MODIFY COLUMN

ALTER TABLE .. MODIFY COLUMN 语句用于修改已有表上的列，包括列的数据类型和属性。若要同时重命名，可改用 CHANGE COLUMN 语句。

从 v5.1.0 版本起，TiDB 开始支持 Reorg 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

AlterTableStmt

MODIFY COLUMN - 图1

ModifyColumnSpec

MODIFY COLUMN - 图2

ColumnType

MODIFY COLUMN - 图3

ColumnOption

MODIFY COLUMN - 图4

ColumnName

MODIFY COLUMN - 图5

1. AlterTableStmt
2. ::= 'ALTER' 'IGNORE'? 'TABLE' TableName ModifyColumnSpec ( ',' ModifyColumnSpec )*

4. ModifyColumnSpec
5. ::= 'MODIFY' ColumnKeywordOpt 'IF EXISTS' ColumnName ColumnType ColumnOption* ( 'FIRST' | 'AFTER' ColumnName )?

7. ColumnType
8. ::= NumericType
9. | StringType
10. | DateAndTimeType
11. | 'SERIAL'

13. ColumnOption
14. ::= 'NOT'? 'NULL'
15. | 'AUTO_INCREMENT'
16. | 'PRIMARY'? 'KEY' ( 'CLUSTERED' | 'NONCLUSTERED' )?
17. | 'UNIQUE' 'KEY'?
18. | 'DEFAULT' ( NowSymOptionFraction | SignedLiteral | NextValueForSequence )
19. | 'SERIAL' 'DEFAULT' 'VALUE'
20. | 'ON' 'UPDATE' NowSymOptionFraction
21. | 'COMMENT' stringLit
22. | ( 'CONSTRAINT' Identifier? )? 'CHECK' '(' Expression ')' ( 'NOT'? ( 'ENFORCED' | 'NULL' ) )?
23. | 'GENERATED' 'ALWAYS' 'AS' '(' Expression ')' ( 'VIRTUAL' | 'STORED' )?
24. | 'REFERENCES' TableName ( '(' IndexPartSpecificationList ')' )? Match? OnDeleteUpdateOpt
25. | 'COLLATE' CollationName
26. | 'COLUMN_FORMAT' ColumnFormat
27. | 'STORAGE' StorageMedia
28. | 'AUTO_RANDOM' ( '(' LengthNum ')' )?

30. ColumnName ::=
31. Identifier ( '.' Identifier ( '.' Identifier )? )?

示例

Meta-Only Change

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 MODIFY col1 BIGINT;

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

1. SHOW CREATE TABLE t1\G;

1. *************************** 1. row ***************************
2. Table: t1
3. Create Table: CREATE TABLE `t1` (
4. `id` int(11) NOT NULL AUTO_INCREMENT,
5. `col1` bigint(20) DEFAULT NULL,
6. PRIMARY KEY (`id`)
7. ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin AUTO_INCREMENT=30001
8. 1 row in set (0.00 sec)

Reorg-Data Change

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 (12345),(67890);

1. Query OK, 2 rows affected (0.00 sec)
2. Records: 2  Duplicates: 0  Warnings: 0

1. ALTER TABLE t1 MODIFY col1 VARCHAR(5);

1. Query OK, 0 rows affected (2.52 sec)

1. SHOW CREATE TABLE t1\G;

1. *************************** 1. row ***************************
2. Table: t1
3. CREATE TABLE `t1` (
4. `id` int(11) NOT NULL AUTO_INCREMENT,
5. `col1` varchar(5) DEFAULT NULL,
6. PRIMARY KEY (`id`) /*T![clustered_index] CLUSTERED */
7. ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin AUTO_INCREMENT=30001
8. 1 row in set (0.00 sec)

注意：

  • 当所变更的类型与已经存在的数据行产生冲突时，TiDB 会进行报错处理。在上述例子中，TiDB 将进行如下报错：

1. alter table t1 modify column col1 varchar(4); 2. ERROR 1406 (22001): Data Too Long, field len 4, data len 5 - 由于和 Async Commit 功能兼容，DDL 在开始进入到 Reorg Data 前会有一定时间（约 2.5s）的等待处理：

1. Query OK, 0 rows affected (2.52 sec)

MySQL 兼容性

  • 不支持修改主键列上需要 Reorg-Data 的类型，但是支持修改 Meta-Only 的类型。例如：

1. CREATE TABLE t (a int primary key); 2. ALTER TABLE t MODIFY COLUMN a VARCHAR(10); 3. ERROR 8200 (HY000): Unsupported modify column: column has primary key flag

1. CREATE TABLE t (a int primary key); 2. ALTER TABLE t MODIFY COLUMN a INT(10) UNSIGNED; 3. ERROR 8200 (HY000): Unsupported modify column: column has primary key flag

1. CREATE TABLE t (a int primary key); 2. ALTER TABLE t MODIFY COLUMN a bigint; 3. Query OK, 0 rows affected (0.01 sec) - 不支持修改生成列的类型。例如：

1. CREATE TABLE t (a INT, b INT as (a+1)); 2. ALTER TABLE t MODIFY COLUMN b VARCHAR(10); 3. ERROR 8200 (HY000): Unsupported modify column: column is generated - 不支持修改分区表上的列类型。例如：

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 MODIFY COLUMN c1 DATETIME; 3. ERROR 8200 (HY000): Unsupported modify column: table is partition table - 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 cast 函数与 MySQL 的行为有一些兼容性问题。例如：

1. CREATE TABLE t (a DECIMAL(13, 7)); 2. ALTER TABLE t MODIFY COLUMN a DATETIME; 3. ERROR 8200 (HY000): Unsupported modify column: change from original type decimal(13,7) to datetime is currently unsupported yet

另请参阅