MySQL 8.0-13.1.9 ALTER TABLE Statement

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原文链接：https://dev.mysql.com/doc/refman/8.0/en/alter-table.html

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本文详细介绍 MySQL 中 ALTER TABLE 命令的使用方法及其各种选项，包括如何修改表结构、添加或删除列、创建或销毁索引等。同时，文章还介绍了如何通过此命令更改表的特性，例如存储引擎和表注释。

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 [CONSTRAINT [symbol]] CHECK (expr) [[NOT] ENFORCED]
  | DROP {CHECK | CONSTRAINT} symbol
  | ALTER {CHECK | CONSTRAINT} symbol [NOT] ENFORCED
  | ALGORITHM [=] {DEFAULT | INSTANT | INPLACE | COPY}
  | ALTER [COLUMN] col_name {
        SET DEFAULT {literal | (expr)}
      | SET {VISIBLE | INVISIBLE}
      | DROP DEFAULT
    }
  | ALTER INDEX index_name {VISIBLE | INVISIBLE}
  | 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 COLUMN old_col_name TO new_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
}

key_part: {col_name [(length)] | (expr)} [ASC | DESC]

index_type:
    USING {BTREE | HASH}

index_option: {
    KEY_BLOCK_SIZE [=] value
  | index_type
  | WITH PARSER parser_name
  | COMMENT 'string'
  | {VISIBLE | INVISIBLE}
}

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

table_option: {
    AUTOEXTEND_SIZE [=] value
  | 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
  | ENGINE_ATTRIBUTE [=] 'string'
  | 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}
  | SECONDARY_ENGINE_ATTRIBUTE [=] 'string'
  | 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.

ALTER TABLE命令用于修改表的结构。例如，您可以添加或删除列，创建或销毁索引，更改现有列的类型，或重命名列或表本身。您还可以更改特征，例如用于表的存储引擎或表注释。

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.
要使用ALTER TABLE，您需要对该表具有ALTER、CREATE和INSERT权限。重命名表需要在旧表上执行ALTER和DROP，在新表上执行ALTER、CREATE和INSERT。
Following the table name, specify the alterations to be made. If none are given, ALTER TABLE does nothing.
在表名之后，指定要进行的更改。如果没有指定，则ALTER TABLE不执行任何操作。
The syntax for many of the permissible alterations is similar to clauses of the CREATE TABLE statement. column_definition clauses use the same syntax for ADD and CHANGE as for CREATE TABLE. For more information, see Section 13.1.20, “CREATE TABLE Statement”.
许多允许的更改的语法类似于CREATE TABLE语句的子句。column_definition子句对ADD和CHANGE使用与CREATE TABLE相同的语法。更多信息，请参见13.1.20节“CREATE TABLE Statement”。
The word COLUMN is optional and can be omitted, except for RENAME COLUMN (to distinguish a column-renaming operation from the RENAME table-renaming operation).
COLUMN这个词是可选的，可以省略，RENAME COLUMN除外(为了区分列重命名操作和RENAME表重命名操作)。
Multiple ADD, ALTER, DROP, and CHANGE clauses are permitted 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:
在一条ALTER TABLE语句中可以使用多个ADD、ALTER、DROP和CHANGE子句，用逗号分隔。这是对标准SQL的MySQL扩展，每个ALTER TABLE语句只允许一个子句。例如，要在一条语句中删除多个列，可以这样做:
```
ALTER TABLE t2 DROP COLUMN c, DROP COLUMN d;
```
If a storage engine does not support an attempted ALTER TABLE operation, a warning may result. Such warnings can be displayed with SHOW WARNINGS. See Section 13.7.7.42, “SHOW WARNINGS Statement”. For information on troubleshooting ALTER TABLE, see Section B.3.6.1, “Problems with ALTER TABLE”.
如果存储引擎不支持尝试的ALTER TABLE操作，可能会导致警告。这些警告可以通过SHOW warnings显示出来。参见13.7.7.42节“显示WARNINGS语句”。有关ALTER TABLE故障排除的信息，请参见B.3.6.1节“ALTER TABLE问题”。
For information about generated columns, see Section 13.1.9.2, “ALTER TABLE and Generated Columns”.
For usage examples, see Section 13.1.9.3, “ALTER TABLE Examples”.
InnoDB in MySQL 8.0.17 and later supports addition of multi-valued indexes on JSON columns using a key_part specification can take the form (CAST json_path AS type ARRAY). See Multi-Valued Indexes, for detailed information regarding multi-valued index creation and usage of, as well as restrictions and limitations on multi-valued indexes.
MySQL 8.0.17及后续版本的InnoDB支持在JSON列上添加多值索引，使用key_part规范，可以采用(CAST json_path AS type ARRAY)的形式。有关多值索引的创建和使用，以及多值索引的限制和限制的详细信息，请参阅多值索引。
With the mysql_info() C API function, you can find out how many rows were copied by ALTER TABLE. See mysql_info().
使用mysql_info() C API函数，您可以知道ALTER TABLE复制了多少行。看到mysql_info()。

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

ALTER TABLE语句还有几个额外的方面，在本节的主题中描述如下:

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.

table_options表示可以在CREATE table语句中使用的表选项，如ENGINE、AUTO_INCREMENT、AVG_ROW_LENGTH、MAX_ROWS、ROW_FORMAT或TABLESPACE。

For descriptions of all table options, see Section 13.1.20, “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 requires that you have the FILE privilege.

所有表选项的描述，请参见13.1.20“CREATE table Statement”。但是，当作为表选项给出时，ALTER TABLE会忽略DATA DIRECTORY和INDEX DIRECTORY。ALTER TABLE只允许它们作为分区选项，并且要求您拥有FILE特权。

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

在ALTER table中使用表选项提供了一种更改单个表特征的方便方法。例如:

If t1 is currently not an InnoDB table, this statement changes its storage engine to InnoDB:如果t1当前不是一个InnoDB表，则该语句将其存储引擎改为InnoDB:
```
ALTER TABLE t1 ENGINE = InnoDB;
```
- See Section 15.6.1.5, “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.
- 当指定ENGINE子句时，ALTER TABLE将重新构建表。即使表已经具有指定的存储引擎，也是如此。
- Running ALTER TABLE tbl_name ENGINE=INNODB on an existing InnoDB table performs a “null” ALTER TABLE operation, which can be used to defragment an InnoDB table, as described in Section 15.11.4, “Defragmenting a Table”. Running ALTER TABLE tbl_name FORCE on an InnoDB table performs the same function.
- 在一个已存在的INNODB表上运行ALTER TABLE tbl_name ENGINE=INNODB会执行一个“null”ALTER TABLE操作，这个操作可以用来对一个INNODB表进行碎片整理，具体操作请参见15.11.4章节中的“碎片整理表”。在InnoDB表上运行ALTER TABLE tbl_name FORCE执行相同的功能。
- ALTER TABLE tbl_name ENGINE=INNODB and ALTER TABLE tbl_name FORCE use online DDL. For more information, see Section 15.12, “InnoDB and Online DDL”.
- The outcome of attempting to change the storage engine of a table is affected by whether the desired storage engine is available and the setting of the NO_ENGINE_SUBSTITUTION SQL mode, as described in Section 5.1.11, “Server SQL Modes”.
- 尝试更改表的存储引擎的结果会受到所需存储引擎是否可用和NO_ENGINE_SUBSTITUTION SQL模式设置的影响，如5.1.11节“服务器SQL模式”所述。
- To prevent inadvertent loss of data, ALTER TABLE cannot be used to change the storage engine of a table to MERGE or BLACKHOLE.
- 为了防止意外丢失数据，不能使用ALTER TABLE将表的存储引擎修改为MERGE或BLACKHOLE。
To change the InnoDB table to use compressed row-storage format:
将InnoDB表改为压缩行存储格式:
```
ALTER TABLE t1 ROW_FORMAT = COMPRESSED;
```
The ENCRYPTION clause enables or disables page-level data encryption for an InnoDB table. A keyring plugin must be installed and configured to enable encryption.

If the table_encryption_privilege_check variable is enabled, the TABLE_ENCRYPTION_ADMIN privilege is required to use an ENCRYPTION clause with a setting that differs from the default schema encryption setting.

Prior to MySQL 8.0.16, the ENCRYPTION clause was only supported when altering tables residing in file-per-table tablespaces. As of MySQL 8.0.16, the ENCRYPTION clause is also supported for tables residing in general tablespaces.

For tables that reside in general tablespaces, table and tablespace encryption must match.

Altering table encryption by moving a table to a different tablespace or changing the storage engine is not permitted without explicitly specifying an ENCRYPTION clause.

As of MySQL 8.0.16, specifying an ENCRYPTION clause with a value other than 'N' or '' is not permitted if the table uses a storage engine that does not support encryption. Previously, the clause was accepted. Attempting to create a table without an ENCRYPTION clause in an encryption-enabled schema using a storage engine that does not support encryption is also not permitted.

For more information, see Section 15.13, “InnoDB Data-at-Rest Encryption”.
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 8.0 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";
```
Bear in mind that ALTER TABLE ... COMMENT ... discards any existing comment for the table. See Setting NDB_TABLE options, for additional information and examples.
ENGINE_ATTRIBUTE and SECONDARY_ENGINE_ATTRIBUTE options (available as of MySQL 8.0.21) are used to specify table, column, and index attributes for primary and secondary storage engines. The options are reserved for future use. Index attributes cannot be altered. An index must be dropped and added back with the desired change, which can be performed in a single ALTER TABLE statement.

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.
INSTANT: Operations only modify metadata in the data dictionary. No exclusive metadata locks are taken on the table during preparation and execution, and table data is unaffected, making operations instantaneous. Concurrent DML is permitted. (Introduced in MySQL 8.0.12)

The ALGORITHM clause is optional. If the ALGORITHM clause is omitted, MySQL uses ALGORITHM=INSTANT for storage engines and ALTER TABLE clauses that support it. Otherwise, ALGORITHM=INPLACE is used. If ALGORITHM=INPLACE is not supported, ALGORITHM=COPY is used.

Note

After adding a column to a partitioned table using ALGORITHM=INSTANT, it is no longer possible to perform ALTER TABLE ... EXCHANGE PARTITION on the table.

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 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, specify ALGORITHM=COPY or enable the old_alter_table system variable. 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 15.12.3, “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 15.12.1, “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 13.1.36, “RENAME TABLE Statement”.) Privileges granted specifically for the renamed table are not migrated to the new name. They must be changed manually.
Operations that modify table metadata only. These operations are immediate because the server does not touch table contents. Metadata-only operations include:
- Renaming a column. In NDB Cluster 8.0.18 and later, this operation can also be performed online.
- 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.
- Changing the definition of a spatial column to remove the SRID attribute. (Adding or changing an SRID attribute does require a rebuild and cannot be done in place because the server must verify that all values have the specified SRID value.)
- As of MySQL 8.0.14, changing a column character set, when these conditions apply:
  - The column data type is CHAR, VARCHAR, a TEXT type, or ENUM.
  - The character set change is from utf8mb3 to utf8mb4, or any character set to binary.
  - There is no index on the column.
- As of MySQL 8.0.14, changing a generated column, when these conditions apply:
  - For InnoDB tables, statements that modify generated stored columns but do not change their type, expression, or nullability.
  - For non-InnoDB tables, statements that modify generated stored or virtual columns but do not change their type, expression, or nullability.
  An example of such a change is a change to the column comment.
Renaming an index.
Adding or dropping a secondary index, for InnoDB and NDB tables. See Section 15.12.1, “Online DDL Operations”.
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 23.6.11, “Online Operations with ALTER TABLE in NDB Cluster”.
Modifying index visibility with an ALTER INDEX operation.
Column modifications of tables containing generated columns that depend on columns with a DEFAULT value if the modified columns are not involved in the generated column expressions. For example, changing the NULL property of a separate column can be done in place without a table rebuild.

ALTER TABLE operations that support the INSTANT algorithm include:

Adding a column. This feature is referred to as “Instant ADD COLUMN”. Limitations apply. See Section 15.12.1, “Online DDL Operations”.
Adding or dropping a virtual column.
Adding or dropping a column default value.
Modifying the definition of an ENUM or SET column. The same restrictions apply as described above for ALGORITHM=INSTANT.
Changing the index type.
Renaming a table. The same restrictions apply as described above for ALGORITHM=INSTANT.

For more information about operations that support ALGORITHM=INSTANT, see Section 15.12.1, “Online DDL Operations”.

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 13.1.9.2, “ALTER TABLE and Generated Columns”.

NDB Cluster 8.0 supports online operations using the same ALGORITHM=INPLACE syntax used with the standard MySQL Server. NDB does not support changing a tablespace online; beginning with NDB 8.0.21, it is disallowed. See Section 23.6.11, “Online Operations with ALTER TABLE in NDB Cluster”, for more information.

NDB 8.0.27 and later, when performing a copying ALTER TABLE, checks to ensure that no concurrent writes have been made to the affected table. If it finds that any have been made, NDB rejects the ALTER TABLE statement and raises ER_TABLE_DEF_CHANGED.

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.

Only LOCK = DEFAULT is permitted for operations that use ALGORITHM=INSTANT. The other LOCK clause parameters are not applicable.

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. 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 ALTER TABLE ... ADD, if the column has an expression default value that uses a nondeterministic function, the statement may produce a warning or error. For further information, see Section 11.6, “Data Type Default Values”, and Section 17.1.3.7, “Restrictions on Replication with GTIDs”.

Renaming, Redefining, and Reordering Columns

The CHANGE, MODIFY, RENAME COLUMN, 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 or RENAME COLUMN, but at the expense of convenience for some operations. CHANGE requires naming the column twice if not renaming it, and requires respecifying the column definition if only 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.
RENAME COLUMN:
- Can change a column name but not its definition.
- More convenient than CHANGE to rename a column without changing its definition.
ALTER: Used only to change a column default value.

CHANGE is a MySQL extension to standard SQL. MODIFY and RENAME COLUMN are MySQL extensions 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 or RENAME COLUMN. With 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;

RENAME COLUMN is more convenient to change the name without changing the definition because it requires only the old and new names:

ALTER TABLE t1 RENAME COLUMN b TO a;

In general, you cannot rename a column to a name that already exists in the table. However, this is sometimes not the case, such as when you swap names or move them through a cycle. If a table has columns named a, b, and c, these are valid operations:

-- swap a and b
ALTER TABLE t1 RENAME COLUMN a TO b,
               RENAME COLUMN b TO a;
-- "rotate" a, b, c through a cycle
ALTER TABLE t1 RENAME COLUMN a TO b,
               RENAME COLUMN b TO c,
               RENAME COLUMN c TO a;

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 5.1.11, “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 or RENAME COLUMN, MySQL automatically renames these references to the renamed column:

Indexes that refer to the old column, including invisible indexes and disabled MyISAM indexes.
Foreign keys that refer to the old column.

For columns renamed by CHANGE or RENAME COLUMN, 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”.

As of MySQL 8.0.23, ALTER ... SET VISIBLE and ALTER ... SET INVISIBLE enable column visibility to be changed. See Section 13.1.20.10, “Invisible Columns”.

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 the sql_require_primary_key system variable is enabled, attempting to drop a primary key produces an error.

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 13.1.27, “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 13.1.15, “CREATE INDEX Statement”. The preferred position is after the column list. 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. USING is one such option. For details about permissible index_option values, see Section 13.1.15, “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 13.7.7.22, “SHOW INDEX Statement”.

The ALTER INDEX operation permits an index to be made visible or invisible. An invisible index is not used by the optimizer. Modification of index visibility applies to indexes other than primary keys (either explicit or implicit). This feature is storage engine neutral (supported for any engine). For more information, see Section 8.3.12, “Invisible Indexes”.

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

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 24.6.2, “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.

Prior to MySQL 8.0.16, ALTER TABLE permits only the following limited version of CHECK constraint-adding syntax, which is parsed and ignored:

ADD CHECK (expr)

As of MySQL 8.0.16, ALTER TABLE permits CHECK constraints for existing tables to be added, dropped, or altered:

Add a new CHECK constraint:
```
ALTER TABLE tbl_name
    ADD CONSTRAINT [symbol] CHECK (expr) [[NOT] ENFORCED];
```
The meaning of constraint syntax elements is the same as for CREATE TABLE. See Section 13.1.20.6, “CHECK Constraints”.

Drop an existing CHECK constraint named symbol:

ALTER TABLE tbl_name
    DROP CHECK symbol;

Alter whether an existing CHECK constraint named symbol is enforced:

ALTER TABLE tbl_name
    ALTER CHECK symbol [NOT] ENFORCED;

The DROP CHECK and ALTER CHECK clauses are MySQL extensions to standard SQL.

As of MySQL 8.0.19, ALTER TABLE permits more general (and SQL standard) syntax for dropping and altering existing constraints of any type, where the constraint type is determined from the constraint name:

Drop an existing constraint named symbol:
```
ALTER TABLE tbl_name
    DROP CONSTRAINT symbol;
```
If the sql_require_primary_key system variable is enabled, attempting to drop a primary key produces an error.
Alter whether an existing constraint named symbol is enforced:
```
ALTER TABLE tbl_name
    ALTER CONSTRAINT symbol [NOT] ENFORCED;
```
Only CHECK constraints can be altered to be unenforced. All other constraint types are always enforced.

The SQL standard specifies that all types of constraints (primary key, unique index, foreign key, check) belong to the same namespace. In MySQL, each constraint type has its own namespace per schema. Consequently, names for each type of constraint must be unique per schema, but constraints of different types can have the same name. When multiple constraints have the same name, DROP CONSTRAINT and ADD CONSTRAINT are ambiguous and an error occurs. In such cases, constraint-specific syntax must be used to modify the constraint. For example, use DROP PRIMARY KEY or DROP FOREIGN KEY to drop a primary key or foreign key.

If a table alteration causes a violation of an enforced CHECK constraint, an error occurs and the table is not modified. Examples of operations for which an error occurs:

Attempts to add the AUTO_INCREMENT attribute to a column that is used in a CHECK constraint.
Attempts to add an enforced CHECK constraint or enforce a nonenforced CHECK constraint for which existing rows violate the constraint condition.
Attempts to modify, rename, or drop a column that is used in a CHECK constraint, unless that constraint is also dropped in the same statement. Exception: If a CHECK constraint refers only to a single column, dropping the column automatically drops the constraint.

ALTER TABLE tbl_name RENAME new_tbl_name changes internally generated and user-defined CHECK constraint names that begin with the string “tbl_name_chk_” to reflect the new table name. MySQL interprets CHECK constraint names that begin with the string “tbl_name_chk_” 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).

Importing InnoDB Tables

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 15.6.1.3, “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.