CHANGE COLUMN

ALTER TABLE.. CHANGE COLUMN 语句用于在已有表上更改列，包括对列进行重命名，和将数据改为兼容类型。

从 v5.1.0 版本起，TiDB 开始支持 Reorg 数据的类型变更，包括但不限于：

  • 从 varchar 转换为 bigint
  • decimal 精度修改
  • 从 varchar(10) 到 varchar(5) 的长度压缩

语法图

1. AlterTableStmt ::=
2. 'ALTER' IgnoreOptional 'TABLE' TableName ( AlterTableSpecListOpt AlterTablePartitionOpt | 'ANALYZE' 'PARTITION' PartitionNameList ( 'INDEX' IndexNameList )? AnalyzeOptionListOpt )

4. AlterTableSpec ::=
5. TableOptionList
6. |   'SET' 'TIFLASH' 'REPLICA' LengthNum LocationLabelList
7. |   'CONVERT' 'TO' CharsetKw ( CharsetName | 'DEFAULT' ) OptCollate
8. |   'ADD' ( ColumnKeywordOpt IfNotExists ( ColumnDef ColumnPosition | '(' TableElementList ')' ) | Constraint | 'PARTITION' IfNotExists NoWriteToBinLogAliasOpt ( PartitionDefinitionListOpt | 'PARTITIONS' NUM ) )
9. |   ( ( 'CHECK' | 'TRUNCATE' ) 'PARTITION' | ( 'OPTIMIZE' | 'REPAIR' | 'REBUILD' ) 'PARTITION' NoWriteToBinLogAliasOpt ) AllOrPartitionNameList
10. |   'COALESCE' 'PARTITION' NoWriteToBinLogAliasOpt NUM
11. |   'DROP' ( ColumnKeywordOpt IfExists ColumnName RestrictOrCascadeOpt | 'PRIMARY' 'KEY' | 'PARTITION' IfExists PartitionNameList | ( KeyOrIndex IfExists | 'CHECK' ) Identifier | 'FOREIGN' 'KEY' IfExists Symbol )
12. |   'EXCHANGE' 'PARTITION' Identifier 'WITH' 'TABLE' TableName WithValidationOpt
13. |   ( 'IMPORT' | 'DISCARD' ) ( 'PARTITION' AllOrPartitionNameList )? 'TABLESPACE'
14. |   'REORGANIZE' 'PARTITION' NoWriteToBinLogAliasOpt ReorganizePartitionRuleOpt
15. |   'ORDER' 'BY' AlterOrderItem ( ',' AlterOrderItem )*
16. |   ( 'DISABLE' | 'ENABLE' ) 'KEYS'
17. |   ( 'MODIFY' ColumnKeywordOpt IfExists | 'CHANGE' ColumnKeywordOpt IfExists ColumnName ) ColumnDef ColumnPosition
18. |   'ALTER' ( ColumnKeywordOpt ColumnName ( 'SET' 'DEFAULT' ( SignedLiteral | '(' Expression ')' ) | 'DROP' 'DEFAULT' ) | 'CHECK' Identifier EnforcedOrNot | 'INDEX' Identifier IndexInvisible )
19. |   'RENAME' ( ( 'COLUMN' | KeyOrIndex ) Identifier 'TO' Identifier | ( 'TO' | '='? | 'AS' ) TableName )
20. |   LockClause
21. |   AlgorithmClause
22. |   'FORCE'
23. |   ( 'WITH' | 'WITHOUT' ) 'VALIDATION'
24. |   'SECONDARY_LOAD'
25. |   'SECONDARY_UNLOAD'

27. ColumnName ::=
28. Identifier ( '.' Identifier ( '.' Identifier )? )?

30. ColumnDef ::=
31. ColumnName ( Type | 'SERIAL' ) ColumnOptionListOpt

33. ColumnPosition ::=
34. ( 'FIRST' | 'AFTER' ColumnName )?

示例

1. CREATE TABLE t1 (id int not null primary key AUTO_INCREMENT, col1 INT);

1. Query OK, 0 rows affected (0.11 sec)

1. INSERT INTO t1 (col1) VALUES (1),(2),(3),(4),(5);

1. Query OK, 5 rows affected (0.02 sec)
2. Records: 5  Duplicates: 0  Warnings: 0

1. ALTER TABLE t1 CHANGE col1 col2 INT;

1. Query OK, 0 rows affected (0.09 sec)

1. ALTER TABLE t1 CHANGE col2 col3 BIGINT, ALGORITHM=INSTANT;

1. Query OK, 0 rows affected (0.08 sec)

1. ALTER TABLE t1 CHANGE col3 col4 BIGINT, CHANGE id id2 INT NOT NULL;

1. ERROR 1105 (HY000): can't run multi schema change

1. CREATE TABLE t (a int primary key);
2. ALTER TABLE t CHANGE COLUMN a a VARCHAR(10);

1. ERROR 8200 (HY000): Unsupported modify column: column has primary key flag

1. CREATE TABLE t (c1 INT, c2 INT, c3 INT) partition by range columns(c1) ( partition p0 values less than (10), partition p1 values less than (maxvalue));
2. ALTER TABLE t CHANGE COLUMN c1 c1 DATETIME;

1. ERROR 8200 (HY000): Unsupported modify column: table is partition table

1. CREATE TABLE t (a INT, b INT as (a+1));
2. ALTER TABLE t CHANGE COLUMN b b VARCHAR(10);

1. ERROR 8200 (HY000): Unsupported modify column: column is generated

1. CREATE TABLE t (a DECIMAL(13, 7));
2. ALTER TABLE t CHANGE COLUMN a a DATETIME;

1. ERROR 8200 (HY000): Unsupported modify column: change from original type decimal(13,7) to datetime is currently unsupported yet

MySQL 兼容性

  • 不支持在单个 ALTER TABLE 语句中进行多个更改。
  • 不支持主键列上 Reorg-Data 类型的变更。
  • 不支持分区表上的列类型变更。
  • 不支持生成列上的列类型变更。
  • 不支持部分数据类型（例如，部分时间类型、Bit、Set、Enum、JSON 等）的变更，因为 TiDB 中 CAST 函数与 MySQL 的行为存在兼容性问题。

另请参阅