ALTER [ONLINE|OFFLINE] [IGNORE] TABLE tbl_name
    [alter_specification [, alter_specification] ...]
    [partition_options]

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

partition_options:
    partition_option [partition_option] ...

partition_option:
    ADD PARTITION (partition_definition)
  | DROP PARTITION partition_names
  | TRUNCATE PARTITION {partition_names | ALL}
  | COALESCE PARTITION number
  | REORGANIZE PARTITION [partition_names INTO (partition_definitions)]
  | ANALYZE PARTITION {partition_names | ALL}
  | CHECK PARTITION {partition_names | ALL}
  | OPTIMIZE PARTITION {partition_names | ALL}
  | REBUILD PARTITION {partition_names | ALL}
  | REPAIR PARTITION {partition_names | ALL}
  | PARTITION BY partitioning_expression
  | REMOVE PARTITIONING

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

index_type:
    USING {BTREE | HASH}

index_option:
    KEY_BLOCK_SIZE [=] value
  | index_type
  | WITH PARSER parser_name
  | COMMENT 'string'

table_options:
    table_option [[,] table_option] ...

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

ALTER TABLE changes the structure of a 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 characteristics such as the storage engine used for the table or the table comment.

There are several additional aspects to the ALTER TABLE statement, described under the following topics in this section:

Table Options

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 descriptions of all table options, see Section 13.1.17, “CREATE TABLE Syntax”. However, ALTER TABLE ignores DATA DIRECTORY and INDEX DIRECTORY when given as table options. ALTER TABLE permits them only as partitioning options, and, as of MySQL 5.5.54, requires that you have the FILE privilege.

Use of table options with ALTER TABLE provides a convenient way of altering single table characteristics. For example:

To verify that the table options were changed as intended, use SHOW CREATE TABLE, or query the INFORMATION_SCHEMA.TABLES table.

Performance and Space Requirements

ALTER TABLE operations are processed using either the table-copy method or in-place method. Operations that use the table-copy method are performed on a temporary copy of the original table, which can require more time, particularly for large tables. Operations that use the in place method do not create temporary copy of the table and tend to be faster.

ALTER TABLE operations that are performed on a temporary copy of the original table wait for other operations that are modifying the table to complete. After alterations are applied to the table copy, data is copied over, the original table is deleted, and the table copy is renamed to the name of the original table. While the ALTER TABLE operation executes, the original table is readable by other sessions (with the exception noted shortly). Updates and writes to the table started after the ALTER TABLE operation begins are stalled until the new table is ready, then are automatically redirected to the new table. The temporary copy of the table is created in the database directory of the original table unless it is a RENAME TO operation that moves the table to a database that resides in a different directory.

The exception referred to earlier is that ALTER TABLE blocks reads (not just writes) at the point where it is ready to install a new version of the table .frm file, discard the old file, and clear outdated table structures from the table and table definition caches. At this point, it must acquire an exclusive lock. To do so, it waits for current readers to finish, and blocks new reads and writes.

To force use of the table-copy method for an ALTER TABLE operation that would otherwise not use it, enable the old_alter_table system variable.

For InnoDB tables, a table-copying ALTER TABLE operation on table that resides in a shared tablespace such as the system tablespace can increase the amount of space used by the tablespace. Such operations require as much additional space as the data in the table plus indexes. For a table residing in a shared tablespace, the additional space used during the operation is not released back to the operating system as it is for a table that resides in a file-per-table tablespace.

ALTER TABLE operations that use the in-place method include:

For MyISAM tables, you can speed up index re-creation (the slowest part of the alteration process) by setting the myisam_sort_buffer_size system variable to a high value.

ALTER TABLE with ADD PARTITION, DROP PARTITION, COALESCE PARTITION, REBUILD PARTITION, or REORGANIZE PARTITION does not create 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.

Adding and Dropping Columns

Use ADD to add new columns to a table, and DROP to remove existing columns. DROP col_name is a MySQL extension to standard SQL.

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.

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

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.

Renaming, Redefining, and Reordering Columns

The CHANGE, MODIFY, and ALTER clauses enable the names and definitions of existing columns to be altered. They have these comparative characteristics:

CHANGE is a MySQL extension to standard SQL. MODIFY is a MySQL extension for Oracle compatibility.

To alter a column to change both its name and definition, use CHANGE, specifying the old and new names and the new definition. For example, to rename an INT NOT NULL column from a to b and change its definition to use the BIGINT data type while retaining the NOT NULL attribute, do this:

ALTER TABLE t1 CHANGE a b BIGINT NOT NULL;

To change a column definition but not its name, use CHANGE or MODIFY. With CHANGE, the syntax requires two column names, so you must specify the same name twice to leave the name unchanged. For example, to change the definition of column b, do this:

ALTER TABLE t1 CHANGE b b INT NOT NULL;

MODIFY is more convenient to change the definition without changing the name because it requires the column name only once:

ALTER TABLE t1 MODIFY b INT NOT NULL;

To change a column name but not its definition, use CHANGE. The syntax requires a column definition, so to leave the definition unchanged, you must respecify the definition the column currently has. For example, to rename an INT NOT NULL column from b to a, do this:

ALTER TABLE t1 CHANGE b a INT NOT NULL;

For column definition changes using CHANGE or MODIFY, the 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, intending to change only INT to BIGINT:

ALTER TABLE t1 MODIFY col1 BIGINT;

That statement changes the data type from INT to BIGINT, but it also drops the UNSIGNED, DEFAULT, and COMMENT attributes. To retain them, the statement must include them explicitly:

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

For data type changes using CHANGE or MODIFY, MySQL tries to convert existing column values to the new type as well as possible.

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.

For columns renamed by CHANGE, MySQL automatically renames these references to the renamed column:

For columns renamed by CHANGE, MySQL does not automatically rename these references to the renamed column:

To reorder columns within a table, use FIRST and AFTER in CHANGE or MODIFY operations.

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”.

Primary Keys and Indexes

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, “Primary Key Optimization”.

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.

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.

SET SESSION old_alter_table=1

DROP INDEX removes an index. This is a MySQL extension to standard SQL. See Section 13.1.24, “DROP INDEX Syntax”. To determine index names, use SHOW INDEX FROM tbl_name.

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. For details about permissible index_option values, see Section 13.1.13, “CREATE INDEX Syntax”.

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.

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”.

Foreign Keys and Other Constraints

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 14.9.1.5, “InnoDB and FOREIGN KEY Constraints”. For other storage engines, the clauses are parsed but ignored.

The CHECK constraint 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;

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.

ALTER TABLE tbl_name RENAME new_tbl_name changes internally generated foreign key constraint names and user-defined foreign key constraint names that begin with the string “tbl_name_ibfk_” to reflect the new table name. InnoDB interprets foreign key constraint names that begin with the string “tbl_name_ibfk_” as internally generated names.

Changing the Character Set

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 changes 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 does not fit in a TEXT column's length bytes, so MySQL converts 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 in a CONVERT TO CHARACTER SET operation, the character set named by the character_set_database system variable 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 the foreign_key_checks system variable 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).

Discarding and Importing InnoDB Tablespaces

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 14.9.3.2, “File-Per-Table Tablespaces”.

Row Order for MyISAM Tables

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 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.

When used on a partitioned table, ALTER TABLE ... ORDER BY orders rows within each partition only.

Partitioning Options

partition_options signifies options that can be used with partitioned tables for repartitioning, to add, drop, discard, merge, and split partitions, and to perform partitioning maintenance.

It is possible for an ALTER TABLE statement to contain a PARTITION BY or REMOVE PARTITIONING clause in an addition to other alter specifications, but the PARTITION BY or REMOVE PARTITIONING clause must be specified last after any other specifications. The ADD PARTITION, DROP PARTITION, COALESCE PARTITION, REORGANIZE PARTITION, ANALYZE PARTITION, CHECK PARTITION, and REPAIR PARTITION options cannot be combined with other alter specifications in a single ALTER TABLE, since the options just listed act on individual partitions.

For a list of partition options and a description of each, see Section 13.1.17, “CREATE TABLE Syntax”. For additional information, see Section 13.1.7.1, “ALTER TABLE Partition Operations”.