation before MySQL 4.1 moves into its beta phase. For a list what is already done in MySQL 4.1, see section 1.5.2.1 Features Available in MySQL 4.1.
The following features are planned for inclusion into MySQL 5.0. Some of the features such as stored procedures are complete and are included in MySQL 5.0 alpha, which is available now. Others such as cursors are only partially available. Expect these and other features to mature and be fully supported in upcoming releases.
Note that because we have many developers that are working on different projects, there will also be many additional features. There is also a small chance that some of these features will be added to MySQL 4.1. For a list what is already done in MySQL 4.1, see section 1.5.2.1 Features Available in MySQL 4.1.
For those wishing to take a look at the bleeding edge of MySQL development, we make our BitKeeper repository for MySQL version 5.0 publicly available. See section 2.3.3 Installing from the Development Source Tree. As of December 2003, binary builds of version 5.0 are also available.
MyISAM tables that an index
should be created as an RTREE index. (In MySQL 4.1, RTREE indexes
are used internally for geometrical data that use GIS datatypes, but cannot be
created on request.)
HEAP tables.
VARCHAR support (column lengths longer than 255, and
no stripping of trailing whitespace).
(There is already support for this in the MyISAM storage engine,
but it is not yet available at the user level.)
SHOW COLUMNS FROM tbl_name (used by mysql client to allow
expansions of column names) should not open the table, only the
definition file. This will require less memory and be much faster.
DELETE on MyISAM tables to use the record cache.
To do this, we need to update the threads record cache when we update
the `.MYD' file.
MEMORY (HEAP) tables:
RENAME TABLE on a table used in an active
MERGE table possibly corrupting the table.
The news section of this manual includes a more in-depth list of features. See section C.1 Changes in release 5.0.x (Development).
FOREIGN KEY support for all table types, not just InnoDB.
BIT type to take 1 bit. (BIT now takes 1 byte;
it is treated as a synonym for TINYINT.)
RENAME DATABASE. To make this safe for all storage engines,
it should work as follows:
RENAME command.
CONNECT BY PRIOR ... to search tree-like (hierarchical)
structures.
SUM(DISTINCT).
INSERT SQL_CONCURRENT and mysqld --concurrent-insert to do
a concurrent insert at the end of a table if the table is read-locked.
UPDATE statements. For example:
UPDATE TABLE foo SET @a=a+b,a=@a, b=@a+c.
GROUP BY, as in the following example:
SELECT id, @a:=COUNT(*), SUM(sum_col)/@a FROM tbl_name GROUP BY id.
IMAGE option to LOAD DATA INFILE to not update
TIMESTAMP and AUTO_INCREMENT fields.
LOAD DATA INFILE ... UPDATE syntax that works like this:
LOAD DATA INFILE ... REPLACE INTO.
LOAD DATA INFILE understand syntax like:
LOAD DATA INFILE 'file_name.txt' INTO TABLE tbl_name
TEXT_FIELDS (text_field1, text_field2, text_field3)
SET table_field1=CONCAT(text_field1, text_field2),
table_field3=23
IGNORE text_field3
This can be used to skip over extra columns in the text file,
or update columns based on expressions of the read data.
SET type columns:
ADD_TO_SET(value,set)
REMOVE_FROM_SET(value,set)
mysql in the middle of a query, you should open
another connection and kill the old running query.
Alternatively, an attempt should be made to detect this in the server.
SHOW INFO FROM tbl_name for basic table information
should be implemented.
SELECT a FROM tbl_name1 LEFT JOIN tbl_name2 USING (a); in this
case a is assumed to come from the tbl_name1 table.
DELETE and REPLACE options to the UPDATE statement
(this will delete rows when a duplicate-key error occurs while updating).
DATETIME to store fractions of seconds.
regexp library instead of the current
one (the new library should be much faster than the current one).
DEFAULT values to columns. Produce an error for
any INSERT statement that is missing a value for a column that has no
DEFAULT.
ANY(), EVERY(), and SOME() group functions. In
standard SQL, these work only on boolean columns, but we can extend these to
work on any columns or expressions by treating 0 values as FALSE and non-zero
values as TRUE.
MAX(column) to be the same as the column type:
mysql> CREATE TABLE t1 (a DATE); mysql> INSERT INTO t1 VALUES (NOW()); mysql> CREATE TABLE t2 SELECT MAX(a) FROM t1; mysql> SHOW COLUMNS FROM t2;
MyISAM
recovery at the same time.
INSERT ... SELECT to optionally use concurrent inserts.
SELECT MIN(column) ... GROUP BY.
long_query_time with a granularity
in microseconds.
myisampack code into the server so that it can perform
PACK or COMPRESS operations.
INSERT/DELETE/UPDATE so that we
can gracefully recover if the index file gets full.
ALTER TABLE on a table that is symlinked to another
disk, create temporary tables on that disk.
DATE/DATETIME type that handles time zone information
properly, to make dealing with dates in different time zones easier.
configure so that all libraries (like MyISAM)
can be compiled without threads.
LIMIT arguments, for example,
LIMIT @a,@b.
mysql to a Web browser.
LOCK DATABASES (with various options).
SHOW STATUS. Records reads and
updates. Selects on a single table and selects with joins. Mean number of
tables in select. Number of ORDER BY and GROUP BY queries.
mysqladmin copy database new-database; this requires a COPY
operation to be added to mysqld.
SHOW HOSTS for printing information about the hostname cache.
NULL for calculated columns.
Item_copy_string on numerical values to avoid
number->string->number conversion in case of:
SELECT COUNT(*)*(id+0) FROM tbl_name GROUP BY id
ALTER TABLE doesn't abort clients
that execute INSERT DELAYED.
UPDATE clause,
they contain the old values from before the update started.
get_changed_tables(timeout,table1,table2,...).
SET TIMESTAMP=#;.
col_name=n
is found in an expression, for some constant n, replace other
occurrences of col_name within the expression with n.
Currently, this is done only for some simple cases.
MINUS, INTERSECT, and FULL OUTER JOIN.
(Currently UNION [in 4.0] and LEFT|RIGHT OUTER JOIN are supported.)
SQL_OPTION MAX_SELECT_TIME=#, for placing a time limit on a query.
LIMIT to allow retrieval of data from the end of a result set.
mysqld_safe: according to FSSTND (which
Debian tries to follow) PID files should go into `/var/run/<progname>.pid'
and log files into `/var/log'. It would be nice if you could put the
"DATADIR" in the first declaration of "pidfile" and "log", so the
placement of these files can be changed with a single statement.
LOAD DATA INFILE statement
to read files that have been compressed with gzip.
BLOB columns (partly solved now).
JOIN with parentheses.
GET_LOCK() to obtain more than one lock. When doing this, it is
also necessary to handle the possible deadlocks this change will introduce.
We aim toward full compliance with ANSI/ISO SQL, so there are no features we plan not to implement.
This section introduces you to the MySQL mailing lists and provides some guidelines as to how the lists should be used. When you subscribe to a mailing list, you will receive all postings to the list as email messages. You can also to send your own questions and answers to the list.
To subscribe to or unsubscribe from any of the mailing lists described in this section, visit http://lists.mysql.com/. Please do not send messages about subscribing or unsubscribing to any of the mailing lists, because such messages are distributed automatically to thousands of other users.
Your local site may have many subscribers to a MySQL mailing list.
If so, the site 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 MySQL mailing lists are as follows:
announcemysqlmysql-digestmysql list in digest form. Subscribing to this list means
you will get all list messages, sent as one large mail message once a day.
bugsMySQL or if you want to be
actively involved in the process of bug hunting and fixing.
See section 1.7.1.3 How to Report Bugs or Problems.
bugs-digestbugs list in digest form.
internalsinternals-digestinternals list in digest form.
mysqldocmysqldoc-digestmysqldoc list in digest form.
benchmarksbenchmarks-digestbenchmarks list in digest form.
packagerspackagers-digestpackagers list in digest form.
javajava-digestjava list in digest form.
win32win32-digestwin32 list in digest form.
myodbcmyodbc-digestmyodbc list in digest form.
mysqlccMySQL Control Center graphical client.
mysqlcc-digestmysqlcc list in digest form.
plusplusplusplus-digestplusplus list in digest form.
msql-mysql-modulesmsql-mysql-modules, which is now named DBD::mysql.
msql-mysql-modules-digestmsql-mysql-modules list in digest form.
If you're unable to get an answer to your questions from a MySQL mailing list, one
option is to purchase support from MySQL AB. This will put you
in direct contact with MySQL developers. See section 1.4.1 Support Offered by MySQL AB.
The following table shows some MySQL mailing lists in languages other than English. These lists are not operated by MySQL AB.
mysql-france-subscribe@yahoogroups.com
list@tinc.net
subscribe mysql your@email.address to this list.
mysql-de-request@lists.4t2.com
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
subscribe mysql-br your@email.address to this list.
mysql-alta@elistas.net
subscribe mysql your@email.address to this list.
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, please follow the guidelines on sending mail to a MySQL mailing list, outlined in the next section, before contacting us.
The normal place to report bugs is http://bugs.mysql.com/, which is the address for our bugs database. This database is public, and can be browsed and searched by anyone. If you log into the system, you will also be able to enter new reports.
Writing a good bug report takes patience, but doing it right the first time saves time both for us and for yourself. A good bug report, containing a full test case for the bug, makes it very likely that we will fix the bug 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). mysqlbug can be
found in the `scripts' directory (source distribution) and in the
`bin' directory under your MySQL installation directory (binary distribution).
If you are unable to use mysqlbug (for instance, if you are running
on Windows), it is still vital that you include all the necessary information
noted in this section (most importantly a description of the operating system
and the MySQL version).
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.
Preferably, you should test the problem using the latest production or
development version of MySQL Server before posting. Anyone should be
able to repeat the bug by just using 'mysql test < script' on the
included test case or by running the shell or Perl script that is included in the
bug report.
All bugs posted in the bugs database at http://bugs.mysql.com/ will be corrected or documented in the next MySQL release. If only minor code changes are needed to correct a problem, we will also post a patch that fixes the problem.
If you have found a sensitive security bug in MySQL, please send an email to security@mysql.com.
If you have a repeatable bug report, please report it to the bugs
database at http://bugs.mysql.com/. Note that even in this case
it's good to run the mysqlbug script first to find information
about your system. Any bug that we are able to repeat has a high chance
of being fixed in the next MySQL release.
To report other problems, you can use one of the MySQL mailing lists.
Remember that it is possible for us to respond to a message containing too much information, but not to one containing too little. People often 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 faster and less troublesome to write a couple more lines in your report than to wait longer for the answer if we must ask you to provide information that was missing from the initial report.
The most common errors made in bug reports are (a) not including the version number of the MySQL distribution used and (b) not fully describing the platform on which the MySQL server is installed (including the platform type and 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 already been fixed in newer MySQL versions. Sometimes the error is platform-dependent; in such cases, it is next to impossible for us to fix anything without knowing the operating system and the version number of the platform.
If you compiled MySQL from source, 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 your problem depends on your compiler, we need to know what compiler you use. Note that every compiling problem should be regarded as a bug and reported accordingly.
It is most helpful when a good description of the problem is included in the bug report. That is, give a good example of everything you did that led to the problem and describe, in exact detail, the problem itself. The best reports are those that include a full example showing how to reproduce the bug or problem. See section D.1.6 Making a Test Case If 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 Connector/ODBC (MyODBC), please try to generate a MyODBC trace file and send it with your report. See section 20.3.7 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 later in this section).
Please include the following information in your report:
mysqladmin version. mysqladmin can be
found in the `bin' directory under your MySQL installation
directory.
uname -a.
mysqld died, you should also report the query that crashed
mysqld. You can usually find this out by running mysqld with
logging enabled. See section D.1.5 Using Log Files to Find Cause of Errors in mysqld.
mysqldump --no-data db_name tbl_name1 tbl_name2 .... This is very easy
to do and is a powerful way to get information about any table in a database.
The information will help us create a situation matching the one you have.
SELECT statements, you
should always include the output of EXPLAIN SELECT ..., and at
least the number of rows that the SELECT statement produces. You
should also include the output from SHOW CREATE TABLE tbl_name
for each involved table. The more information you give about your
situation, the more likely it is that someone can help you.
The following is an example of a very good bug report. It should be posted
with the mysqlbug script. The example uses the mysql
command-line tool. Note the use of the \G statement terminator for
statements whose output width would otherwise exceed that of an 80-column
display device.
mysql> SHOW VARIABLES;
mysql> SHOW COLUMNS FROM ...\G
<output from SHOW COLUMNS>
mysql> EXPLAIN SELECT ...\G
<output from EXPLAIN>
mysql> FLUSH STATUS;
mysql> SELECT ...;
<A short version of the output from SELECT,
including the time taken to run the query>
mysql> SHOW STATUS;
<output from SHOW STATUS>
mysqld, try to provide an
input script that will reproduce the anomaly. This script should include any
necessary source files. The more closely the script can reproduce your
situation, the better. If you can make a reproducible test case, you should
post it on http://bugs.mysql.com/ for high-priority treatment.
If you can't provide a script, you should at least include the output
from mysqladmin variables extended-status processlist in your mail to
provide some information on how your system is performing.
mysqldump and create a `README' file
that describes your problem.
Create a compressed archive of your files using
tar and gzip or zip, and use ftp to transfer the
archive to ftp://support.mysql.com/pub/mysql/secret/. Then enter
the problem into our bugs database at http://bugs.mysql.com/.
ftp to transfer it to
ftp://support.mysql.com/pub/mysql/secret/. If the data is really top
secret and you don't want to show it even to us, then go ahead and provide
an example using other names, but please regard this as the last choice.
mysqld
server as well as the options that you use to run any MySQL client programs.
The options to programs like mysqld and mysql, and to the
configure script, are often keys to answers and are very relevant.
It is never a bad idea to include them. If you use any modules, such
as Perl or PHP, please include the version numbers of those as well.
mysqlaccess, the output of mysqladmin reload, and all
the error messages you get when trying to connect. When you test your
privileges, you should first run mysqlaccess. After this, execute
mysqladmin reload version and try to connect with the program that
gives you trouble. mysqlaccess can be found in the `bin'
directory under your MySQL installation directory.
parse error, please check your syntax closely. If
you can't find something wrong with it, it's extremely likely that your
current version of MySQL Server doesn't support the syntax you are
using. If you are using the current version and the manual at
http://www.mysql.com/doc/ doesn't cover the
syntax you are using, MySQL Server doesn't support your query. In this
case, your only options are to implement the syntax yourself or email
licensing@mysql.com and ask for an offer to implement it.
If the manual covers the syntax you are using, but you have an older version
of MySQL Server, you should check the MySQL change history to see
when the syntax was implemented. In this case, you have the option of
upgrading to a newer version of MySQL Server. See section C MySQL Change History.
CHECK TABLE and REPAIR TABLE
or with myisamchk.
See section 5 Database Administration.
If you are running Windows, please verify that lower_case_table_names
is 1 or 2 with SHOW VARIABLES LIKE 'lower_case_table_names'.
mysqld should never crash a table if nothing killed it in the
middle of an update. If you can find the cause of mysqld dying,
it's much easier for us to provide you with a fix for the problem.
See section A.1 How to Determine What Is Causing a Problem.
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 whether someone else has experienced (and perhaps solved) the problem.
For information on reporting bugs in MyODBC, see section 20.3.4 How to Report Problems with MyODBC.
For solutions to some common problems, 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.
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.
In addition to the various MySQL mailing lists, you can find experienced
community people on IRC (Internet Relay Chat).
These are the best networks/channels currently known to us:
#mysql
Primarily MySQL questions but other database and SQL questions welcome.
#mysqlphp
Questions about MySQL+PHP, a popular combination.
#mysqlperl
Questions about MySQL+Perl, another popular combination.
#mysql
MySQL questions.
If you are looking for IRC client software to connect to an IRC network,
take a look at X-Chat (http://www.xchat.org/).
X-Chat (GPL licensed) is available for Unix as well as for Windows platforms.
This section describes how MySQL relates to the ANSI/ISO SQL standards. MySQL Server has many extensions to the SQL standard, and here you will find out what they are and how to use them. You will also find information about functionality missing from MySQL Server, and how to work around some differences.
Our goal is to not restrict MySQL Server usability for any usage without a very good reason for doing so. Even if we don't have the resources to perform development for every possible use, we are always willing to help and offer suggestions to people who are trying to use MySQL Server in new territories.
One of our main goals with the product is to continue to work toward
compliance with the SQL standard, but without sacrificing speed or reliability.
We are not afraid to add extensions to SQL or support for non-SQL
features if this greatly increases the usability of MySQL Server for a large
segment of our user base.
(The HANDLER interface in MySQL Server 4.0 is an example of this
strategy. See section 14.1.3 HANDLER Syntax.)
We will continue to support transactional and non-transactional databases to satisfy both mission-critical 24/7 usage and heavy Web or logging usage.
MySQL Server was originally designed to work with medium size databases (10-100 million rows, or about 100MB per table) on small computer systems. Today MySQL Server handles terabyte-size databases, but the code can also be compiled in a reduced version suitable for hand-held and embedded devices. The compact design of the MySQL server makes development in both directions possible without any conflicts in the source tree.
We are currently not targeting realtime support, though the MySQL replication capabilities already offer significant functionality.
Database cluster support is planned through integration of our acquired NDB Cluster technology into a new storage engine, available early 2004.
We are also looking at providing XML support in the database server.
ODBC levels 0-3.51.
We are aiming toward supporting the full ANSI/ISO SQL standard, but without making concessions to speed and quality of the code.
The MySQL server can operate in different SQL modes, and can apply these modes differentially for different clients. This allows applications to tailor server operation to their own requirements.
Modes define what SQL syntax MySQL should support and what kind of validation checks it should perform on the data. This makes it easier to use MySQL in a lot of different environments and to use MySQL together with other database servers.
You can set the default SQL mode by starting mysqld with the
--sql-mode="modes" option. Beginning with MySQL 4.1, you can also
change the mode after startup time by setting the sql_mode variable
with a SET [SESSION|GLOBAL] sql_mode='modes' statement.
For more information on setting the server mode, see section 5.2.2 The Server SQL Mode.
You can tell mysqld to use the ANSI mode with the --ansi
startup option. See section 5.2.1 mysqld Command-line Options.
Running the server in ANSI mode is the same as starting it with these options:
--sql-mode=REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,ONLY_FULL_GROUP_BY --transaction-isolation=SERIALIZABLE
In MySQL 4.1, you can achieve the same effect with these two statements:
SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE; SET GLOBAL sql_mode = 'REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,ONLY_FULL_GROUP_BY';
See section 1.8.2 Selecting SQL Modes.
In MySQL 4.1.1, the sql_mode options shown can be also be set with:
SET GLOBAL sql_mode='ansi';
In this case, the value of the sql_mode variable will be set to all
options that are relevant for ANSI mode. You can check the result by doing:
mysql> SET GLOBAL sql_mode='ansi';
mysql> SELECT @@GLOBAL.sql_mode;
-> 'REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,ONLY_FULL_GROUP_BY,ANSI'
MySQL Server 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 Server 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 '!' character, the syntax within
the comment will be
executed only if the MySQL version is equal to or newer than the specified
version number:
CREATE /*!32302 TEMPORARY */ TABLE t (a INT);
This means that if you have Version 3.23.02 or newer, MySQL
Server will use the TEMPORARY keyword.
The following descriptions list MySQL extensions, organized by category.
MyISAM or ISAM storage engines.
For example, to rename a MyISAM table, rename the `.MYD',
`.MYI', and `.frm' files to which the table corresponds.
db_name.tbl_name syntax. Some SQL servers provide
the same functionality but call this User space.
MySQL Server doesn't support tablespaces such as used in statements like this:
CREATE TABLE ralph.my_table...IN my_tablespace.
ANALYZE TABLE, CHECK TABLE, OPTIMIZE TABLE, and
REPAIR TABLE statements.
CREATE DATABASE and DROP DATABASE statements.
See section 14.2.3 CREATE DATABASE Syntax.
DO statement.
EXPLAIN SELECT to get a description of how tables are joined.
FLUSH and RESET statements.
SET statement. See section 14.5.3.1 SET Syntax.
SHOW statement.
See section 14.5.3 SET and SHOW Syntax.
LOAD DATA INFILE. In many cases, this syntax is compatible with
Oracle's LOAD DATA INFILE. See section 14.1.5 LOAD DATA INFILE Syntax.
RENAME TABLE. See section 14.2.9 RENAME TABLE Syntax.
REPLACE instead of DELETE + INSERT.
See section 14.1.6 REPLACE Syntax.
CHANGE col_name, DROP col_name, or DROP
INDEX, IGNORE or RENAME in an ALTER TABLE
statement.
Use of multiple ADD, ALTER, DROP, or CHANGE
clauses in an ALTER TABLE statement.
See section 14.2.2 ALTER TABLE Syntax.
INDEX or KEY in a CREATE TABLE
statement. See section 14.2.5 CREATE TABLE Syntax.
TEMPORARY or IF NOT EXISTS with CREATE TABLE.
IF EXISTS with DROP TABLE.
DROP TABLE statement.
ORDER BY and LIMIT clauses of the UPDATE and
DELETE statements.
INSERT INTO ... SET col_name = ... syntax.
DELAYED clause of the INSERT and REPLACE
statements.
LOW_PRIORITY clause of the INSERT, REPLACE,
DELETE, and UPDATE statements.
INTO OUTFILE and STRAIGHT_JOIN in a SELECT
statement. See section 14.1.7 SELECT Syntax.
SQL_SMALL_RESULT option in a SELECT statement.
GROUP BY part.
This gives better performance for some very specific, but quite normal
queries.
See section 13.9 Functions and Modifiers for Use with GROUP BY Clauses.
ASC and DESC with GROUP BY.
:= assignment
operator:
mysql> SELECT @a:=SUM(total),@b=COUNT(*),@a/@b AS avg
-> FROM test_table;
mysql> SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;
MEDIUMINT, SET, ENUM, and the
different BLOB and TEXT types.
AUTO_INCREMENT, BINARY, NULL,
UNSIGNED, and ZEROFILL.
|| and && operators to mean
logical OR and AND, as in the C programming language. In MySQL Server,
|| and OR are synonyms, as are && and AND.
Because of this nice syntax, MySQL Server doesn't support
the standard SQL || operator for string concatenation; use
CONCAT() instead. Because CONCAT() takes any number
of arguments, it's easy to convert use of the || operator to
MySQL Server.
COUNT(DISTINCT list) where list has more than one element.
BINARY attribute or use the BINARY cast, which causes
comparisons to be done using the underlying character code values rather
then a lexical ordering.
% operator is a synonym for MOD(). That is,
N % M is equivalent to MOD(N,M). % is supported
for C programmers and for compatibility with PostgreSQL.
=, <>, <= ,<, >=,>,
<<, >>, <=>, AND, OR, or LIKE
operators may be used in column comparisons to the left of the
FROM in SELECT statements. For example:
mysql> SELECT col1=1 AND col2=2 FROM tbl_name;
LAST_INSERT_ID() function.
See section 13.8.3 Information Functions.
LIKE is allowed on numeric columns.
REGEXP and NOT REGEXP extended regular expression
operators.
CONCAT() or CHAR() with one argument or more than two
arguments. (In MySQL Server, these functions can take any number of
arguments.)
BIT_COUNT(), CASE, ELT(),
FROM_DAYS(), FORMAT(), IF(), PASSWORD(),
ENCRYPT(), MD5(), ENCODE(), DECODE(),
PERIOD_ADD(), PERIOD_DIFF(), TO_DAYS(), or
WEEKDAY() functions.
TRIM() to trim substrings. Standard SQL supports removal
of single characters only.
GROUP BY functions STD(), BIT_OR(),
BIT_AND(), BIT_XOR(), and GROUP_CONCAT().
See section 13.9 Functions and Modifiers for Use with GROUP BY Clauses.
For a prioritized list indicating when new extensions will be added to MySQL Server, you should consult the online MySQL TODO list at http://www.mysql.com/doc/en/TODO.php. That is the latest version of the TODO list in this manual. See section 1.6 MySQL and the Future (The TODO).
We try to make MySQL Server follow the ANSI SQL standard and the ODBC SQL standard, but MySQL Server performs operations differently in some cases:
VARCHAR columns, trailing spaces are removed when the value is
stored. See section 1.8.7 Known Errors and Design Deficiencies in MySQL.
CHAR columns are silently converted to VARCHAR
columns when you define a table or alter its structure.
See section 14.2.5.1 Silent Column Specification Changes.
REVOKE statement to revoke
privileges for a table. See section 14.5.1.2 GRANT and REVOKE Syntax.
MySQL Version 4.1 supports subqueries and derived tables.
A subquery is a SELECT statement nested within another statement.
A derived table (an unnamed view) is a subquery in the FROM clause
of another statement.
See section 14.1.8 Subquery Syntax.
For MySQL versions older than 4.1, most subqueries can be rewritten using joins or other methods. See section 14.1.8.11 Rewriting Subqueries for Earlier MySQL Versions for examples that show how to do this.
SELECT INTO TABLE
MySQL Server doesn't support the Sybase SQL extension:
SELECT ... INTO TABLE .... Instead, MySQL Server supports the
standard SQL syntax INSERT INTO ... SELECT ..., which is basically
the same thing. See section 14.1.4.1 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.
From version 5.0, MySQL supports SELECT ... INTO with user
variables. The same syntax may also be used inside stored procedures using
cursors and local variables.
See section 19.1.6.3 SELECT ... INTO Statement.
MySQL Server (version 3.23-max and all versions 4.0 and above) supports
transactions with the InnoDB and BDB
transactional storage engines.
InnoDB provides full ACID compliance.
See section 15 MySQL Storage Engines and Table Types.
The other non-transactional storage engines in MySQL Server (such as
MyISAM) follow a different paradigm for data integrity called
``Atomic Operations.'' In transactional terms, MyISAM
tables effectively always operate in AUTOCOMMIT=1 mode.
Atomic operations often offer comparable integrity with higher performance.
With MySQL Server supporting both paradigms, you can decide whether your applications are best served by the speed of atomic operations or the use of transactional features. This choice can be made on a per-table basis.
As noted, the trade off for transactional vs. non-transactional table
types lies mostly in performance. Transactional tables have significantly
higher memory and diskspace requirements, and more CPU overhead.
On the other hand, transactional table types such as InnoDB also
offer many significant features. MySQL Server's modular design allows the
concurrent use of different storage engines to suit different
requirements and deliver optimum performance in all situations.
But how do you use the features of MySQL Server to maintain rigorous
integrity even with the non-transactional MyISAM tables, and how
do these features compare with the transactional table types?
ROLLBACK rather than
COMMIT in critical situations, transactions are more
convenient. Transactions also 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.
If you use non-transactional tables,
MySQL Server in almost all cases allows you to resolve 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 an inconsistency occurs. Note that just by using the
MySQL log or even adding one extra log, you can normally fix tables
perfectly with no data integrity loss.
LOCK TABLES or atomic updates, ensuring
that you never will get an automatic abort from the server, which is
a common problem with transactional database systems.
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. However, even if you are new to the atomic operations paradigm, or more familiar with transactions, do consider the speed benefit that non-transactional tables can offer on the order of three to five times the speed of the fastest and most optimally tuned transactional tables.
In situations where integrity is of highest importance, MySQL Server offers
transaction-level reliability and integrity even for non-transactional tables.
If you lock tables with LOCK TABLES, all updates will stall
until any integrity checks are made. If you obtain a READ LOCAL lock
(as opposed to a write lock) for a table that allows concurrent inserts at the
end of the table, reads are allowed, as are inserts by other clients.
The new inserted records will not be seen by the
client that has the read lock until it releases the lock.
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 14.1.4.2 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 with transactional tables if you are not very careful). MySQL Server also guarantees that there will not be any dirty reads.
Following are some techniques for working with non-transactional tables:
LOCK TABLES, and you don't need cursors to update
records on the fly.
ROLLBACK, you can use the following strategy:
LOCK TABLES ... to lock all the tables you want to access.
UNLOCK TABLES to release your locks.
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
has 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
to 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+125;
UPDATE customer
SET
customer_date='current_date',
address='new address',
phone='new phone',
money_he_owes_us=money_he_owes_us-125
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.
LOCK TABLES and/or ROLLBACK
for the purpose of managing unique identifiers. This can be handled much
more efficiently without locking or rolling back by using an
AUTO_INCREMENT column and either the SQL function
LAST_INSERT_ID() or the C API function mysql_insert_id().
See section 13.8.3 Information Functions.
See section 20.2.3.32 mysql_insert_id().
You can generally code around the need for row-level locking. Some situations
really do need it, and InnoDB tables support row-level locking. With
MyISAM tables, you can use a flag column in the table and do something
like the following:
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 though MySQL Server changed the preceding query to:
UPDATE tbl_name SET row_flag=1 WHERE id=ID AND row_flag <> 1;
Stored procedures are implemented in MySQL version 5.0. See section 19 Stored Procedures and Functions.
Triggers are scheduled for implementation in MySQL version 5.1. A trigger is effectively a type of stored procedure, one that is invoked when a particular event occurs. For example, you could set up a stored procedure that is triggered each time a record is deleted from a transactional table and that stored procedure automatically deletes the corresponding customer from a customer table when all their transactions are deleted.
In MySQL Server 3.23.44 and up, the InnoDB storage engine supports
checking of foreign key constraints, including CASCADE, ON
DELETE, and ON UPDATE. See section 16.7.4 FOREIGN KEY Constraints.
For storage engines other than InnoDB, MySQL Server parses the
FOREIGN KEY syntax in CREATE TABLE statements, but does not
use or store it. In the future, the implementation will be
extended to store this information in the table specification file so that it
may be retrieved by mysqldump and ODBC. At a later stage,
foreign key constraints will be implemented for MyISAM tables as well.
Foreign key enforcement offers several benefits to database developers:
Do keep in mind that these benefits come at the cost of additional overhead for the database server to perform the necessary checks. Additional checking by the server affects performance, which for some applications may be sufficiently undesirable as to be avoided if possible. (Some major commercial applications have coded the foreign-key logic at the application level for this reason.)
MySQL gives database developers the choice of which approach to use. If you
don't need foreign keys and want to avoid the overhead associated with
enforcing referential integrity, you can choose another table type instead,
such as MyISAM. (For example, the MyISAM storage engine offers
very fast performance for applications that perform only INSERT and
SELECT operations, because the inserts can be performed concurrently
with retrievals. See section 7.3.2 Table Locking Issues.)
If you choose not to take advantage of referential integrity checks, keep the following considerations in mind:
ON DELETE is the only referential integrity capability an
application needs, note that as of MySQL Server 4.0, you can use
multiple-table DELETE statements to delete rows from many
tables with a single statement. See section 14.1.1 DELETE Syntax.
ON DELETE is to add
the appropriate DELETE statement to your application when you
delete records from a table that has a foreign key. In practice, this is often
as quick as using foreign keys, and is more portable.
Be aware that the use of foreign keys can in some instances lead to problems:
FOREIGN KEY Constraints.
As of MySQL 4.1.1,
mysqldump generates dump files that take advantage of this
capability automatically when reloaded.)
Note that foreign keys in SQL are used to check and enforce referential
integrity, not to join tables. If you want to get results from multiple
tables from a SELECT statement, you do this by performing a join
between them:
SELECT * FROM table1,table2 WHERE table1.id = table2.id;
See section 14.1.7.1 JOIN Syntax. See section 3.6.6 Using Foreign Keys.
The FOREIGN KEY syntax without ON DELETE ... is often used
by ODBC applications to produce automatic WHERE clauses.
Views are currently being implemented, and will appear in the 5.0 or 5.1
version of MySQL Server.
Unnamed views (derived tables, a subquery in the FROM
clause of a SELECT) are already implemented in version 4.1.
Historically, MySQL Server has been most used in applications and on Web systems where the application writer has full control over database usage. Usage has shifted over time, and so we find that an increasing number of users now regard views as an important feature.
Views are useful for allowing users to access a set of relations (tables) as if it were a single table, and limiting their access to just that. Views can also be used to restrict access to rows (a subset of a particular table). One does not require views to restrict access to columns, as MySQL Server has a sophisticated privilege system. See section 5.4 The MySQL Access Privilege System.
Many DBMS don't allow updates to a view. Instead, you have to perform the updates on the individual tables. In designing an implementation of views, our goal, as much as is possible within the confines of SQL, is full compliance with ``Codd's Rule #6'' for relational database systems: All views that are theoretically updatable, should in practice also be updatable.
Some other SQL databases use `--' to start comments.
MySQL Server uses `#' as the start comment character. You can also use
the C comment style /* this is a comment */ with MySQL Server.
See section 10.5 Comment Syntax.
MySQL Server Version 3.23.3 and above support the `--' comment style,
provided the comment is followed by a space (or by a control character such
as a newline). The requirement for a space is to prevent 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!
Think about what happens if the value of payment is a negative value
such as -1:
UPDATE tbl_name SET credit=credit--1
credit--1 is a legal expression in SQL, but if -- is interpreted
as the start of a comment, part of the expression is discarded. The result is a
statement that has a completely different meaning than intended:
UPDATE tbl_name SET credit=credit
The statement produces no change in value at all! This illustrates that allowing comments to start with `--' can have serious consequences.
Using our implementation of this method of commenting in MySQL Server
Version 3.23.3 and up, credit--1 is actually safe.
Another safe feature is that the mysql command-line client
removes all lines that start with `--'.
The following information is relevant only if you are running a MySQL version earlier than 3.23.3:
If you have an SQL program in a text file that contains `--'
comments, you should use the replace utility as follows to convert the
comments to use `#' characters:
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
MySQL allows you to work with both transactional tables that allow rollback and non-transactional tables that do not, so constraint handling is a bit different in MySQL than in other databases.
We have to handle the case when you have updated a lot of rows in a non-transactional table that cannot roll back when an error occurs.
The basic philosophy is to try to give an error for anything that we can detect at compile time but try to recover from any errors we get at runtime. We do this in most cases, but not yet for all. See section 1.6.4 New Features Planned for the Near Future.
The options MySQL has when an error occurs are to stop the statement in the middle or to recover as well as possible from the problem and continue.
The following sections describe what happens for the different types of constraints.
Normally you will get an error when you try to INSERT or
UPDATE a row that causes a primary key, unique key or foreign key
violation. If you are using a transactional storage engine such as
InnoDB, MySQL will automatically roll back the transaction. If you are
using a non-transactional storage engine, MySQL will stop at the incorrect
row and leave any remaining rows unprocessed.
To make life easier, MySQL supports an IGNORE keyword for
most commands that can cause a key violation (such as INSERT IGNORE
and UPDATE IGNORE). In this case, MySQL will ignore any key
violation and continue with processing the next row. You can get
information about what MySQL did with the mysql_info() API function.
See section 20.2.3.30 mysql_info().
In MySQL 4.1 and up, you also can use the SHOW WARNINGS statement.
See section 14.5.3.20 SHOW WARNINGS Syntax.
Note that for the moment only InnoDB tables support foreign keys.
See section 16.7.4 FOREIGN KEY Constraints.
Foreign key support in MyISAM tables is scheduled for implementation
in MySQL 5.1.
NOT NULL and DEFAULT valuesTo be able to support easy handling of non-transactional tables all columns in MySQL have default values.
If you insert an ``incorrect'' value in a column, such as a NULL in a
NOT NULL column or a too-large numerical value in a numerical
column, MySQL sets the column to the ``best possible value''
instead of producing an error. For numerical values, this is either 0, the
smallest possible value or the largest possible value. For strings, this is
either the empty string or the longest possible string that can be in
the column.
This means that if you try to store NULL into a column that
doesn't take NULL values, MySQL Server instead stores 0 or ''
(the empty string). This last behavior can, for single row
inserts, be changed with the -DDONT_USE_DEFAULT_FIELDS compile
option.) See section 2.3.2 Typical configure Options.
This causes INSERT statements to generate an error unless you
explicitly specify values for all columns that require a non-NULL
value.
The reason for the preceding rules is that we can't check these conditions until the query has begun executing. We can't just roll back if we encounter a problem after updating a few rows, because the table type may not support rollback. The option of terminating the statement is not that good; in this case, the update would be ``half done,'' which is probably the worst possible scenario. In this case it's better to ``do the best you can'' and then continue as if nothing happened.
This means that you should generally not use MySQL to check column content. Instead, the application should ensure that is passes only legal values to MySQL.
In MySQL 5.0, we plan to improve this by providing warnings when automatic column conversions occur, plus an option to let you roll back statements that attempt to perform a disallowed column value assignment, as long as the statement uses only transactional tables.
ENUM and SET
In MySQL 4.x, ENUM is not a real constraint, but is a more efficient
way to define columns that can only contain a given set of values.
This is because of the same reasons NOT NULL is not honored.
See section 1.8.6.2 Constraint NOT NULL and DEFAULT values.
If you insert an incorrect value into an ENUM column, it will be set to
the reserved enumeration value 0, which will be displayed as an empty
string in string context. See section 12.6.3 The ENUM Type.
If you insert an incorrect value into a SET column, the incorrect value
is ignored. For example, if the column can contain the values
'a', 'b', and 'c', an attempt to assign 'a,x,b,y'
results in a value of 'a,b'.
See section 12.6.4 The SET Type.
The following known errors or bugs are not fixed in MySQL 3.23 because fixing them would involve changing a lot of code that could introduce other even worse bugs. The bugs are also classified as ``not fatal'' or ``bearable.''
LOCK TABLE to lock multiple tables
and then in the same connection use DROP TABLE to drop one of
them while another thread is trying to lock it. (To break the deadlock, you
can use KILL to terminate any of the threads involved.) This issue is
resolved in MySQL 4.0.12.
SELECT MAX(key_column) FROM t1,t2,t3... where one of the tables are
empty doesn't return NULL but instead returns the maximum value for the
column. This issue is resolved in MySQL 4.0.11.
DELETE FROM heap_table without a WHERE clause doesn't work on
a locked HEAP table.
The following known errors or bugs are not fixed in MySQL 4.0 because fixing them would involve changing a lot of code that could introduce other even worse bugs. The bugs are also classified as ``not fatal'' or ``bearable.''
UNION, the first SELECT determines the type,
max_length and NULL properties for the resulting
columns. This issue is resolved in MySQL 4.1.1; the property values are based
on the rows from all UNION parts.
DELETE with many tables, one can't refer to tables to be
deleted through an alias. This is fixed in 4.1.
UNION ALL and UNION DISTINCT in the same query.
If you use ALL for one UNION, it is used for all
of them.
The following problems are known and fixing them is a high priority:
FOREIGN KEY constraint doesn't work in replication because
the constraint may have another name on the slave.
REPLACE (and LOAD DATA with REPLACE option) does not
trigger ON DELETE CASCADE.
DISTINCT with ORDER BY doesn't work inside GROUP_CONCAT()
if you don't use all and only those columns that are in the
DISTINCT list.
GROUP_CONCAT() doesn't work with BLOB/TEXT columns
when you use DISTINCT or ORDER BY inside
GROUP_CONCAT(). To work around this limitation, use
MID(expr, 1, 255) instead.
DROP TABLE command before the table is
used in the transaction itself. We plan to fix this in 5.0 by
having the DROP TABLE wait until the table is not used in any
transaction.
FLUSH TABLES WITH READ LOCK does not block CREATE TABLE or
COMMIT, which make cause a problem with the binary log position when
doing a full backup of tables and the binary log.
ANALYZE TABLE on a BDB table may in some cases make the table
unusable until you restart mysqld. If this happens, you will
see errors of the following form in the MySQL error file:
001207 22:07:56 bdb: log_flush: LSN past current end-of-log
FROM part of a SELECT
statement, but silently
ignores them. The reason for not giving an error is that many clients
that automatically generate queries add parentheses in the FROM
part even where they are not needed.
RIGHT JOINS or combining LEFT and
RIGHT join in the same query may not give a correct answer as
MySQL only generates NULL rows for the table preceding a LEFT or
before a RIGHT join. This will be fixed in 5.0 at the same time
we add support for parentheses in the FROM part.
ALTER TABLE on a BDB table on which you are
running multiple-statement transactions until all those transactions complete.
(The transaction will probably be ignored.)
ANALYZE TABLE, OPTIMIZE TABLE, and REPAIR TABLE may
cause problems on tables for which you are using INSERT DELAYED.
LOCK TABLE ... and FLUSH TABLES ... doesn't
guarantee that there isn't a half-finished transaction in progress on the
table.
BDB tables are a bit slow to open. If you have many BDB tables
in a database, it will take a long time to use the mysql client on
the database if you are not using the -A option or if you are using
rehash. This is especially notable when you have a large table cache.
CREATE ... SELECT or
INSERT ... SELECT statements that
insert zero or NULL values into an AUTO_INCREMENT column.
DELETE if you are
deleting rows from a table which has foreign keys with ON DELETE
CASCADE properties.
REPLACE ... SELECT,
INSERT IGNORE ... SELECT if you have
duplicate key values in the inserted data.
ORDER BY
clause guaranteeing a deterministic order.
Indeed, for example for INSERT ... SELECT with no ORDER
BY, the SELECT may return rows in a different order
(which will result in a row having different ranks, hence getting a
different number in the AUTO_INCREMENT column),
depending on the choices made by the optimizers on the master and
slave. A query will be optimized differently on the master and slave only if:
OPTIMIZE TABLE was run on the master tables and not on
the slave tables. (To fix this, OPTIMIZE, ANALYZE
and REPAIR are written to the binary log, as of MySQL 4.1.1).
InnoDB on the master,
but MyISAM on the slave if the slave has less available disk
space.)
key_buffer_size, etc.) are different on
the master and slave.
mysqlbinlog|mysql.
The easiest way to avoid this problem in all cases is add an
ORDER BY clause to
such non-deterministic queries to ensure that the rows are always
stored or modified in the same order.
In future MySQL versions, we will automatically add an ORDER BY
clause when needed.
The following problems are known and will be fixed in due time:
--log-bin=old_host_name-bin if you change your host name to
something else. Another option is to just rename the old files to
reflect your hostname change. See section 5.2.1 mysqld Command-line Options.
mysqlbinlog will not delete temporary files left after a
LOAD DATA INFILE command. See section 8.5 The mysqlbinlog Binary Log Utility.
RENAME doesn't work with TEMPORARY tables or tables used in a
MERGE table.
RPAD() function in a query that has to be
resolved by using a temporary table, all resulting strings will
have rightmost spaces removed. This is an example of such a query:
SELECT RPAD(t1.column1, 50, ' ') AS f2, RPAD(t2.column2, 50, ' ') AS f1 FROM table1 as t1 LEFT JOIN table2 AS t2 ON t1.record=t2.joinID ORDER BY t2.record;The final result of this bug is that you will not be able to get spaces on the right side of the resulting values. The problem also occurs for any other string function that adds spaces to the right. The reason for this is due to the fact that
HEAP tables, which are used
first for temporary tables, are not capable of handling VARCHAR columns.
This behavior exists in all versions of MySQL.
It will be fixed in one of the 4.1 series releases.
CHAR(255)) in table names, column names, or enumerations.
This is scheduled to be fixed in version 5.1 when we have new table
definition format files.
SET CHARACTER SET, you can't use translated
characters in database, table, and column names.
_ or % with ESCAPE in LIKE
... ESCAPE.
DECIMAL column with a number stored in different
formats (+01.00, 1.00, 01.00), GROUP BY may regard each value
as a different value.
DELETE FROM merge_table used without a WHERE clause
will clear only the mapping for the table, not delete everything in the
mapped tables.
BLOB values can't ``reliably'' be used in GROUP BY or
ORDER BY or DISTINCT. Only the first max_sort_length
bytes are used when comparing BLOB values in these cases.
The default value of max_sort_length value is 1024. It can be changed
at server startup time. A workaround for most cases is to use a substring.
For example:
SELECT DISTINCT LEFT(blob,2048) FROM tbl_name.
BIGINT or DOUBLE (both are
normally 64 bits long). It depends on the function which precision one
gets. The general rule is that bit functions are done with BIGINT
precision, IF, and ELT() with BIGINT or DOUBLE
precision and the rest with DOUBLE precision. You should try to
avoid using unsigned long long values if they resolve to be bigger than
63 bits (9223372036854775807) for anything other than bit fields.
MySQL Server 4.0 has better BIGINT handling than 3.23.
BLOB and TEXT columns, automatically
have all trailing spaces removed when retrieved. For CHAR types this
is okay. The bug is
that in MySQL Server, VARCHAR columns are treated the same way.
ENUM and SET columns in one table.
MIN(), MAX(), and other aggregate functions, MySQL
currently compares ENUM and SET columns by their string
value rather than by the string's relative position in the set.
mysqld_safe redirects all messages from mysqld to the
mysqld log. One problem with this is that if you execute
mysqladmin refresh to close and reopen the log,
stdout and stderr are still redirected to the old log.
If you use --log extensively, you should edit mysqld_safe to
log to `'hostname'.err' instead of `'hostname'.log' so you can
easily reclaim the space for the old log by deleting the old one and
executing mysqladmin refresh.
UPDATE statement, columns are updated from left to right. If
you refer to an updated column, you will get the updated value instead of the
original value. For example:
mysql> UPDATE tbl_name SET KEY=KEY+1,KEY=KEY+1;This will increment
KEY by 2, not 1.
mysql> SELECT * FROM temporary_table, temporary_table AS t2;
DISTINCT differently if you are using
'hidden' columns in a join or not. In a join, hidden columns are
counted as part of the result (even if they are not shown) while in
normal queries hidden columns don't participate in the DISTINCT
comparison. We will probably change this in the future to never compare
the hidden columns when executing DISTINCT.
An example of this is:
SELECT DISTINCT mp3id FROM band_downloads
WHERE userid = 9 ORDER BY id DESC;
and
SELECT DISTINCT band_downloads.mp3id
FROM band_downloads,band_mp3
WHERE band_downloads.userid = 9
AND band_mp3.id = band_downloads.mp3id
ORDER BY band_downloads.id DESC;
In the second case you may in MySQL Server 3.23.x get two identical rows in
the result set (because the values in the hidden id column may differ).
Note that this happens only for queries where you don't have the
ORDER BY columns in the result.
best possible value in the column:
NULL into a column that doesn't allow
NULL values, MySQL Server stores 0 or '' (the empty
string) in it instead. (This behavior can, however, be changed with the
-DDONT_USE_DEFAULT_FIELDS compile option.)
DATE and
DATETIME columns (like '2000-02-31' or '2000-02-00').
The idea is that it's not the job of the SQL server to validate dates. If
MySQL can store a date value and retrieve exactly the same value, MySQL
stores it as given. If the date is totally wrong (outside the server's
ability to store it), the special date value '0000-00-00' is stored
in the column instead.
ENUM column to an unsupported value, it is set to
the error value empty string, with numeric value 0.
SET column to an unsupported value, the value is ignored.
PROCEDURE on a query that returns an empty set,
in some cases the PROCEDURE will not transform the columns.
MERGE doesn't check whether the underlying
tables are of compatible types.
NaN, -Inf, and Inf
values in DOUBLE columns. Using these will cause problems when trying to export
and import data. We should as an intermediate solution change NaN to
NULL (if possible) and -Inf and Inf to the
minimum respective maximum possible double value.
ALTER TABLE to first add a UNIQUE index to a
table used in a MERGE table and then use ALTER TABLE to
add a normal index on the MERGE table, the key order will be
different for the tables if there was an old key that was not unique in the
table. This is because ALTER TABLE puts UNIQUE indexes before
normal indexes to be able to detect duplicate keys as early as possible.
The following are known bugs in earlier versions of MySQL:
DROP TABLE on a table that is
one among many tables that is locked with LOCK TABLES.
LOCK table with WRITE.
FLUSH TABLES.
UPDATE that updated a key with
a WHERE on the same key may have failed because the key was used to
search for records and the same row may have been found multiple times:
UPDATE tbl_name SET KEY=KEY+1 WHERE KEY > 100;A workaround is to use:
mysql> UPDATE tbl_name SET KEY=KEY+1 WHERE KEY+0 > 100;This will work because MySQL Server will not use an index on expressions in the
WHERE clause.
For platform-specific bugs, see the sections about compiling and porting. See section 2.3 MySQL Installation Using a Source Distribution. See section D Porting to Other Systems.
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