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MySQL 5.7 Reference Manual  /  ...  /  ALTER TABLE Statement

13.1.8 ALTER TABLE Statement

13.1.8.1 ALTER TABLE Partition Operations
13.1.8.2 ALTER TABLE and Generated Columns
13.1.8.3 ALTER TABLE Examples
ALTER TABLE tbl_name
    [alter_option [, alter_option] ...]
    [partition_options]

alter_option: {
    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)
  | ALGORITHM [=] {DEFAULT | INPLACE | COPY}
  | ALTER [COLUMN] col_name {
        SET DEFAULT {literal | (expr)}
      | 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
  | LOCK [=] {DEFAULT | NONE | SHARED | EXCLUSIVE}
  | MODIFY [COLUMN] col_name column_definition
        [FIRST | AFTER col_name]
  | ORDER BY col_name [, col_name] ...
  | RENAME {INDEX | KEY} old_index_name TO new_index_name
  | RENAME [TO | AS] new_tbl_name
  | {WITHOUT | WITH} VALIDATION
}

partition_options:
    partition_option [partition_option] ...

partition_option: {
    ADD PARTITION (partition_definition)
  | DROP PARTITION partition_names
  | DISCARD PARTITION {partition_names | ALL} TABLESPACE
  | IMPORT PARTITION {partition_names | ALL} TABLESPACE
  | TRUNCATE PARTITION {partition_names | ALL}
  | COALESCE PARTITION number
  | REORGANIZE PARTITION partition_names INTO (partition_definitions)
  | EXCHANGE PARTITION partition_name WITH TABLE tbl_name [{WITH | WITHOUT} VALIDATION]
  | 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}
  | REMOVE PARTITIONING
  | UPGRADE 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'
  | COMPRESSION [=] {'ZLIB' | 'LZ4' | 'NONE'}
  | CONNECTION [=] 'connect_string'
  | {DATA | INDEX} DIRECTORY [=] 'absolute path to directory'
  | DELAY_KEY_WRITE [=] {0 | 1}
  | ENCRYPTION [=] {'Y' | 'N'}
  | 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}
  | STATS_AUTO_RECALC [=] {DEFAULT | 0 | 1}
  | STATS_PERSISTENT [=] {DEFAULT | 0 | 1}
  | STATS_SAMPLE_PAGES [=] value
  | TABLESPACE tablespace_name [STORAGE {DISK | MEMORY}]
  | UNION [=] (tbl_name[,tbl_name]...)
}

partition_options:
    (see CREATE TABLE options)

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, ROW_FORMAT, or TABLESPACE.

For descriptions of all table options, see Section 13.1.18, “CREATE TABLE Statement”. 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 5.7.17, 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:

  • If t1 is currently not an InnoDB table, this statement changes its storage engine to InnoDB: 

    ALTER TABLE t1 ENGINE = InnoDB;

  • To change the InnoDB table to use compressed row-storage format: 

    ALTER TABLE t1 ROW_FORMAT = COMPRESSED;

  • To enable or disable encryption for an InnoDB table in a file-per-table tablespace: 

    ALTER TABLE t1 ENCRYPTION='Y';
ALTER TABLE t1 ENCRYPTION='N';

    A keyring plugin must be installed and configured to use the ENCRYPTION option. For more information, see Section 14.14, “InnoDB Data-at-Rest Encryption”.

    The ENCRYPTION option is supported only by the InnoDB storage engine; thus it works only if the table already uses InnoDB (and you do not change the table's storage engine), or if the ALTER TABLE statement also specifies ENGINE=InnoDB. Otherwise the statement is rejected with ER_CHECK_NOT_IMPLEMENTED.

  • To reset the current auto-increment value: 

    ALTER TABLE t1 AUTO_INCREMENT = 13;

    You cannot reset the counter to a value less than or equal to the value that is currently in use. For both InnoDB and 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 AUTO_INCREMENT column value plus one.

  • To change the default table character set: 

    ALTER TABLE t1 CHARACTER SET = utf8;

    See also Changing the Character Set.

  • To add (or change) a table comment: 

    ALTER TABLE t1 COMMENT = 'New table comment';

  • Use ALTER TABLE with the TABLESPACE option to move InnoDB tables between existing general tablespaces, file-per-table tablespaces, and the system tablespace. See Moving Tables Between Tablespaces Using ALTER TABLE.

    • ALTER TABLE ... TABLESPACE operations always cause a full table rebuild, even if the TABLESPACE attribute has not changed from its previous value.

    • ALTER TABLE ... TABLESPACE syntax does not support moving a table from a temporary tablespace to a persistent tablespace.

    • The DATA DIRECTORY clause, which is supported with CREATE TABLE ... TABLESPACE, is not supported with ALTER TABLE ... TABLESPACE, and is ignored if specified.

    • For more information about the capabilities and limitations of the TABLESPACE option, see CREATE TABLE.

  • MySQL NDB Cluster 7.5.2 and later supports setting NDB_TABLE options for controlling a table's partition balance (fragment count type), read-from-any-replica capability, full replication, or any combination of these, as part of the table comment for an ALTER TABLE statement in the same manner as for CREATE TABLE, as shown in this example: 

    ALTER TABLE t1 COMMENT = "NDB_TABLE=READ_BACKUP=0,PARTITION_BALANCE=FOR_RA_BY_NODE";

    It is also possible to set NDB_COMMENT options for columns of NDB tables as part of an ALTER TABLE statement, like this one: 

    ALTER TABLE t1 
  CHANGE COLUMN c1 c1 BLOB 
    COMMENT = 'NDB_COLUMN=MAX_BLOB_PART_SIZE';

    Bear in mind that ALTER TABLE ... COMMENT ... discards any existing comment for the table. See Setting NDB_TABLE options, for additional information and examples.

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 one of the following algorithms:

  • COPY: Operations are performed on a copy of the original table, and table data is copied from the original table to the new table row by row. Concurrent DML is not permitted.

  • INPLACE: Operations avoid copying table data but may rebuild the table in place. An exclusive metadata lock on the table may be taken briefly during preparation and execution phases of the operation. Typically, concurrent DML is supported.

For tables using the NDB storage engine, these algorithms work as follows:

  • COPY: NDB creates a copy of the table and alters it; the NDB Cluster handler then copies the data between the old and new versions of the table. Subsequently, NDB deletes the old table and renames the new one.

    This is sometimes also referred to as a “copying” or “offline” ALTER TABLE.

  • INPLACE: The data nodes make the required changes; the NDB Cluster handler does not copy data or otherwise take part.

    This is sometimes also referred to as a “non-copying” or “online” ALTER TABLE.

See Section 21.6.12, “Online Operations with ALTER TABLE in NDB Cluster”, for more information.

The ALGORITHM clause is optional. If the ALGORITHM clause is omitted, MySQL uses ALGORITHM=INPLACE for storage engines and ALTER TABLE clauses that support it. Otherwise, ALGORITHM=COPY is used.

Specifying an ALGORITHM clause requires the operation to use the specified algorithm for clauses and storage engines that support it, or fail with an error otherwise. Specifying ALGORITHM=DEFAULT is the same as omitting the ALGORITHM clause.

ALTER TABLE operations that use the COPY algorithm 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.

An ALTER TABLE operation that uses the COPY algorithm prevents concurrent DML operations. Concurrent queries are still allowed. That is, a table-copying operation always includes at least the concurrency restrictions of LOCK=SHARED (allow queries but not DML). You can further restrict concurrency for operations that support the LOCK clause by specifying LOCK=EXCLUSIVE, which prevents DML and queries. For more information, see Concurrency Control.

To force use of the COPY algorithm for an ALTER TABLE operation that would otherwise not use it, enable the old_alter_table system variable or specify ALGORITHM=COPY. If there is a conflict between the old_alter_table setting and an ALGORITHM clause with a value other than DEFAULT, the ALGORITHM clause takes precedence.

For InnoDB tables, an ALTER TABLE operation that uses the COPY algorithm on a table that resides in a shared 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.

For information about space requirements for online DDL operations, see Section 14.13.3, “Online DDL Space Requirements”.

ALTER TABLE operations that support the INPLACE algorithm include:

  • ALTER TABLE operations supported by the InnoDB online DDL feature. See Section 14.13.1, “Online DDL Operations”.

  • Renaming a table. MySQL renames 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 13.1.33, “RENAME TABLE Statement”.) Privileges granted specifically for the renamed table are not migrated to the new name. They must be changed manually.

  • Operations that only modify table metadata. These operations are immediate because the server only alters the table .frm file, not touch table contents. Metadata-only operations include:

    • Renaming a column.

    • Changing the default value of a column (except for NDB tables).

    • Modifying the definition of an ENUM or SET column by adding new enumeration or set members to the end of the list of valid member values, as long as the storage size of the data type does not change. For example, adding a member to a SET column that has 8 members changes the required storage per value from 1 byte to 2 bytes; this requires a table copy. Adding members in the middle of the list causes renumbering of existing members, which requires a table copy.

  • Renaming an index.

  • Adding or dropping a secondary index, for InnoDB and NDB tables. See Section 14.13, “InnoDB and Online DDL”.

  • For NDB tables, operations that add and drop indexes on variable-width columns. These operations occur online, without table copying and without blocking concurrent DML actions for most of their duration. See Section 21.6.12, “Online Operations with ALTER TABLE in NDB Cluster”.

ALTER TABLE upgrades MySQL 5.5 temporal columns to 5.6 format for ADD COLUMN, CHANGE COLUMN, MODIFY COLUMN, ADD INDEX, and FORCE operations. This conversion cannot be done using the INPLACE algorithm because the table must be rebuilt, so specifying ALGORITHM=INPLACE in these cases results in an error. Specify ALGORITHM=COPY if necessary.

If an ALTER TABLE operation on a multicolumn index used to partition a table by KEY changes the order of the columns, it can only be performed using ALGORITHM=COPY.

The WITHOUT VALIDATION and WITH VALIDATION clauses affect whether ALTER TABLE performs an in-place operation for virtual generated column modifications. See Section 13.1.8.2, “ALTER TABLE and Generated Columns”.

NDB Cluster formerly supported online ALTER TABLE operations using the ONLINE and OFFLINE keywords. These keywords are no longer supported; their use causes a syntax error. MySQL NDB Cluster 7.5 (and later) supports online operations using the same ALGORITHM=INPLACE syntax used with the standard MySQL Server. NDB does not support changing a tablespace online. See Section 21.6.12, “Online Operations with ALTER TABLE in NDB Cluster”, for more information.

ALTER TABLE with DISCARD ... PARTITION ... TABLESPACE or IMPORT ... PARTITION ... TABLESPACE does not create any temporary tables or temporary partition files.

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.

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.

Concurrency Control

For ALTER TABLE operations that support it, you can use the LOCK clause to control the level of concurrent reads and writes on a table while it is being altered. Specifying a non-default value for this clause enables you to require a certain amount of concurrent access or exclusivity during the alter operation, and halts the operation if the requested degree of locking is not available. The parameters for the LOCK clause are:

  • LOCK = DEFAULT

    Maximum level of concurrency for the given ALGORITHM clause (if any) and ALTER TABLE operation: Permit concurrent reads and writes if supported. If not, permit concurrent reads if supported. If not, enforce exclusive access.

  • LOCK = NONE

    If supported, permit concurrent reads and writes. Otherwise, an error occurs.

  • LOCK = SHARED

    If supported, permit concurrent reads but block writes. Writes are blocked even if concurrent writes are supported by the storage engine for the given ALGORITHM clause (if any) and ALTER TABLE operation. If concurrent reads are not supported, an error occurs.

  • LOCK = EXCLUSIVE

    Enforce exclusive access. This is done even if concurrent reads/writes are supported by the storage engine for the given ALGORITHM clause (if any) and ALTER TABLE operation.

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:

    • Can rename a column and change its definition, or both.

    • Has more capability than MODIFY, but at the expense of convenience for some operations. CHANGE requires naming the column twice if not renaming it.

    • With FIRST or AFTER, can reorder columns.

  • MODIFY:

    • Can change a column definition but not its name.

    • More convenient than CHANGE to change a column definition without renaming it.

    • With FIRST or AFTER, can reorder columns.

  • ALTER: Used only to change a column default value.

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.

Warning

This conversion may result in alteration of data. For example, if you shorten a string column, values may be truncated. To prevent the operation from succeeding if conversions to the new data type would result in loss of data, enable strict SQL mode before using ALTER TABLE (see Section 5.1.10, “Server SQL Modes”).

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:

  • Indexes that refer to the old column, including indexes and disabled MyISAM indexes.

  • Foreign keys that refer to the old column.

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

  • Generated column and partition expressions that refer to the renamed column. You must use CHANGE to redefine such expressions in the same ALTER TABLE statement as the one that renames the column.

  • Views and stored programs that refer to the renamed column. You must manually alter the definition of these objects to refer to the new column name.

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”.

ALTER ... SET DEFAULT cannot be used with the CURRENT_TIMESTAMP function.

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.

DROP INDEX removes an index. This is a MySQL extension to standard SQL. See Section 13.1.25, “DROP INDEX Statement”. 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 13.1.14, “CREATE INDEX Statement”. The preferred position is after the column list. You should expect support for use of the option before the column list to 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 13.1.14, “CREATE INDEX Statement”.

RENAME INDEX old_index_name TO new_index_name renames an index. This is a MySQL extension to standard SQL. The content of the table remains unchanged. old_index_name must be the name of an existing index in the table that is not dropped by the same ALTER TABLE statement. new_index_name is the new index name, which cannot duplicate the name of an index in the resulting table after changes have been applied. Neither index name can be PRIMARY.

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 13.7.5.22, “SHOW INDEX Statement”.

Foreign Keys and Other Constraints

The FOREIGN KEY and REFERENCES clauses are supported by the InnoDB and NDB storage engines, which implement ADD [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (...) REFERENCES ... (...). See Section 1.6.3.2, “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 13.1.18, “CREATE TABLE Statement”. 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 1.6.2, “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 (....) ...
Important

MySQL silently ignores inline REFERENCES specifications, where the references are defined as part of the column specification. MySQL accepts only REFERENCES clauses defined as part of a separate FOREIGN KEY specification.

Note

Partitioned InnoDB tables do not support foreign keys. This restriction does not apply to NDB tables, including those explicitly partitioned by [LINEAR] KEY. For more information, see Section 22.6.2, “Partitioning Limitations Relating to Storage Engines”.

MySQL Server and NDB Cluster both support 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 the same ALTER TABLE statement is supported for ALTER TABLE ... ALGORITHM=INPLACE but not for ALTER TABLE ... ALGORITHM=COPY.

The server prohibits changes to foreign key columns that have the potential to cause loss of referential integrity. A workaround is to use ALTER TABLE ... DROP FOREIGN KEY before changing the column definition and ALTER TABLE ... ADD FOREIGN KEY afterward. Examples of prohibited changes include:

  • Changes to the data type of foreign key columns that may be unsafe. For example, changing VARCHAR(20) to VARCHAR(30) is permitted, but changing it to VARCHAR(1024) is not because that alters the number of length bytes required to store individual values.

  • Changing a NULL column to NOT NULL in non-strict mode is prohibited to prevent converting NULL values to default non-NULL values, for which there are no corresponding values in the referenced table. The operation is permitted in strict mode, but an error is returned if any such conversion is required.

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;

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 have a character set and a subsequent CONVERT TO operation does 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.

Warning

The CONVERT TO operation converts column values between the original and named character sets. This is not what you want if you have a column in one character set (like latin1) but the stored values actually use some other, incompatible character set (like utf8). In this case, you have to do the following for each such column: 

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

The reason this works is that there is no conversion when you convert to or from BLOB columns.

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

An InnoDB table created in its own file-per-table tablespace can be imported from a backup or from another MySQL server instance using DISCARD TABLEPACE and IMPORT TABLESPACE clauses. See Section 14.6.1.3, “Importing InnoDB Tables”.

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.

Note

The table does not remain in the specified order after inserts and deletes.

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, import, 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, DISCARD PARTITION, IMPORT PARTITION, COALESCE PARTITION, REORGANIZE PARTITION, EXCHANGE 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 more information about partition options, see Section 13.1.18, “CREATE TABLE Statement”, and Section 13.1.8.1, “ALTER TABLE Partition Operations”. For information about and examples of ALTER TABLE ... EXCHANGE PARTITION statements, see Section 22.3.3, “Exchanging Partitions and Subpartitions with Tables”.

Prior to MySQL 5.7.6, partitioned InnoDB tables used the generic ha_partition partitioning handler employed by MyISAM and other storage engines not supplying their own partitioning handlers; in MySQL 5.7.6 and later, such tables are created using the InnoDB storage engine's own (or “native”) partitioning handler. Beginning with MySQL 5.7.9, you can upgrade an InnoDB table that was created in MySQL 5.7.6 or earlier (that is, created using ha_partition) to the InnoDB native partition handler using ALTER TABLE ... UPGRADE PARTITIONING. (Bug #76734, Bug #20727344) This ALTER TABLE syntax does not accept any other options and can be used only on a single table at a time. You can also use mysql_upgrade in MySQL 5.7.9 or later to upgrade older partitioned InnoDB tables to the native partitioning handler.


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