For ALTER TABLE tbl_name RENAME TO new_tbl_name without any other options, MySQL simply renames any files that correspond to the table tbl_name without making a copy. (You can also use the RENAME TABLE statement to rename tables. See Section 13.1.32, “RENAME TABLE Syntax”.) Any privileges granted specifically for the renamed table are not migrated to the new name. They must be changed manually.

Alterations that modify only table metadata and not table data are immediate because the server only needs to alter the table .frm file, not touch table contents. The following changes are fast alterations that can be made this way:

ALTER TABLE with ADD PARTITION, DROP PARTITION, COALESCE PARTITION, REBUILD PARTITION, or REORGANIZE PARTITION does not create any temporary tables (except when used with NDB tables); however, these operations can and do create temporary partition files.

ADD or DROP operations for RANGE or LIST partitions are immediate operations or nearly so. ADD or COALESCE operations for HASH or KEY partitions copy data between all partitions, unless LINEAR HASH or LINEAR KEY was used; this is effectively the same as creating a new table, although the ADD or COALESCE operation is performed partition by partition. REORGANIZE operations copy only changed partitions and do not touch unchanged ones.

Renaming an index, except for InnoDB.

Adding or dropping an index, for InnoDB and NDB.

To use ALTER TABLE, you need ALTER, CREATE, and INSERT privileges for the table. Renaming a table requires ALTER and DROP on the old table, ALTER, CREATE, and INSERT on the new table.

IGNORE is a MySQL extension to standard SQL. It controls how ALTER TABLE works if there are duplicates on unique keys in the new table or if warnings occur when strict mode is enabled. If IGNORE is not specified, the copy is aborted and rolled back if duplicate-key errors occur. If IGNORE is specified, only one row is used of rows with duplicates on a unique key. The other conflicting rows are deleted. Incorrect values are truncated to the closest matching acceptable value.

ERROR 23000: Duplicate entry '347' for key 'pl'

            SET SESSION old_alter_table=1
          

Pending INSERT DELAYED statements are lost if a table is write locked and ALTER TABLE is used to modify the table structure.

table_options signifies table options of the kind that can be used in the CREATE TABLE statement, such as ENGINE, AUTO_INCREMENT, AVG_ROW_LENGTH, MAX_ROWS, or ROW_FORMAT. For a list of all table options and a description of each, see Section 13.1.17, “CREATE TABLE Syntax”. However, ALTER TABLE ignores the DATA DIRECTORY and INDEX DIRECTORY table options.

For example, to convert a table to be an InnoDB table, use this statement:

ALTER TABLE t1 ENGINE = InnoDB;

See Section 14.11.5, “Converting Tables from MyISAM to InnoDB” for considerations when switching tables to the InnoDB storage engine.

When you specify an ENGINE clause, ALTER TABLE rebuilds the table. This is true even if the table already has the specified storage engine.

The outcome of attempting to change a table's storage engine is affected by whether the desired storage engine is available and the setting of the NO_ENGINE_SUBSTITUTION SQL mode, as described in Section 5.1.8, “Server SQL Modes”.

To prevent inadvertent loss of data, ALTER TABLE cannot be used to change the storage engine of a table to MERGE or BLACKHOLE.

To change the value of the AUTO_INCREMENT counter to be used for new rows, do this:

ALTER TABLE t2 AUTO_INCREMENT = value;

You cannot reset the counter to a value less than or equal to any that have already been used. For MyISAM, if the value is less than or equal to the maximum value currently in the AUTO_INCREMENT column, the value is reset to the current maximum plus one. For InnoDB, if the value is less than the current maximum value in the column, no error occurs and the current sequence value is not changed.

You can issue multiple ADD, ALTER, DROP, and CHANGE clauses in a single ALTER TABLE statement, separated by commas. This is a MySQL extension to standard SQL, which permits only one of each clause per ALTER TABLE statement. For example, to drop multiple columns in a single statement, do this:

ALTER TABLE t2 DROP COLUMN c, DROP COLUMN d;

CHANGE col_name, DROP col_name, and DROP INDEX are MySQL extensions to standard SQL.

The word COLUMN is optional and can be omitted.

column_definition clauses use the same syntax for ADD and CHANGE as for CREATE TABLE. See Section 13.1.17, “CREATE TABLE Syntax”.

You can rename a column using a CHANGE old_col_name new_col_name column_definition clause. To do so, specify the old and new column names and the definition that the column currently has. For example, to rename an INTEGER column from a to b, you can do this:

ALTER TABLE t1 CHANGE a b INTEGER;

To change a column's type but not the name, CHANGE syntax still requires an old and new column name, even if they are the same. For example:

ALTER TABLE t1 CHANGE b b BIGINT NOT NULL;

You can also use MODIFY to change a column's type without renaming it:

ALTER TABLE t1 MODIFY b BIGINT NOT NULL;

MODIFY is an extension to ALTER TABLE for Oracle compatibility.

When you use CHANGE or MODIFY, column_definition must include the data type and all attributes that should apply to the new column, other than index attributes such as PRIMARY KEY or UNIQUE. Attributes present in the original definition but not specified for the new definition are not carried forward. Suppose that a column col1 is defined as INT UNSIGNED DEFAULT 1 COMMENT 'my column' and you modify the column as follows:

ALTER TABLE t1 MODIFY col1 BIGINT;

The resulting column will be defined as BIGINT, but will not include the attributes UNSIGNED DEFAULT 1 COMMENT 'my column'. To retain them, the statement should be:

ALTER TABLE t1 MODIFY col1 BIGINT UNSIGNED DEFAULT 1 COMMENT 'my column';

When you change a data type using CHANGE or MODIFY, MySQL tries to convert existing column values to the new type as well as possible.

To add a column at a specific position within a table row, use FIRST or AFTER col_name. The default is to add the column last. You can also use FIRST and AFTER in CHANGE or MODIFY operations to reorder columns within a table.

ALTER ... SET DEFAULT or ALTER ... DROP DEFAULT specify a new default value for a column or remove the old default value, respectively. If the old default is removed and the column can be NULL, the new default is NULL. If the column cannot be NULL, MySQL assigns a default value as described in Section 11.6, “Data Type Default Values”.

DROP INDEX removes an index. This is a MySQL extension to standard SQL. See Section 13.1.24, “DROP INDEX Syntax”. If you are unsure of the index name, use SHOW INDEX FROM tbl_name.

If columns are dropped from a table, the columns are also removed from any index of which they are a part. If all columns that make up an index are dropped, the index is dropped as well. If you use CHANGE or MODIFY to shorten a column for which an index exists on the column, and the resulting column length is less than the index length, MySQL shortens the index automatically.

If a table contains only one column, the column cannot be dropped. If what you intend is to remove the table, use DROP TABLE instead.

DROP PRIMARY KEY drops the primary key. If there is no primary key, an error occurs. For information about the performance characteristics of primary keys, especially for InnoDB tables, see Section 8.3.2, “Using Primary Keys”.

If you add a UNIQUE INDEX or PRIMARY KEY to a table, MySQL stores it before any nonunique index to permit detection of duplicate keys as early as possible.

Some storage engines permit you to specify an index type when creating an index. The syntax for the index_type specifier is USING type_name. For details about USING, see Section 13.1.13, “CREATE INDEX Syntax”. The preferred position is after the column list. Support for use of the option before the column list will be removed in a future MySQL release.

index_option values specify additional options for an index. USING is one such option. For details about permissible index_option values, see Section 13.1.13, “CREATE INDEX Syntax”.

After an ALTER TABLE statement, it may be necessary to run ANALYZE TABLE to update index cardinality information. See Section 13.7.5.23, “SHOW INDEX Syntax”.

ORDER BY enables you to create the new table with the rows in a specific order. This option is useful primarily when you know that you are mostly to query the rows in a certain order most of the time. By using this option after major changes to the table, you might be able to get higher performance. In some cases, it might make sorting easier for MySQL if the table is in order by the column that you want to order it by later.

ORDER BY syntax permits one or more column names to be specified for sorting, each of which optionally can be followed by ASC or DESC to indicate ascending or descending sort order, respectively. The default is ascending order. Only column names are permitted as sort criteria; arbitrary expressions are not permitted. This clause should be given last after any other clauses.

ORDER BY does not make sense for InnoDB tables because InnoDB always orders table rows according to the clustered index.

If you use ALTER TABLE on a MyISAM table, all nonunique indexes are created in a separate batch (as for REPAIR TABLE). This should make ALTER TABLE much faster when you have many indexes.

For MyISAM tables, key updating can be controlled explicitly. Use ALTER TABLE ... DISABLE KEYS to tell MySQL to stop updating nonunique indexes. Then use ALTER TABLE ... ENABLE KEYS to re-create missing indexes. MyISAM does this with a special algorithm that is much faster than inserting keys one by one, so disabling keys before performing bulk insert operations should give a considerable speedup. Using ALTER TABLE ... DISABLE KEYS requires the INDEX privilege in addition to the privileges mentioned earlier.

While the nonunique indexes are disabled, they are ignored for statements such as SELECT and EXPLAIN that otherwise would use them.

If ALTER TABLE for an InnoDB table results in changes to column values (for example, because a column is truncated), InnoDB's FOREIGN KEY constraint checks do not notice possible violations caused by changing the values.

The FOREIGN KEY and REFERENCES clauses are supported by the InnoDB storage engine, which implements ADD [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (...) REFERENCES ... (...). See Section 14.11.7, “InnoDB and FOREIGN KEY Constraints”. For other storage engines, the clauses are parsed but ignored. The CHECK clause is parsed but ignored by all storage engines. See Section 13.1.17, “CREATE TABLE Syntax”. The reason for accepting but ignoring syntax clauses is for compatibility, to make it easier to port code from other SQL servers, and to run applications that create tables with references. See Section 1.7.2, “MySQL Differences from Standard SQL”.

For ALTER TABLE, unlike CREATE TABLE, ADD FOREIGN KEY ignores index_name if given and uses an automatically generated foreign key name. As a workaround, include the CONSTRAINT clause to specify the foreign key name:

ADD CONSTRAINT name FOREIGN KEY (....) ...

InnoDB supports the use of ALTER TABLE to drop foreign keys:

ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol;

For more information, see Section 14.11.7, “InnoDB and FOREIGN KEY Constraints”.

Adding and dropping a foreign key in separate clauses of a single ALTER TABLE statement may be problematic in some cases and is therefore unsupported. Use separate statements for each operation.

For an InnoDB table that is created with its own tablespace in an .ibd file, that file can be discarded and imported. To discard the .ibd file, use this statement:

ALTER TABLE tbl_name DISCARD TABLESPACE;

This deletes the current .ibd file, so be sure that you have a backup first. Attempting to access the table while the tablespace file is discarded results in an error.

To import the backup .ibd file back into the table, copy it into the database directory, and then issue this statement:

ALTER TABLE tbl_name IMPORT TABLESPACE;

The tablespace file must have been created on the server into which it is imported later.

See Section 14.10.4, “InnoDB File-Per-Table Tablespaces”.

To change the table default character set and all character columns (CHAR, VARCHAR, TEXT) to a new character set, use a statement like this:

ALTER TABLE tbl_name
CONVERT TO CHARACTER SET charset_name [COLLATE collation_name];

The statement also changes the collation of all character columns. If you specify no COLLATE clause to indicate which collation to use, the statement uses default collation for the character set. If this collation is inappropriate for the intended table use (for example, if it would change from a case-sensitive collation to a case-insensitive collation), specify a collation explicitly.

For a column that has a data type of VARCHAR or one of the TEXT types, CONVERT TO CHARACTER SET will change the data type as necessary to ensure that the new column is long enough to store as many characters as the original column. For example, a TEXT column has two length bytes, which store the byte-length of values in the column, up to a maximum of 65,535. For a latin1 TEXT column, each character requires a single byte, so the column can store up to 65,535 characters. If the column is converted to utf8, each character might require up to three bytes, for a maximum possible length of 3 × 65,535 = 196,605 bytes. That length will not fit in a TEXT column's length bytes, so MySQL will convert the data type to MEDIUMTEXT, which is the smallest string type for which the length bytes can record a value of 196,605. Similarly, a VARCHAR column might be converted to MEDIUMTEXT.

To avoid data type changes of the type just described, do not use CONVERT TO CHARACTER SET. Instead, use MODIFY to change individual columns. For example:

ALTER TABLE t MODIFY latin1_text_col TEXT CHARACTER SET utf8;
ALTER TABLE t MODIFY latin1_varchar_col VARCHAR(M) CHARACTER SET utf8;

If you specify CONVERT TO CHARACTER SET binary, the CHAR, VARCHAR, and TEXT columns are converted to their corresponding binary string types (BINARY, VARBINARY, BLOB). This means that the columns no longer will have a character set and a subsequent CONVERT TO operation will not apply to them.

If charset_name is DEFAULT, the database character set is used.

ALTER TABLE t1 CHANGE c1 c1 BLOB;
ALTER TABLE t1 CHANGE c1 c1 TEXT CHARACTER SET utf8;

To change only the default character set for a table, use this statement:

ALTER TABLE tbl_name DEFAULT CHARACTER SET charset_name;

The word DEFAULT is optional. The default character set is the character set that is used if you do not specify the character set for columns that you add to a table later (for example, with ALTER TABLE ... ADD column).

When foreign_key_checks is enabled, which is the default setting, character set conversion is not permitted on tables that include a character string column used in a foreign key constraint. The workaround is to disable foreign_key_checks before performing the character set conversion. You must perform the conversion on both tables involved in the foreign key constraint before re-enabling foreign_key_checks. If you re-enable foreign_key_checks after converting only one of the tables, an ON DELETE CASCADE or ON UPDATE CASCADE operation could corrupt data in the referencing table due to implicit conversion that occurs during these operations (Bug #45290, Bug #74816).