ALTER {DATABASE | SCHEMA} [db_name]
    alter_specification [, alter_specification] ...

alter_specification:
    [DEFAULT] CHARACTER SET charset_name
  | [DEFAULT] COLLATE collation_name

ALTER DATABASE allows you to change the overall characteristics of a database. These characteristics are stored in the db.opt file in the database directory. To use ALTER DATABASE, you need the ALTER privilege on the database.

The CHARACTER SET clause changes the default database character set. The COLLATE clause changes the default database collation. Character set and collation names are discussed in Chapter 10, Character Set Support.

In MySQL 5.0, the database name can be omitted. The statement applies to the default database in this case. ALTER SCHEMA can be used as of MySQL 5.0.2.

ALTER [IGNORE] TABLE tbl_name
    alter_specification [, alter_specification] ...

alter_specification:
    ADD [COLUMN] column_definition [FIRST | AFTER col_name ]
  | ADD [COLUMN] (column_definition,...)
  | ADD INDEX [index_name] [index_type] (index_col_name,...)
  | ADD [CONSTRAINT [symbol]]
        PRIMARY KEY [index_type] (index_col_name,...)
  | ADD [CONSTRAINT [symbol]]
        UNIQUE [index_name] [index_type] (index_col_name,...)
  | ADD [FULLTEXT|SPATIAL] [index_name] (index_col_name,...)
  | ADD [CONSTRAINT [symbol]]
        FOREIGN KEY [index_name] (index_col_name,...)
        [reference_definition]
  | ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
  | CHANGE [COLUMN] old_col_name column_definition
        [FIRST|AFTER col_name]
  | MODIFY [COLUMN] column_definition [FIRST | AFTER col_name]
  | DROP [COLUMN] col_name
  | DROP PRIMARY KEY
  | DROP INDEX index_name
  | DROP FOREIGN KEY fk_symbol
  | DISABLE KEYS
  | ENABLE KEYS
  | RENAME [TO] new_tbl_name
  | ORDER BY col_name
  | CONVERT TO CHARACTER SET charset_name [COLLATE collation_name]
  | [DEFAULT] CHARACTER SET charset_name [COLLATE collation_name]
  | DISCARD TABLESPACE
  | IMPORT TABLESPACE
  | 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.

The syntax for many of the allowable alterations is similar to clauses of the CREATE TABLE statement. This includes table_options modifications, for options such as ENGINE, AUTO_INCREMENT, and AVG_ROW_LENGTH. See Section 13.1.5, “CREATE TABLE Syntax”.

Some operations may result in warnings if attempted on a table for which the storage engine does not support the operation. These warnings can be displayed with SHOW WARNINGS. See Section 13.5.4.22, “SHOW WARNINGS 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 13.1.5.1, “Silent Column Specification Changes”. For example, if you try to change a VARCHAR column to CHAR, MySQL still uses 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. 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, then are automatically redirected to the new table without any failed updates.

Note that if you use any other option to ALTER TABLE than RENAME, MySQL always creates a temporary table, even if the data wouldn't strictly need to be copied (such as when you change the name of a column). For MyISAM tables, you can speed up the index re-creation operation (which is the slowest part of the alteration process) by setting the myisam_sort_buffer_size system variable to a high value.

Here are some examples that show uses of ALTER TABLE. Begin with a table t1 that is created as shown here:

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 indexes on column d and on column a:

mysql> ALTER TABLE t2 ADD INDEX (d), ADD INDEX (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 PRIMARY KEY (c);

Note that we indexed c (as a PRIMARY KEY), because AUTO_INCREMENT columns must be indexed, and also that we declare c as NOT NULL, because primary key columns cannot be NULL.

When you add an AUTO_INCREMENT column, column values are filled in with sequence numbers for you automatically. For MyISAM tables, you can set the first sequence number by executing SET INSERT_ID=value before ALTER TABLE or by using the AUTO_INCREMENT=value table option. See Section 13.5.3, “SET Syntax”.

From MySQL 5.0.3, you can use the ALTER TABLE ... AUTO_INCREMENT=value table option for InnoDB tables to set the sequence number for new rows if the value is greater than the maximum value in the AUTO_INCREMENT column. If the value is less than the current maximum value in the column, no error message is given and the current sequence value is not changed.

With MyISAM tables, if you do not change the AUTO_INCREMENT column, the sequence number is not affected. If you drop an AUTO_INCREMENT column and then add another AUTO_INCREMENT column, the numbers are resequenced beginning with 1.

See Section A.7.1, “Problems with ALTER TABLE”.

CREATE {DATABASE | SCHEMA} [IF NOT EXISTS] db_name
    [create_specification [, create_specification] ...]

create_specification:
    [DEFAULT] CHARACTER SET charset_name
  | [DEFAULT] COLLATE collation_name

CREATE DATABASE creates a database with the given name. To use CREATE DATABASE, you need the CREATE privilege on the database.

Rules for allowable database names are given in Section 9.2, “Database, Table, Index, Column, and Alias Names”. An error occurs if the database exists and you did not specify IF NOT EXISTS.

In MySQL 5.0, create_specification options can be given to specify database characteristics. Database characteristics are stored in the db.opt file in the database directory. The CHARACTER SET clause specifies the default database character set. The COLLATE clause specifies the default database collation. Character set and collation names are discussed in Chapter 10, Character Set Support.

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 in MySQL 5.0 creates only a directory under the MySQL data directory and the db.opt file.

If you manually create a directory under the data directory (for example, with mkdir), the server considers it a database directory and it shows up in the output of SHOW DATABASES.

CREATE SCHEMA can be used as of MySQL 5.0.2.

You can also use the mysqladmin program to create databases. See Section 8.5, “mysqladmin — Client for Administering a MySQL Server”.

CREATE [UNIQUE|FULLTEXT|SPATIAL] INDEX index_name
    [USING index_type]
    ON tbl_name (index_col_name,...)

index_col_name:
    col_name [(length)] [ASC | DESC]

In MySQL 5.0, CREATE INDEX is mapped to an ALTER TABLE statement to create indexes. See Section 13.1.2, “ALTER TABLE Syntax”.

Normally, you create all indexes on a table at the time the table itself is created with CREATE TABLE. See Section 13.1.5, “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 to index a prefix consisting of the first length characters of each column value. BLOB and TEXT columns also can be indexed, but a prefix length must be given.

The statement shown here creates an index using the first 10 characters of the name column:

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.

Prefixes can be up to 255 bytes long. For MyISAM and InnoDB tables in MySQL 5.0, they may be up to 1000 bytes in length. Note that prefix limits are measured in bytes, whereas the prefix length in CREATE INDEX statements is interpreted as number of characters. Take this into account when specifying a prefix length for a column that uses a multi-byte character set.

In MySQL 5.0:

An index_col_name specification can end with ASC or DESC. These keywords are allowed for future extensions for specifying ascending or descending index value storage. Currently they are parsed but ignored; index values are always stored in ascending order.

In MySQL 5.0, some storage engines allow you to specify an index type when creating an index. The syntax for the index_type specifier is USING type_name. The allowable type_name values supported by different storage engines are shown in the following table. Where multiple index types are listed, the first one is the default when no index_type specifier is given.

Example:

CREATE TABLE lookup (id INT) ENGINE = MEMORY;
CREATE INDEX id_index USING BTREE ON lookup (id);

TYPE type_name can be used as a synonym for USING type_name to specify an index type. However, USING is the preferred form. In addition, the index name that precedes the index type in the index specification syntax is not optional with TYPE. This is because, unlike USING, TYPE is not a reserved word and thus is interpreted as an index name.

If you specify an index type that is not legal for a given storage engine, but there is another index type available that the engine can use without affecting query results, the engine uses the available type.

For more information about how MySQL uses indexes, see Section 7.4.5, “How MySQL Uses Indexes”.

FULLTEXT indexes in MySQL 5.0 can index only CHAR, VARCHAR, and TEXT columns, and only in MyISAM tables. See Section 12.7, “Full-Text Search Functions”.

SPATIAL indexes in MySQL 5.0 can index only spatial columns, and only in MyISAM tables. Spatial column types are described in Chapter 17, Spatial Extensions in MySQL.

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    [(create_definition,...)]
    [table_options] [select_statement]

Or:

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    [(] LIKE old_tbl_name [)];

create_definition:
    column_definition
  | [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (index_col_name,...)
  | KEY [index_name] [index_type] (index_col_name,...)
  | INDEX [index_name] [index_type] (index_col_name,...)
  | [CONSTRAINT [symbol]] UNIQUE [INDEX]
        [index_name] [index_type] (index_col_name,...)
  | [FULLTEXT|SPATIAL] [INDEX] [index_name] (index_col_name,...)
  | [CONSTRAINT [symbol]] FOREIGN KEY
        [index_name] (index_col_name,...) [reference_definition]
  | CHECK (expr)

column_definition:
    col_name type [NOT NULL | NULL] [DEFAULT default_value]
        [AUTO_INCREMENT] [UNIQUE [KEY] | [PRIMARY] KEY]
        [COMMENT 'string'] [reference_definition]

type:
    TINYINT[(length)] [UNSIGNED] [ZEROFILL]
  | SMALLINT[(length)] [UNSIGNED] [ZEROFILL]
  | MEDIUMINT[(length)] [UNSIGNED] [ZEROFILL]
  | INT[(length)] [UNSIGNED] [ZEROFILL]
  | INTEGER[(length)] [UNSIGNED] [ZEROFILL]
  | BIGINT[(length)] [UNSIGNED] [ZEROFILL]
  | REAL[(length,decimals)] [UNSIGNED] [ZEROFILL]
  | DOUBLE[(length,decimals)] [UNSIGNED] [ZEROFILL]
  | FLOAT[(length,decimals)] [UNSIGNED] [ZEROFILL]
  | DECIMAL(length,decimals) [UNSIGNED] [ZEROFILL]
  | NUMERIC(length,decimals) [UNSIGNED] [ZEROFILL]
  | DATE
  | TIME
  | TIMESTAMP
  | DATETIME
  | CHAR(length) [BINARY | ASCII | UNICODE]
  | VARCHAR(length) [BINARY]
  | TINYBLOB
  | BLOB
  | MEDIUMBLOB
  | LONGBLOB
  | TINYTEXT [BINARY]
  | TEXT [BINARY]
  | MEDIUMTEXT [BINARY]
  | LONGTEXT [BINARY]
  | ENUM(value1,value2,value3,...)
  | SET(value1,value2,value3,...)
  | spatial_type

index_col_name:
    col_name [(length)] [ASC | DESC]

reference_definition:
    REFERENCES tbl_name [(index_col_name,...)]
               [MATCH FULL | MATCH PARTIAL | MATCH SIMPLE]
               [ON DELETE reference_option]
               [ON UPDATE reference_option]

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION

table_options: table_option [table_option] ...

table_option:
    {ENGINE|TYPE} = engine_name
  | AUTO_INCREMENT = value
  | AVG_ROW_LENGTH = value
  | [DEFAULT] CHARACTER SET charset_name [COLLATE collation_name]
  | CHECKSUM = {0 | 1}
  | COMMENT = 'string'
  | CONNECTION = 'connect_string'
  | MAX_ROWS = value
  | MIN_ROWS = value
  | PACK_KEYS = {0 | 1 | DEFAULT}
  | PASSWORD = 'string'
  | DELAY_KEY_WRITE = {0 | 1}
  | ROW_FORMAT = {DEFAULT|DYNAMIC|FIXED|COMPRESSED|REDUNDANT|COMPACT}
  | UNION = (tbl_name[,tbl_name]...)
  | INSERT_METHOD = { NO | FIRST | LAST }
  | DATA DIRECTORY = 'absolute path to directory'
  | INDEX DIRECTORY = 'absolute path to directory'

select_statement:
    [IGNORE | REPLACE] [AS] SELECT ...   (Some legal select statement)

CREATE TABLE creates a table with the given name. You must have the CREATE privilege for the table.

Rules for allowable table names are given in Section 9.2, “Database, Table, Index, Column, and Alias Names”. By default, the table is created in the current database. An error occurs if the table exists, if there is no current database, or if the database does not exist.

In MySQL 5.0, the table name can be specified as db_name.tbl_name to create the table in a specific database. This works whether or not there is a current database. If you use quoted identifiers, quote the database and table names separately. For example, `mydb`.`mytbl` is legal, but `mydb.mytbl` is not.

You can use the TEMPORARY keyword when creating a table. A TEMPORARY table is visible only to the current connection, and is dropped automatically when the connection is closed. This means that two different connections can use the same temporary table name without conflicting with each other or with an existing non-TEMPORARY table of the same name. (The existing table is hidden until the temporary table is dropped.) In MySQL 5.0, you must have the CREATE TEMPORARY TABLES privilege to be able to create temporary tables.

MySQL 5.0 also supports the keywords IF NOT EXISTS so that an error does not occur if the table exists. Note that there is no verification that the existing table has a structure identical to that indicated by the CREATE TABLE statement. Note: If you use IF NOT EXISTS in a CREATE TABLE ... SELECT statement, any records selected by the SELECT part are inserted whether or not the table already exists.

MySQL represents each table by an .frm table format (definition) file in the database directory. The storage engine for the table might create other files as well. In the case of MyISAM tables, the storage engine creates data and index files. Thus, for each MyISAM table tbl_name, there are three disk files:

The files created by each storage engine to represent tables are described in Chapter 14, Storage Engines and Table Types.

For general information on the properties of the various column types, see Chapter 11, Column Types. For information about spatial column types, see Chapter 17, Spatial Extensions in MySQL.

The table_options part of the CREATE TABLE syntax can be used in MySQL 3.23 and above.

The ENGINE and TYPE options specify the storage engine for the table. ENGINE is the preferred option name in MySQL 5.0.

The ENGINE and TYPE options take the following values:

For more information about MySQL's storage engines, see Chapter 14, Storage Engines and Table Types.

If a storage engine is specified that is not available, MySQL uses MyISAM instead. For example, if a table definition includes the ENGINE=BDB option but the MySQL server does not support BDB tables, the table is created as a MyISAM table. This makes it possible to have a replication setup where you have transactional tables on the master but tables created on the slave are non-transactional (to get more speed). In MySQL 5.0, a warning occurs if the storage engine specification is not honored.

The other table options are used to optimize the behavior of the table. In most cases, you do not have to specify any of them. These options work for all storage engines unless otherwise indicated:

In MySQL 5.0, you can create one table from another by adding a SELECT statement at the end of the CREATE TABLE statement:

CREATE TABLE new_tbl SELECT * FROM orig_tbl;

MySQL creates new columns for all elements in the SELECT. For example:

mysql> CREATE TABLE test (a INT NOT NULL AUTO_INCREMENT,
    ->        PRIMARY KEY (a), KEY(b))
    ->        TYPE=MyISAM SELECT b,c FROM test2;

This creates a MyISAM table with three columns, a, b, and c. Notice that the columns from the SELECT statement are appended to the right side of the table, not overlapped onto it. Take the following example:

mysql> SELECT * FROM foo;
+---+
| n |
+---+
| 1 |
+---+

mysql> CREATE TABLE bar (m INT) SELECT n FROM foo;
Query OK, 1 row affected (0.02 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM bar;
+------+---+
| m    | n |
+------+---+
| NULL | 1 |
+------+---+
1 row in set (0.00 sec)

For each row in table foo, a row is inserted in bar with the values from foo and default values for the new columns.

In a table resulting from CREATE TABLE ... SELECT, columns named only in the CREATE TABLE part come first. Columns named in both parts or only in the SELECT part come after that. The data type of SELECT columns can be overridden by also specifying the column in the CREATE TABLE part.

If any errors occur while copying the data to the table, it is automatically dropped and not created.

CREATE TABLE ... SELECT does not automatically create any indexes for you. This is done intentionally to make the statement as flexible as possible. If you want to have indexes in the created table, you should specify these before the SELECT statement:

mysql> CREATE TABLE bar (UNIQUE (n)) SELECT n FROM foo;

Some conversion of column types might occur. For example, the AUTO_INCREMENT attribute is not preserved, and VARCHAR columns can become CHAR columns.

When creating a table with CREATE ... SELECT, make sure to alias any function calls or expressions in the query. If you do not, the CREATE statement might fail or result in undesirable column names.

CREATE TABLE artists_and_works
SELECT artist.name, COUNT(work.artist_id) AS number_of_works
FROM artist LEFT JOIN work ON artist.id = work.artist_id
GROUP BY artist.id;

You can also explicitly specify the type for a generated column:

CREATE TABLE foo (a TINYINT NOT NULL) SELECT b+1 AS a FROM bar;

In MySQL 5.0, use LIKE to create an empty table based on the definition of another table, including any column attributes and indexes defined in the original table:

CREATE TABLE new_tbl LIKE orig_tbl;

CREATE TABLE ... LIKE does not copy any DATA DIRECTORY or INDEX DIRECTORY table options that were specified for the original table, or any foreign key definitions.

You can precede the SELECT by IGNORE or REPLACE to indicate how to handle records that duplicate unique key values. With IGNORE, new records that duplicate an existing record on a unique key value are discarded. With REPLACE, new records replace records that have the same unique key value. If neither IGNORE nor REPLACE is specified, duplicate unique key values result in an error.

To ensure that the update log/binary log can be used to re-create the original tables, MySQL does not allow concurrent inserts during CREATE TABLE ... SELECT.

DROP {DATABASE | SCHEMA} [IF EXISTS] db_name

DROP DATABASE drops all tables in the database and deletes the database. Be very careful with this statement! To use DROP DATABASE, you need the DROP privilege on the database.

IF EXISTS is used to prevent an error from occurring if the database does not exist.

DROP SCHEMA can be used as of MySQL 5.0.2.

If you use DROP DATABASE on a symbolically linked database, both the link and the original database are deleted.

DROP DATABASE returns the number of tables that were removed. This corresponds to the number of .frm files removed.

The DROP DATABASE statement removes from the given database directory those files and directories that MySQL itself may create during normal operation:

If other files or directories remain in the database directory after MySQL removes those just listed, the database directory cannot be removed. In this case, you must remove any remaining files or directories manually and issue the DROP DATABASE statement again.

You can also drop databases with mysqladmin. See Section 8.5, “mysqladmin — Client for Administering a MySQL Server”.

DROP INDEX index_name ON tbl_name

DROP INDEX drops the index named index_name from the table tbl_name. In MySQL 5.0, DROP INDEX is mapped to an ALTER TABLE statement to drop the index. See Section 13.1.2, “ALTER TABLE Syntax”.

DROP [TEMPORARY] TABLE [IF EXISTS]
    tbl_name [, tbl_name] ...
    [RESTRICT | CASCADE]

DROP TABLE removes one or more tables. You must have the DROP privilege for each table. All table data and the table definition are removed, so be careful with this statement!

Use IF EXISTS to prevent an error from occurring for tables that do not exist. A NOTE is generated for each non-existent table when using IF EXISTS. See Section 13.5.4.22, “SHOW WARNINGS Syntax”.

RESTRICT and CASCADE are allowed to make porting easier. For the moment, they do nothing.

Note: DROP TABLE automatically commits the current active transaction, unless you use the TEMPORARY keyword.

The TEMPORARY keyword has the following effects:

Using TEMPORARY is a good way to ensure that you do not accidentally drop a non-TEMPORARY table.

RENAME TABLE tbl_name TO new_tbl_name
    [, tbl_name2 TO new_tbl_name2] ...

This statement renames one or more tables.

The rename operation is done atomically, which means that no other thread can access any of the tables while the rename is running. For example, if you have an existing table old_table, you can create another table new_table that has the same structure but is empty, and then replace the existing table with the empty one as follows:

CREATE TABLE new_table (...);
RENAME TABLE old_table TO backup_table, new_table TO old_table;

If the statement renames more than one table, renaming operations are done from left to right. If you want to swap two table names, you can do so like this (assuming that no table named tmp_table already exists):

RENAME TABLE old_table TO tmp_table,
             new_table TO old_table,
             tmp_table TO new_table;

As long as two databases are on the same filesystem you can also rename a table to move it from one database to another:

RENAME TABLE current_db.tbl_name TO other_db.tbl_name;

When you execute RENAME, you cannot have any locked tables or active transactions. You must also have the ALTER and DROP privileges on the original table, and the CREATE and INSERT privileges on the new table.

If MySQL encounters any errors in a multiple-table rename, it does a reverse rename for all renamed tables to return everything to its original state.

As of MySQL 5.0.14, RENAME TABLE works for views as well, as long as you do not try to rename a view into a different database.

Single-table syntax:

DELETE [LOW_PRIORITY] [QUICK] [IGNORE] FROM tbl_name
         [WHERE where_definition]
         [ORDER BY ...]
         [LIMIT row_count]

Multiple-table syntax:

DELETE [LOW_PRIORITY] [QUICK] [IGNORE]
       tbl_name[.*] [, tbl_name[.*] ...]
       FROM table_references
       [WHERE where_definition]

Or:

DELETE [LOW_PRIORITY] [QUICK] [IGNORE]
       FROM tbl_name[.*] [, tbl_name[.*] ...]
       USING table_references
       [WHERE where_definition]

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 statement with no WHERE clause, all rows are deleted. A faster way to do this, when you do not want to know the number of deleted rows, is to use TRUNCATE TABLE. See Section 13.2.9, “TRUNCATE Syntax”.

If you delete the row containing the maximum value for an AUTO_INCREMENT column, the value is reused for a BDB table, but not for a MyISAM or InnoDB table. If you delete all rows in the table with DELETE FROM tbl_name (without a WHERE clause) in AUTOCOMMIT mode, the sequence starts over for all table types except InnoDB and MyISAM. There are some exceptions to this behavior for InnoDB tables, as discussed in Section 15.6.3, “How an AUTO_INCREMENT Column Works in InnoDB”.

For MyISAM and BDB tables, you can specify an AUTO_INCREMENT secondary column in a multiple-column key. In this case, reuse of values deleted from the top of the sequence occurs even for MyISAM tables. See Section 3.6.9, “Using AUTO_INCREMENT”.

The DELETE statement supports the following modifiers:

The speed of delete operations may also be affected by factors discussed in Section 7.2.18, “Speed of DELETE Statements”.

In MyISAM tables, 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 13.5.2.5, “OPTIMIZE TABLE Syntax” and Chapter 7, Optimization.

The QUICK modifier affects whether index leaves are merged for delete operations. DELETE QUICK is most useful for applications where index values for deleted rows are replaced by similar index values from rows inserted later. In this case, the holes left by deleted values are reused.

DELETE QUICK is not useful when deleted values lead to underfilled index blocks spanning a range of index values for which new inserts occur again. In this case, use of QUICK can lead to wasted space in the index that remains unreclaimed. Here is an example of such a scenario:

In this scenario, the index blocks associated with the deleted index values become underfilled but are not merged with other index blocks due to the use of QUICK. They remain underfilled when new inserts occur, because new records does not have index values in the deleted range. Furthermore, they remain underfilled even if you later use DELETE without QUICK, unless some of the deleted index values happen to lie in index blocks within or adjacent to the underfilled blocks. To reclaim unused index space under these circumstances, use OPTIMIZE TABLE.

If you are going to delete many rows from a table, it might be faster to use DELETE QUICK followed by OPTIMIZE TABLE. This rebuilds the index rather than performing many index block merge operations.

The MySQL-specific LIMIT row_count 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 statement does not take too much time. You can simply repeat the DELETE statement until the number of affected rows is less than the LIMIT value.

If the DELETE statement includes an ORDER BY clause, the rows are deleted in the order specified by the clause. This is really useful only in conjunction with LIMIT. For example, the following statement finds rows matching the WHERE clause, sorts them by timestamp_column, and deletes the first (oldest) one:

DELETE FROM somelog
WHERE user = 'jcole'
ORDER BY timestamp_column
LIMIT 1;

You can specify multiple tables in a DELETE statement to delete rows from one or more tables depending on a particular condition in multiple tables. However, you cannot use ORDER BY or LIMIT in a multiple-table DELETE.

The table_references part lists the tables involved in the join. Its syntax is described in Section 13.2.7.1, “JOIN Syntax”.

For the first syntax, only matching rows from the tables listed before the FROM clause are deleted. For the second syntax, only matching rows from the tables listed in the FROM clause (before the USING clause) are deleted. The effect is that you can delete rows from many tables at the same time and have additional tables that are used for searching:

DELETE t1, t2 FROM t1, t2, t3 WHERE t1.id=t2.id AND t2.id=t3.id;

Or:

DELETE FROM t1, t2 USING t1, t2, t3 WHERE t1.id=t2.id AND t2.id=t3.id;

These statements use all three tables when searching for rows to delete, but delete matching rows only from tables t1 and t2.

The examples above show inner joins using the comma operator, but multiple-table DELETE statements can use any type of join allowed in SELECT statements, such as LEFT JOIN.

The syntax allows .* after the table names for compatibility with Access.

If you use a multiple-table DELETE statement involving InnoDB tables for which there are foreign key constraints, the MySQL optimizer might process tables in an order that differs from that of their parent/child relationship. In this case, the statement fails and rolls back. Instead, you should delete from a single table and rely on the ON DELETE capabilities that InnoDB provides to cause the other tables to be modified accordingly.

Note: In MySQL 5.0, you must use the alias (if one was given) when referring to a table name:

DELETE t1 FROM test AS t1, test2 WHERE ...

Cross-database deletes are supported for multiple-table deletes, but in this case, you must refer to the tables without using aliases. For example:

DELETE test1.tmp1, test2.tmp2 FROM test1.tmp1, test2.tmp2 WHERE ...

Currently, you cannot delete from a table and select from the same table in a subquery.

DO expr [, expr] ...

DO executes the expressions but does not return any results. This is shorthand for SELECT expr, ..., but has the advantage that it is slightly faster when you do not care about the result.

DO is useful mainly with functions that have side effects, such as RELEASE_LOCK().

HANDLER tbl_name OPEN [ AS alias ]
HANDLER tbl_name READ index_name { = | >= | <= | < } (value1,value2,...)
    [ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name READ index_name { FIRST | NEXT | PREV | LAST }
    [ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name READ { FIRST | NEXT }
    [ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name CLOSE

The HANDLER statement provides direct access to table storage engine interfaces. In MySQL 5.0, it is available for MyISAM and InnoDB tables.

The HANDLER ... OPEN statement opens a table, making it accessible via subsequent HANDLER ... READ statements. This table object is not shared by other threads and is not closed until the thread calls HANDLER ... CLOSE or the thread terminates. If you open the table using an alias, further references to the table with other HANDLER statements must use the alias rather than the table name.

The first HANDLER ... READ syntax fetches a row where the index specified satisfies the given values and the WHERE condition is met. If you have a multiple-column index, specify the index column values as a comma-separated list. Either specify values for all the columns in the index, or specify values for a leftmost prefix of the index columns. Suppose that an index includes three columns named col_a, col_b, and col_c, in that order. The HANDLER statement can specify values for all three columns in the index, or for the columns in a leftmost prefix. For example:

HANDLER ... index_name = (col_a_val,col_b_val,col_c_val) ...
HANDLER ... index_name = (col_a_val,col_b_val) ...
HANDLER ... index_name = (col_a_val) ...

The second HANDLER ... READ syntax fetches a row from the table in index order that matches the WHERE condition.

The third HANDLER ... READ syntax fetches a row from the table in natural row order that matches the WHERE condition. It is faster than HANDLER tbl_name READ index_name when a full table scan is desired. Natural row order is the order in which rows are stored in a MyISAM table data file. This statement works for InnoDB tables as well, but there is no such concept because there is no separate data file.

Without a LIMIT clause, all forms of HANDLER ... READ fetch a single row if one is available. To return a specific number of rows, include a LIMIT clause. It has the same syntax as for the SELECT statement. See Section 13.2.7, “SELECT Syntax”.

HANDLER ... CLOSE closes a table that was opened with HANDLER ... OPEN.

Note: To employ the HANDLER interface to refer to a table's PRIMARY KEY, use the quoted identifier `PRIMARY`:

HANDLER tbl_name READ `PRIMARY` > (...);

HANDLER is a somewhat low-level statement. For example, it does not provide consistency. That is, HANDLER ... OPEN does not take a snapshot of the table, and does not lock the table. This means that after a HANDLER ... OPEN statement is issued, table data can be modified (by this or any other thread) and these modifications might appear only partially in HANDLER ... NEXT or HANDLER ... PREV scans.

There are several reasons to use the HANDLER interface instead of normal SELECT statements:

INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
    [INTO] tbl_name [(col_name,...)]
    VALUES ({expr | DEFAULT},...),(...),...
    [ ON DUPLICATE KEY UPDATE col_name=expr, ... ]

Or:

INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
    [INTO] tbl_name
    SET col_name={expr | DEFAULT}, ...
    [ ON DUPLICATE KEY UPDATE col_name=expr, ... ]

Or:

INSERT [LOW_PRIORITY | HIGH_PRIORITY] [IGNORE]
    [INTO] tbl_name [(col_name,...)]
    SELECT ...
    [ ON DUPLICATE KEY UPDATE col_name=expr, ... ]

INSERT inserts new rows into an existing table. The INSERT ... VALUES and INSERT ... SET forms of the statement insert rows based on explicitly specified values. The INSERT ... SELECT form inserts rows selected from another table or tables. INSERT ... SELECT is discussed further in Section 13.2.4.1, “INSERT ... SELECT Syntax”.

tbl_name is the table into which rows should be inserted. The columns for which the statement provides values can be specified as follows:

Column values can be given in several ways:

The INSERT statement supports the following modifiers:

If you specify ON DUPLICATE KEY UPDATE, and a row is inserted that would cause a duplicate value in a UNIQUE index or PRIMARY KEY, an UPDATE of the old row is performed. For example, if column a is declared as UNIQUE and contains the value 1, the following two statements have identical effect:

mysql> INSERT INTO table (a,b,c) VALUES (1,2,3)
    -> ON DUPLICATE KEY UPDATE c=c+1;

mysql> UPDATE table SET c=c+1 WHERE a=1;

The rows-affected value is 1 if the row is inserted as a new record and 2 if an existing record is updated.

Note: If column b is also unique, the INSERT would be equivalent to this UPDATE statement instead:

mysql> UPDATE table SET c=c+1 WHERE a=1 OR b=2 LIMIT 1;

If a=1 OR b=2 matches several rows, only one row is updated. In general, you should try to avoid using an ON DUPLICATE KEY clause on tables with multiple unique keys.

MySQL 5.0 permits the use of the VALUES(col_name) function in the UPDATE clause to refer to column values from the INSERT portion of the INSERT ... UPDATE statement. In other words, VALUES(col_name) in the UPDATE clause refers to the value of col_name that would be inserted, had no duplicate-key conflict occurred. This function is especially useful in multiple-row inserts. The VALUES() function is meaningful only in INSERT ... UPDATE statements and returns NULL otherwise.

Example:

mysql> INSERT INTO table (a,b,c) VALUES (1,2,3),(4,5,6)
    -> ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b);

That statement is identical to the following two statements:

mysql> INSERT INTO table (a,b,c) VALUES (1,2,3)
    -> ON DUPLICATE KEY UPDATE c=3;
mysql> INSERT INTO table (a,b,c) VALUES (4,5,6)
    -> ON DUPLICATE KEY UPDATE c=9;

When you use ON DUPLICATE KEY UPDATE, the DELAYED option is ignored.

You can find the value used for an AUTO_INCREMENT column by using the SQL LAST_INSERT_ID() function. From within the C API, use the mysql_insert_id() function. However, you should note that the two functions do not always behave identically. The behavior of INSERT statements with respect to AUTO_INCREMENT columns is discussed further in Section 12.9.3, “Information Functions” and Section 23.2.3.36, “mysql_insert_id()”.

If you use an INSERT ... VALUES statement with multiple value lists or INSERT ... SELECT, the statement returns an information string in this format:

Records: 100 Duplicates: 0 Warnings: 0

Records indicates the number of rows processed by the statement. (This is not necessarily the number of rows actually inserted, since Duplicates can be non-zero.) Duplicates indicates the number of rows that could not 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:

If you are using the C API, the information string can be obtained by invoking the mysql_info() function. See Section 23.2.3.34, “mysql_info()”.

INSERT [LOW_PRIORITY | HIGH_PRIORITY] [IGNORE]
    [INTO] tbl_name [(col_name,...)]
    SELECT ...
    [ ON DUPLICATE KEY UPDATE col_name=expr, ... ]

INSERT INTO tbl_temp2 (fld_id)
    SELECT tbl_temp1.fld_order_id
    FROM tbl_temp1 WHERE tbl_temp1.fld_order_id > 100;

INSERT DELAYED ...

LOAD DATA [LOW_PRIORITY | CONCURRENT] [LOCAL] INFILE 'file_name.txt'
    [REPLACE | IGNORE]
    INTO TABLE tbl_name
    [FIELDS
        [TERMINATED BY 'string']
        [[OPTIONALLY] ENCLOSED BY 'char']
        [ESCAPED BY 'char' ]
    ]
    [LINES
        [STARTING BY 'string']
        [TERMINATED BY 'string']
    ]
    [IGNORE number LINES]
    [(col_name_or_user_var,...)]
    [SET col_name = expr,...)]

The LOAD DATA INFILE statement reads rows from a text file into a table at a very high speed. The filename must be given as a literal string.

For more information about the efficiency of INSERT versus LOAD DATA INFILE and speeding up LOAD DATA INFILE, see Section 7.2.16, “Speed of INSERT Statements”.

In MySQL 5.0, the character set indicated by the character_set_database system variable is used to interpret the information in the file. SET NAMES and the setting of character_set_client do not affect interpretation of input.

Note that it's currently not possible to load UCS2 data files.

You can also load data files by using the mysqlimport utility; it operates by sending a LOAD DATA INFILE statement 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. See Section 8.10, “mysqlimport — A Data Import Program”.

If you use LOW_PRIORITY, execution of the LOAD DATA statement is delayed until no other clients are reading from the table.

If you specify CONCURRENT with a MyISAM table that satisfies the condition for concurrent inserts (that is, it contains no free blocks in the middle), then other threads can retrieve data from the table while LOAD DATA is executing. Using this option affects the performance of LOAD DATA a bit, even if no other thread is using the table at the same time.

If LOCAL is specified, it is interpreted with respect to the client end of the connection:

When locating files on the server host, the server uses the following rules:

Note that these rules mean that a file named as ./myfile.txt is read from the server's data directory, whereas the same file named as myfile.txt is read from the database directory of the default 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;

Note that Windows pathnames are specified using forward slashes rather than backslashes. If you do use backslashes, you must double them.

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.

See Section 5.7.3, “Privileges Provided by MySQL”.

Using LOCAL is a bit slower than letting the server access the files directly, because the contents of the file must be sent over the connection by the client to the server. On the other hand, you do not need the FILE privilege to load local files.

In MySQL 5.0, LOCAL works only if your server and your client both have been enabled to allow it. For example, if mysqld was started with --local-infile=0, then LOCAL does not work. See Section 5.6.4, “Security Issues with LOAD DATA LOCAL”.

If you need LOAD DATA to read from a pipe, you can use the following technique (here we load the listing of the / directory into a table):

mkfifo /mysql/db/x/x
chmod 666 /mysql/db/x/x
find / -ls > /mysql/db/x/x
mysql -e "LOAD DATA INFILE 'x' INTO TABLE x" x

The REPLACE and IGNORE keywords control handling of input records that duplicate existing records on unique key values.

If you specify REPLACE, input rows replace existing rows (in other words, rows that have the same value for a primary or unique index as an existing row). See Section 13.2.6, “REPLACE Syntax”.

If you specify IGNORE, input rows that duplicate an existing row on a unique key value are skipped. If you do not specify either option, the behavior depends on whether or not the LOCAL keyword is specified. Without LOCAL, an error occurs when a duplicate key value is found, and the rest of the text file is ignored. With LOCAL, the default behavior is the same as if IGNORE is specified; this is because the server has no way to stop transmission of the file in the middle of the operation.

If you want to ignore foreign key constraints during the load operation, you can issue a SET FOREIGN_KEY_CHECKS=0 statement before executing LOAD DATA.

If you use LOAD DATA INFILE on an empty MyISAM table, all non-unique indexes are created in a separate batch (as for REPAIR TABLE). This normally makes LOAD DATA INFILE much faster when you have many indexes. Normally this is very fast, but in some extreme cases, you can create the indexes even faster by turning them off with ALTER TABLE ... DISABLE KEYS before loading the file into the table and using ALTER TABLE ... ENABLE KEYS to re-create the indexes after loading the file. See Section 7.2.16, “Speed of INSERT Statements”.

LOAD DATA INFILE is the complement of SELECT ... INTO OUTFILE. (See Section 13.2.7, “SELECT Syntax”.) To write data from a table to a file, use SELECT ... INTO OUTFILE. To read the file back into a table, use LOAD DATA INFILE. The syntax of the FIELDS and LINES clauses is the same for both statements. 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 do not 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 do not specify a LINES clause, the default is the same as if you had written this:

LINES TERMINATED BY '\n' STARTING BY ''

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.

Note: If you have generated the text file on a Windows system, you might have to use LINES TERMINATED BY '\r\n' to read the file properly, because Windows programs typically use two characters as a line terminator. Some programs, such as WordPad, might use \r as a line terminator when writing files. To read such files, use LINES TERMINATED BY '\r'.

If all the lines you want to read in have a common prefix that you want to ignore, you can use LINES STARTING BY 'prefix_string' to skip over the prefix (and anything before it). If a line does not include the prefix, the entire line is skipped. Note: prefix_string may occur in the middle of a line.

Example:

mysql> LOAD DATA INFILE '/tmp/test.txt'
    -> INTO TABLE test LINES STARTING BY "xxx";

With this you can read in a file that contains something like:

xxx"row",1
something xxx"row",2

And just get the data ("row",1) and ("row",2).

The IGNORE number LINES option can be used to ignore lines at the start of the file. For example, you can use IGNORE 1 LINES to skip over an initial header line containing column names:

mysql> LOAD DATA INFILE '/tmp/test.txt'
    -> 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 statements must match. Otherwise, LOAD DATA INFILE does not interpret the contents of the file properly. Suppose that you use SELECT ... INTO OUTFILE to write a file with fields delimited by commas:

mysql> SELECT * INTO OUTFILE 'data.txt'
    ->          FIELDS TERMINATED BY ','
    ->          FROM table2;

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 here, 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 has fields separated by commas and enclosed within double quotes. If lines in the file are terminated by newlines, the statement shown here 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 can 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, LINES STARTING BY, and LINES TERMINATED BY values can 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.

To read a file containing jokes that are separated by lines consisting of %%, you can do this

mysql> CREATE TABLE jokes
    ->     (a INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    ->     joke TEXT NOT NULL);
mysql> 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 here:

"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 values in columns that have a string data type (such as CHAR, BINARY, TEXT, or ENUM):

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 preceding output just shown would appear as follows 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.

If the field begins with the ENCLOSED BY character, instances of that character are recognized as terminating a field value only if followed by the field or line TERMINATED BY sequence. To avoid ambiguity, occurrences of the ENCLOSED BY character within a field value can be doubled and are interpreted as a single instance of the character. For example, if ENCLOSED BY '"' is specified, quotes are handled as shown here:

"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 and NULL is output as NULL, not \N. 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 NUL (a zero-valued byte) and NULL. The rules for NULL handling are described later in this section.

For more information about ‘\’-escape syntax, see Section 9.1, “Literal Values”.

In certain cases, field- and line-handling options interact:

Handling of NULL values varies according to the FIELDS and LINES options in 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;

By default, when no column list is provided at the end of the LOAD DATA INFILE statement, input lines are expected to contain a field for each table column. If you want to load only some of a table's columns, specify a column list:

mysql> LOAD DATA INFILE 'persondata.txt'
    ->           INTO TABLE persondata (col1,col2,...);

You must also specify a column 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.

Before MySQL 5.0.3, the column list must contain only names of columns in the table being loaded, and the SET clause is not supported. As of MySQL 5.0.3, the column list can contain either column names or user variables, and the SET clause is supported. This enables you to assign input values to user variables, and then perform transformations on those values before assigning the result to columns.

User variables in the SET clause can be used in several ways. The following example uses the first column in the data file directly for the value of t1.column1, and assigns the second column to a user variable that is subjected to a division operation before being used for the value of t2.column2:

LOAD DATA INFILE 'file.txt'
  INTO TABLE t1
  (column1, @var1)
  SET column2 = @var1/100;

The SET clause can be used to supply values not derived from the input file. THe following statement sets column3 to the current date and time:

LOAD DATA INFILE 'file.txt'
  INTO TABLE t1
  (column1, column2)
  SET column3 = CURRENT_TIMESTAMP;

You can also discard an input value by assigning it to a user variable and not assigning the variable to a table column:

LOAD DATA INFILE 'file.txt'
  INTO TABLE t1
  (column1, @dummy, column2, @dummy, column3);

Use of the column/variable list and SET clause is subject to the following restrictions:

When processing an input line, LOAD DATA splits it into fields and uses the values according to the column/variable list and the SET clause, if they are present. Then the resulting row is inserted into the table. If there are BEFORE INSERT or AFTER INSERT triggers for the table, they are activated before or after inserting the row, respectively.

If an input line has too many fields, the extra fields are ignored and the number of warnings is incremented.

If an input line has too few fields, the table columns for which input fields are missing are set to their default values. Default value assignment is described in Section 13.1.5, “CREATE TABLE Syntax”.

An empty field value is interpreted differently than if the field value is missing:

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 set to the current date and time only if there is a NULL value for the column (that is, \N), or (for the first TIMESTAMP column only) if the TIMESTAMP column is omitted from the field list when a field list is specified.

LOAD DATA INFILE regards all input as strings, so you cannot 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.

When the LOAD DATA INFILE statement finishes, it returns an information string in the following format:

Records: 1  Deleted: 0  Skipped: 0  Warnings: 0

If you are using the C API, you can get information about the statement by calling the mysql_info() function. See Section 23.2.3.34, “mysql_info()”.

Warnings occur under the same circumstances as when values are inserted via the INSERT statement (see Section 13.2.4, “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 be used only as an indication of whether everything went well.

In MySQL 5.0, you can use SHOW WARNINGS to get a list of the first max_error_count warnings as information about what went wrong. See Section 13.5.4.22, “SHOW WARNINGS Syntax”.

REPLACE [LOW_PRIORITY | DELAYED]
    [INTO] tbl_name [(col_name,...)]
    VALUES ({expr | DEFAULT},...),(...),...

Or:

REPLACE [LOW_PRIORITY | DELAYED]
    [INTO] tbl_name
    SET col_name={expr | DEFAULT}, ...

Or:

REPLACE [LOW_PRIORITY | DELAYED]
    [INTO] tbl_name [(col_name,...)]
    SELECT ...

REPLACE works exactly like INSERT, except that if an old record in the table has the same value as a new record for a PRIMARY KEY or a UNIQUE index, the old record is deleted before the new record is inserted. See Section 13.2.4, “INSERT Syntax”.

Note that unless the table has a PRIMARY KEY or UNIQUE index, using a REPLACE statement makes no sense. It becomes equivalent to INSERT, because there is no index to be used to determine whether a new row duplicates another.

Values for all columns are taken from the values specified in the REPLACE statement. Any missing columns are set to their default values, just as happens for INSERT. You cannot refer to values from the current row and use them in the new row. If you use an assignment such as SET col_name = col_name + 1, the reference to the column name on the right hand side is treated as DEFAULT(col_name), so the assignment is equivalent to SET col_name = DEFAULT(col_name) + 1.

To be able to use REPLACE, you must have both the INSERT and DELETE privileges for the table.

The REPLACE statement returns a count to indicate the number of rows affected. This is the sum of the rows deleted and inserted. If the count is 1 for a single-row REPLACE, a row was inserted and no rows were deleted. If the count is greater than 1, one or more old rows were deleted before the new row was inserted. It is possible for a single row to replace more than one old row if the table contains multiple unique indexes and the new row duplicates values for different old rows in different unique indexes.

The affected-rows count makes it easy to determine whether REPLACE only added a row or whether it also replaced any rows: Check whether the count is 1 (added) or greater (replaced).

If you are using the C API, the affected-rows count can be obtained using the mysql_affected_rows() function.

Currently, you cannot replace into a table and select from the same table in a subquery.

Here follows in more detail the algorithm that is used (it is also used with LOAD DATA ... REPLACE):

SELECT
    [ALL | DISTINCT | DISTINCTROW ]
      [HIGH_PRIORITY]
      [STRAIGHT_JOIN]
      [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
      [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
    select_expr, ...
    [INTO OUTFILE 'file_name' export_options
      | INTO DUMPFILE 'file_name']
    [FROM table_references
      [WHERE where_definition]
      [GROUP BY {col_name | expr | position}
        [ASC | DESC], ... [WITH ROLLUP]]
      [HAVING where_definition]
      [ORDER BY {col_name | expr | position}
        [ASC | DESC] , ...]
      [LIMIT {[offset,] row_count | row_count OFFSET offset}]
      [PROCEDURE procedure_name(argument_list)]
      [FOR UPDATE | LOCK IN SHARE MODE]]

SELECT is used to retrieve rows selected from one or more tables. MySQL 5.0 includes support for UNION statements and subqueries. See Section 13.2.7.2, “UNION Syntax” and Section 13.2.8, “Subquery Syntax”.

SELECT can also be used to retrieve rows computed without reference to any table.

For example:

mysql> SELECT 1 + 1;
        -> 2

All clauses used must be given in exactly the order shown in the syntax description. For example, a HAVING clause must come after any GROUP BY clause and before any ORDER BY clause.

Following the SELECT keyword, you can use a number of options that affect the operation of the statement.

The ALL, DISTINCT, and DISTINCTROW options specify whether duplicate rows should be returned. If none of these options are given, the default is ALL (all matching rows are returned). DISTINCT and DISTINCTROW are synonyms and specify that duplicate rows in the result set should be removed.

HIGH_PRIORITY, STRAIGHT_JOIN, and options beginning with SQL_ are MySQL extensions to standard SQL.

table_references:
    table_reference [, table_reference] …

table_reference:
    table_factor
  | join_table

table_factor:
    tbl_name [[AS] alias]
        [{USE|IGNORE|FORCE} INDEX (key_list)]
  | ( table_references )
  | { OJ table_reference LEFT OUTER JOIN table_reference
        ON conditional_expr }

join_table:
    table_reference [INNER | CROSS] JOIN table_factor [join_condition]
  | table_reference STRAIGHT_JOIN table_factor
  | table_reference STRAIGHT_JOIN table_factor ON condition
  | table_reference LEFT [OUTER] JOIN table_reference join_condition
  | table_reference NATURAL [LEFT [OUTER]] JOIN table_factor
  | table_reference RIGHT [OUTER] JOIN table_reference join_condition
  | table_reference NATURAL [RIGHT [OUTER]] JOIN table_factor

join_condition:
    ON conditional_expr
  | USING (column_list)

SELECT * FROM t1 LEFT JOIN (t2, t3, t4)
                 ON (t2.a=t1.a AND t3.b=t1.b AND t4.c=t1.c)

SELECT * FROM t1 LEFT JOIN (t2 CROSS JOIN t3 CROSS JOIN t4)
                 ON (t2.a=t1.a AND t3.b=t1.b AND t4.c=t1.c)

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;

SELECT ...
UNION [ALL | DISTINCT]
SELECT ...
[UNION [ALL | DISTINCT]
SELECT ...]

(SELECT a FROM tbl_name WHERE a=10 AND B=1)
UNION
(SELECT a FROM tbl_name WHERE a=11 AND B=2)
ORDER BY a LIMIT 10;

(SELECT a AS b FROM t) UNION (SELECT ...) ORDER BY b;
(SELECT a AS b FROM t) UNION (SELECT ...) ORDER BY a;

(SELECT a FROM tbl_name WHERE a=10 AND B=1 ORDER BY a LIMIT 10)
UNION
(SELECT a FROM tbl_name WHERE a=11 AND B=2 ORDER BY a LIMIT 10);

mysql> SELECT REPEAT('a',1) UNION SELECT REPEAT('b',10);
+---------------+
| REPEAT('a',1) |
+---------------+
| a             |
| bbbbbbbbbb    |
+---------------+

A subquery is a SELECT statement within another statement.

MySQL 5.0 supports all subquery forms and operations that the SQL standard requires, as well as a few features that are MySQL-specific.

Here is an example of a subquery:

SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2);

In this example, SELECT * FROM t1 ... is the outer query (or outer statement), and (SELECT column1 FROM t2) is the subquery. We say that the subquery is nested within the outer query, and in fact it is possible to nest subqueries within other subqueries, to a considerable depth. A subquery must always appear within parentheses.

The main advantages of subqueries are:

Here is an example statement that shows the major points about subquery syntax as specified by the SQL standard and supported in MySQL:

DELETE FROM t1
WHERE s11 > ANY
(SELECT COUNT(*) /* no hint */ FROM t2
WHERE NOT EXISTS
(SELECT * FROM t3
WHERE ROW(5*t2.s1,77)=
(SELECT 50,11*s1 FROM t4 UNION SELECT 50,77 FROM
(SELECT * FROM t5) AS t5)));

A subquery can return a scalar (a single value), a single row, a single column, or a table (one or more rows of one or more columns). These are called scalar, column, row, and table subqueries. Subqueries that return a particular kind of result often can be used only in certain contexts, as described in the following sections.

There are few restrictions on the type of statements in which subqueries can be used. A subquery can contain any of the keywords or clauses that an ordinary SELECT can contain: DISTINCT, GROUP BY, ORDER BY, LIMIT, joins, index hints, UNION constructs, comments, functions, and so on.

One restriction is that a subquery's outer statement must be one of: SELECT, INSERT, UPDATE, DELETE, SET, or DO. Another restriction is that currently you cannot modify a table and select from the same table in a subquery. This applies to statements such as DELETE, INSERT, REPLACE, and UPDATE. A more comprehensive discussion of restrictions on subquery use is given in Appendix I, Feature Restrictions.

CREATE TABLE t1 (s1 INT, s2 CHAR(5) NOT NULL);
INSERT INTO t1 VALUES(100, 'abcde');
SELECT (SELECT s2 FROM t1);

CREATE TABLE t1 (s1 INT);
INSERT INTO t1 VALUES (1);
CREATE TABLE t2 (s1 INT);
INSERT INTO t2 VALUES (2);

SELECT (SELECT s1 FROM t2) FROM t1;

SELECT UPPER((SELECT s1 FROM t1)) FROM t2;

non_subquery_operand comparison_operator (subquery)

=  >  <  >=  <=  <>

  ... 'a' = (SELECT column1 FROM t1)

SELECT column1 FROM t1
WHERE column1 = (SELECT MAX(column2) FROM t2);

SELECT * FROM t1 AS t
WHERE 2 = (SELECT COUNT(*) FROM t1 WHERE t1.id = t.id);

operand comparison_operator ANY (subquery)
operand IN (subquery)
operand comparison_operator SOME (subquery)

SELECT s1 FROM t1 WHERE s1 > ANY (SELECT s1 FROM t2);

SELECT s1 FROM t1 WHERE s1 = ANY (SELECT s1 FROM t2);
SELECT s1 FROM t1 WHERE s1 IN    (SELECT s1 FROM t2);

SELECT s1 FROM t1 WHERE s1 <> ANY  (SELECT s1 FROM t2);
SELECT s1 FROM t1 WHERE s1 <> SOME (SELECT s1 FROM t2);

operand comparison_operator ALL (subquery)

SELECT s1 FROM t1 WHERE s1 > ALL (SELECT s1 FROM t2);

SELECT * FROM t1 WHERE 1 > ALL (SELECT s1 FROM t2);

SELECT * FROM t1 WHERE 1 > (SELECT s1 FROM t2);

SELECT * FROM t1 WHERE 1 > ALL (SELECT MAX(s1) FROM t2);

SELECT s1 FROM t1 WHERE s1 <> ALL (SELECT s1 FROM t2);
SELECT s1 FROM t1 WHERE s1 NOT IN (SELECT s1 FROM t2);

SELECT * FROM t1 WHERE (1,2) = (SELECT column1, column2 FROM t2);
SELECT * FROM t1 WHERE ROW(1,2) = (SELECT column1, column2 FROM t2);

  SELECT * FROM t1 WHERE (column1,column2) = (1,1);
  SELECT * FROM t1 WHERE column1 = 1 AND column2 = 1;

SELECT column1,column2,column3
FROM t1
WHERE (column1,column2,column3) IN
(SELECT column1,column2,column3 FROM t2);

SELECT column1 FROM t1 WHERE EXISTS (SELECT * FROM t2);

SELECT * FROM t1 WHERE column1 = ANY
(SELECT column1 FROM t2 WHERE t2.column2 = t1.column2);

SELECT column1 FROM t1 AS x
WHERE x.column1 = (SELECT column1 FROM t2 AS x
WHERE x.column1 = (SELECT column1 FROM t3
WHERE x.column2 = t3.column1));

val IN (SELECT key_val FROM tbl_name WHERE correlated_condition)

SELECT ... FROM (subquery) [AS] name ...

CREATE TABLE t1 (s1 INT, s2 CHAR(5), s3 FLOAT);

INSERT INTO t1 VALUES (1,'1',1.0);
INSERT INTO t1 VALUES (2,'2',2.0);
SELECT sb1,sb2,sb3
FROM (SELECT s1 AS sb1, s2 AS sb2, s3*2 AS sb3 FROM t1) AS sb
WHERE sb1 > 1;

SELECT AVG(SUM(column1)) FROM t1 GROUP BY column1;

SELECT AVG(sum_column1)
FROM (SELECT SUM(column1) AS sum_column1
FROM t1 GROUP BY column1) AS t1;

SELECT * FROM t1 WHERE id IN (SELECT id FROM t2);

SELECT DISTINCT t1.* FROM t1, t2 WHERE t1.id=t2.id;

SELECT * FROM t1 WHERE id NOT IN (SELECT id FROM t2);
SELECT * FROM t1 WHERE NOT EXISTS (SELECT id FROM t2 WHERE t1.id=t2.id);

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

TRUNCATE [TABLE] tbl_name

TRUNCATE TABLE empties a table completely. Logically, this is equivalent to a DELETE statement that deletes all rows, but there are practical differences under some circumstances.

For InnoDB before version 5.0.3, TRUNCATE TABLE is mapped to DELETE, so there is no difference. Starting with MySQL 5.0.3, fast TRUNCATE TABLE is available. However, the operation is still mapped to DELETE if there are foreign key constraints that reference the table. (When fast truncate is used, it resets any AUTO_INCREMENT counter. From MySQL 5.0.13 on, the AUTO_INCREMENT counter is reset by TRUNCATE TABLE, regardless of whether there is a foreign key constraint.)

For other storage engines, TRUNCATE TABLE differs from DELETE FROM in the following ways in MySQL 5.0:

TRUNCATE TABLE is an Oracle SQL extension adopted in MySQL.

Single-table syntax:

UPDATE [LOW_PRIORITY] [IGNORE] tbl_name
    SET col_name1=expr1 [, col_name2=expr2 ...]
    [WHERE where_definition]
    [ORDER BY ...]
    [LIMIT row_count]

Multiple-table syntax:

UPDATE [LOW_PRIORITY] [IGNORE] table_references
    SET col_name1=expr1 [, col_name2=expr2 ...]
    [WHERE where_definition]

The UPDATE statement 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 are updated in the order that is specified. The LIMIT clause places a limit on the number of rows that can be updated.

The UPDATE statement supports the following modifiers:

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 does not update it.

If you update a column that has been declared NOT NULL by setting to NULL, the column is set to the default value appropriate for the column type and the warning count is incremented. The default value is 0 for numeric types, the empty string ('') for string types, and the “zero” value for date and time types.

UPDATE returns the number of rows that were actually changed. In MySQL 5.0, the mysql_info() C API function returns the number of rows that were matched and updated and the number of warnings that occurred during the UPDATE.

You can use LIMIT row_count to restrict the scope of the UPDATE. A LIMIT clause is a rows-matched restriction. The statement stops as soon as it has found row_count rows that satisfy the WHERE clause, whether or not they actually were changed.

If an UPDATE statement includes an ORDER BY clause, the rows are updated in the order specified by the clause.

You can also perform UPDATE operations covering multiple tables. The table_references clause lists the tables involved in the join. Its syntax is described in Section 13.2.7.1, “JOIN Syntax”. Here is an example:

UPDATE items,month SET items.price=month.price
WHERE items.id=month.id;

The example above shows an inner join using the comma operator, but multiple-table UPDATE statements can use any type of join allowed in SELECT statements, such as LEFT JOIN.

Note: You cannot use ORDER BY or LIMIT with a multiple-table UPDATE.

In MySQL 5.0, you need the UPDATE privilege only for columns referenced in a multiple-table UPDATE that are actually updated. You need only the SELECT privilege for any columns that are read but not modified.

If you use a multiple-table UPDATE statement involving InnoDB tables for which there are foreign key constraints, the MySQL optimizer might process tables in an order that differs from that of their parent/child relationship. In this case, the statement fails and rolls back. Instead, update a single table and rely on the ON UPDATE capabilities that InnoDB provides to cause the other tables to be modified accordingly. See Section 15.6.4, “FOREIGN KEY Constraints”.

Currently, you cannot update a table and select from the same table in a subquery.

{DESCRIBE | DESC} tbl_name [col_name | wild]

DESCRIBE provides information about the columns in a table. It is a shortcut for SHOW COLUMNS FROM. As of MySQL 5.0.1, these statements also display information for views.

See Section 13.5.4.3, “SHOW COLUMNS Syntax”.

col_name can be a column name, or a string containing the SQL ‘%’ and ‘_’ wildcard characters to obtain output only for the columns with names matching the string. There is no need to enclose the string in quotes unless it contains spaces or other special characters.

mysql> DESCRIBE city;
+------------+----------+------+-----+---------+----------------+
| Field      | Type     | Null | Key | Default | Extra          |
+------------+----------+------+-----+---------+----------------+
| Id         | int(11)  | NO   | PRI | NULL    | auto_increment |
| Name       | char(35) | NO   |     |         |                |
| Country    | char(3)  | NO   | UNI |         |                |
| District   | char(20) | YES  | MUL |         |                |
| Population | int(11)  | NO   |     | 0       |                |
+------------+----------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

The Null field indicates whether NULL values can be stored in the column.

The Key field indicates whether the column is indexed. A value of PRI indicates that the column is part of the table's primary key. UNI indicates that the column is part of a UNIQUE index. The MUL value indicates that multiple occurrences of a given value are allowed within the column.

One reason for MUL to be displayed on a UNIQUE index is that several columns form a composite UNIQUE index; although the combination of the columns is unique, each column can still hold multiple occurrences of a given value. Note that in a composite index, only the leftmost column of the index has an entry in the Key field.

Before MySQL 5.0.11, if the column allows NULL values, the Key value can be MUL even when a UNIQUE index is used. The rationale was that multiple rows in a UNIQUE index can hold a NULL value if the column is not declared NOT NULL. As of MySQL 5.0.11, the display is UNI rather than MUL regardless of whether the column allows NULL; you can see from the Null field whether or not the column can contain NULL.

The Default field indicates the default value that is assigned to the column.

The Extra field contains any additional information that is available about a given column. In our example, the Extra field indicates that the Id column was created with the AUTO_INCREMENT keyword.

If the column types are different from what you expect them to be based on a CREATE TABLE statement, note that MySQL sometimes changes column types. See Section 13.1.5.1, “Silent Column Specification Changes”.

The DESCRIBE statement is provided for compatibility with Oracle.

The SHOW CREATE TABLE and SHOW TABLE STATUS statements also provide information about tables. See Section 13.5.4, “SHOW Syntax”.

USE db_name

The USE db_name statement tells MySQL to use the db_name database as the default (current) database for subsequent statements. The database remains the default until the end of the session or until a different 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 following example 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 compatibility with Sybase.

START TRANSACTION | BEGIN [WORK]
COMMIT [WORK] [AND [NO] CHAIN] [[NO] RELEASE]
ROLLBACK [WORK] [AND [NO] CHAIN] [[NO] RELEASE]
SET AUTOCOMMIT = {0 | 1}

The START TRANSACTION or BEGIN statement begin a new transaction. COMMIT commits the current transaction, making its changes permanent. ROLLBACK rolls back the current transaction, canceling its changes. The SET AUTOCOMMIT statement disables or enables the default autocommit mode for the current connection.

Beginning with MySQL 5.0.3, the optional WORK keyword is supported for COMMIT and RELEASE, as are the CHAIN and RELEASE clauses. CHAIN and RELEASE can be used for additional control over transaction completion. The value of the completion_type system variable determines the default completion behavior. See Section 5.3.3, “Server System Variables”.

The AND CHAIN clause causes a new transaction to begin as soon as the current one end, and the new transaction has the same isolation level as the just-terminated transaction. The RELEASE clause causes the server to disconnect the current client connection after terminating the current transaction. Including the NO keyword suppresses CHAIN or RELEASE completion, which can be useful if the completion_type system variable is set to cause chaining or release completion by default.

By default, MySQL runs with autocommit mode enabled. This means that as soon as you execute a statement that updates (modifies) a table, MySQL stores the update on disk.

If you are using a transaction-safe storage engine (like InnoDB, BDB or NDB Cluster), you can disable autocommit mode with the following statement:

SET AUTOCOMMIT=0;

After disabling autocommit mode by setting the AUTOCOMMIT variable to zero, 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 disable autocommit mode for a single series of statements, you can use the START TRANSACTION statement:

START TRANSACTION;
SELECT @A:=SUM(salary) FROM table1 WHERE type=1;
UPDATE table2 SET summary=@A WHERE type=1;
COMMIT;

With START TRANSACTION, autocommit remains disabled until you end the transaction with COMMIT or ROLLBACK. The autocommit mode then reverts to its previous state.

BEGIN and BEGIN WORK are supported as aliases of START TRANSACTION for initiating a transaction. START TRANSACTION is standard SQL syntax and is the recommended way to start an ad-hoc transaction. The BEGIN statement differs from the use of the BEGIN keyword that starts a BEGIN ... END compound statement. The latter does not begin a transaction. See Section 18.2.7, “BEGIN ... END Compound Statement”.

You can also begin a transaction like this:

START TRANSACTION WITH CONSISTENT SNAPSHOT;

The WITH CONSISTENT SNAPSHOT clause starts a consistent read for storage engines that are capable of it. Currently, this applies only to InnoDB. The effect is the same as issuing a START TRANSACTION followed by a SELECT from any InnoDB table. See Section 15.10.4, “Consistent Non-Locking Read”.

Beginning a transaction causes an implicit UNLOCK TABLES to be performed.

For best results, transactions should be performed using only tables managed by a single transactional storage engine. Otherwise, the following problems can occur:

Each transaction is stored in the binary log in one chunk, upon COMMIT. Transactions that are rolled back are not logged. (Exception: Modifications to non-transactional tables cannot be rolled back. If a transaction that is rolled back includes modifications to non-transactional tables, the entire transaction is logged with a ROLLBACK statement at the end to ensure that the modifications to those tables are replicated.) See Section 5.11.3, “The Binary Log”.

You can change the isolation level for transactions with SET TRANSACTION ISOLATION LEVEL. See Section 13.4.6, “SET TRANSACTION Syntax”.

Rolling back can be a slow operation that may occur without the user having explicitly asked for it (for example, when an error occurs). Because of this, SHOW PROCESSLIST in MySQL 5.0 displays Rolling back in the State column for the connection during implicit and explicit (ROLLBACK SQL command) rollbacks.

Some statements cannot be rolled back. In general, these include data definition language (DDL) statements, such as those that create or drop databases, those that create, drop, or alter tables or stored routines.

You should design your transactions not to include such statements. If you issue a statement early in a transaction that cannot be rolled back, and then another statement later fails, the full effect of the transaction cannot be rolled back in such cases by issuing a ROLLBACK statement.

Each of the following statements (and any synonyms for them) implicitly end a transaction, as if you had done a COMMIT before executing the statement:

Transactions cannot be nested. This is a consequence of the implicit COMMIT performed for any current transaction when you issue a START TRANSACTION statement or one of its synonyms.

SAVEPOINT identifier
ROLLBACK [WORK] TO SAVEPOINT identifier
RELEASE SAVEPOINT identifier

In MySQL 5.0, InnoDB supports the SQL statements SAVEPOINT and ROLLBACK TO SAVEPOINT. Starting from MySQL 5.0.3, RELEASE SAVEPOINT and the optional WORK keyword for ROLLBACK are supported as well.

The SAVEPOINT statement sets a named transaction save point with a name of identifier. If the current transaction has a save point with the same name, the old save point is deleted and a new one is set.

The ROLLBACK TO SAVEPOINT statement rolls back a transaction to the named save point. Modifications that the current transaction made to rows after the save point was set are undone in the rollback, but InnoDB does not release the row locks that were stored in memory after the save point. (Note that for a new inserted row, the lock information is carried by the transaction ID stored in the row; the lock is not separately stored in memory. In this case, the row lock is released in the undo.) Savepoints that were set at a later time than the named save point are deleted.

If the statement returns the following error, it means that no save point with the specified name exists:

ERROR 1181: Got error 153 during ROLLBACK

The RELEASE SAVEPOINT statement removes the named save point from the set of save points of the current transaction. No commit or rollback occurs. It is an error if the save point does not exist.

All save points of the current transaction are deleted if you execute a COMMIT, or a ROLLBACK that does not name a save point.

LOCK TABLES
    tbl_name [AS alias] {READ [LOCAL] | [LOW_PRIORITY] WRITE}
    [, tbl_name [AS alias] {READ [LOCAL] | [LOW_PRIORITY] WRITE}] ...
UNLOCK TABLES

LOCK TABLES locks tables for the current thread. If any of the tables are locked by other threads, it blocks until all locks can be acquired. UNLOCK TABLES releases any locks held by the current thread. All tables that are locked by the current thread are implicitly unlocked when the thread issues another LOCK TABLES, or when the connection to the server is closed.

A table lock protects only against inappropriate reads or writes by other clients. The client holding the lock, even a read lock, can perform table-level operations such as DROP TABLE.

Note the following regarding the use of LOCK TABLES with transactional tables:

To use LOCK TABLES in MySQL 5.0, you must have the LOCK TABLES privilege and the SELECT privilege for the involved tables.

The main reasons to use LOCK TABLES are for emulating transactions or to get 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, only the thread holding the lock can write to the table. Other threads are blocked from doing so until the lock has been released.

The difference between READ LOCAL and READ is that READ LOCAL allows non-conflicting INSERT statements (concurrent inserts) to execute while the lock is held. However, this cannot be used if you are going to manipulate the database files outside MySQL while you hold the lock. For InnoDB tables, READ LOCAL is the same as READ as of MySQL 5.0.13. (Before that, READ LOCAL essentially does nothing: It does not lock the table at all, so for InnoDB tables, the use of READ LOCAL is deprecated because a plain consistent-read SELECT does the same thing, and no locks are needed.)

When you use LOCK TABLES, you must lock all tables that you are going to use in your queries. While the locks obtained with a LOCK TABLES statement are in effect, you cannot access any tables that were not locked by the statement. Also, you cannot use a locked table multiple times in a single query — use aliases instead, in which case you must obtain a lock for each alias separately.

mysql> LOCK TABLE t WRITE, t AS t1 WRITE;
mysql> INSERT INTO t SELECT * FROM t;
ERROR 1100: Table 't' was not locked with LOCK TABLES
mysql> INSERT INTO t SELECT * FROM t AS t1;

If your queries refer to a table using an alias, then you must lock the table using that same alias. It does not work to lock the table without specifying the alias:

mysql> LOCK TABLE t READ;
mysql> SELECT * FROM t AS myalias;
ERROR 1100: Table 'myalias' was not locked with LOCK TABLES

Conversely, if you lock a table using an alias, you must refer to it in your queries using that alias:

mysql> LOCK TABLE t AS myalias READ;
mysql> SELECT * FROM t;
ERROR 1100: Table 't' was not locked with LOCK TABLES
mysql> SELECT * FROM t AS myalias;

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 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 use LOW_PRIORITY WRITE locks only if you are sure that eventually there will be a time when no threads have a READ lock.

LOCK TABLES works as follows:

This policy ensures that table locking is deadlock free. There are, however, other things you need to be aware of about this policy:

If you are using a LOW_PRIORITY WRITE lock for a table, it means only that MySQL waits for this particular lock until there are no threads that want a READ lock. When the thread has gotten the WRITE lock and is waiting to get the lock for the next table in the lock table list, all other threads 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 transaction-safe tables.

You can safely use KILL to terminate a thread that is waiting for a table lock. See Section 13.5.5.3, “KILL Syntax”.

Note that you should not lock any tables that you are using with INSERT DELAYED because in that case the INSERT is performed by a separate thread.

Normally, you do not need to lock tables, because all single UPDATE statements are atomic; no other thread can interfere with any other currently executing SQL statement. However, there are a few cases when locking tables may provide an advantage:

You can avoid using LOCK TABLES in many cases by using relative updates (UPDATE customer SET value=value+new_value) or the LAST_INSERT_ID() function, See Section 1.8.5.3, “Transactions and Atomic Operations”.

You can also avoid locking tables in some cases by using the user-level advisory 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 12.9.4, “Miscellaneous Functions”.

See Section 7.3.1, “Locking Methods”, for more information on locking policy.

You can lock all tables in all databases with read locks with the FLUSH TABLES WITH READ LOCK statement. See Section 13.5.5.2, “FLUSH Syntax”. This is a very convenient way to get backups if you have a filesystem such as Veritas that can take snapshots in time.

Note: If you use ALTER TABLE on a locked table, it may become unlocked. See Section A.7.1, “Problems with ALTER TABLE”.

SET [GLOBAL | SESSION] TRANSACTION ISOLATION LEVEL
{ READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE }

This statement sets the transaction isolation level for the next transaction, globally, or for the current session.

The default behavior of SET TRANSACTION is to set the isolation level for the next (not yet started) transaction. If you use the GLOBAL keyword, the statement sets the default transaction level globally for all new connections created from that point on. Existing connections are unaffected. You need the SUPER privilege to do this. Using the SESSION keyword sets the default transaction level for all future transactions performed on the current connection.

For descriptions of each InnoDB transaction isolation level, see Section 15.10.3, “InnoDB and TRANSACTION ISOLATION LEVEL”. InnoDB supports each of these levels in MySQL 5.0. The default level is REPEATABLE READ.

You can set the initial default global isolation level for mysqld with the --transaction-isolation option. See Section 5.3.1, “mysqld Command-Line Options”.

MySQL 5.0.3 and up provides server-side support for XA transactions. Currently, this support is available for the InnoDB storage engine. The MySQL XA implementation is based on the X/Open CAE document Distributed Transaction Processing: The XA Specification. This document is published by The Open Group and available at http://www.opengroup.org/public/pubs/catalog/c193.htm. Limitations of the current XA implementation are described in Section I.5, “Restrictions on XA Transactions”.

On the client side, there are no special requirements. The XA interface to a MySQL server consists of SQL statements that begin with the XA keyword. MySQL client programs must be able to send SQL statements and to understand the semantics of the XA statement interface, but they need not be linked against a client library from MySQL 5.0.3 or newer.

Currently, among the MySQL Connectors, MySQL Connector/J 5.0.0 supports XA directly (that is, through a class interface that handles the XA SQL statement interface for you).

XA supports distributed transactions, that is, the ability to allow multiple separate transactional resources to participate in a global transaction. Transactional resources often are RDBMSs but may be other kinds of resources.

A global transaction involves several actions that are transactional in themselves, but that all must either complete successfully as a group, or all be rolled back as a group. In essence, this extends ACID properties “up a level” so that multiple ACID transactions can be executed in concert as components of a global operation that also has ACID properties. (However, for a distributed transaction, you mus use the SERIALIZABLE isolation level to achieve ACID properties. It is enough to use REPEATABLE READ for a non-distributed transaction, but not for a distributed transaction.)

Some examples of distributed transactions:

Applications that use global transactions involve one or more Resource Managers and a Transaction Manager:

The MySQL implementation of XA MySQL allows a MySQL server to act as a Resource Manager that handles XA transactions within a global transaction. A client program that connects to the MySQL server acts as the Transaction Manager.

To carry out a global transaction, it's necessary to know which components are involved, and bring each component to a point when it can be committed or rolled back. Depending on what each component reports about its ability to succeed, they must all commit or roll back as an atomic group. That is, either all components must commit, or all components musts roll back. To manage a global transaction, it's necessary to take into account that any component or the connecting network might fail.

The process for executing a global transaction uses two-phase commit (2PC). This takes place after the actions performed by the branches of the global transaction have been executed.

In some cases, a global transaction might use one-phase commit (1PC). For example, When a Transaction Manager finds that a global transaction consists of only one transactional resource (that is, a single branch), that resource can be told to prepare and commit at the same time.

XA {START|BEGIN} xid [JOIN|RESUME]

XA END xid [SUSPEND [FOR MIGRATE]]

XA PREPARE xid

XA COMMIT xid [ONE PHASE]

XA ROLLBACK xid

XA RECOVER

xid: gtrid [, bqual [, formatID ]]

mysql> XA RECOVER;
+----------+--------------+--------------+--------+
| formatID | gtrid_length | bqual_length | data   |
+----------+--------------+--------------+--------+
|        7 |            3 |            3 | abcdef |
+----------+--------------+--------------+--------+

mysql> XA START 'xatest';
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO mytable (i) VALUES(10);
Query OK, 1 row affected (0.04 sec)

mysql> XA END 'xatest';
Query OK, 0 rows affected (0.00 sec)

mysql> XA PREPARE 'xatest';
Query OK, 0 rows affected (0.00 sec)

mysql> XA COMMIT 'xatest';
Query OK, 0 rows affected (0.00 sec)