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
ModifyColumnSpec
ColumnType
ColumnOption
ColumnName

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

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

ColumnType
         ::= NumericType
           | StringType
           | DateAndTimeType
           | 'SERIAL'

ColumnOption
         ::= 'NOT'? 'NULL'
           | 'AUTO_INCREMENT'
           | 'PRIMARY'? 'KEY' ( 'CLUSTERED' | 'NONCLUSTERED' )?
           | 'UNIQUE' 'KEY'?
           | 'DEFAULT' ( NowSymOptionFraction | SignedLiteral | NextValueForSequence )
           | 'SERIAL' 'DEFAULT' 'VALUE'
           | 'ON' 'UPDATE' NowSymOptionFraction
           | 'COMMENT' stringLit
           | ( 'CONSTRAINT' Identifier? )? 'CHECK' '(' Expression ')' ( 'NOT'? ( 'ENFORCED' | 'NULL' ) )?
           | 'GENERATED' 'ALWAYS' 'AS' '(' Expression ')' ( 'VIRTUAL' | 'STORED' )?
           | 'REFERENCES' TableName ( '(' IndexPartSpecificationList ')' )? Match? OnDeleteUpdateOpt
           | 'COLLATE' CollationName
           | 'COLUMN_FORMAT' ColumnFormat
           | 'STORAGE' StorageMedia
           | 'AUTO_RANDOM' ( '(' LengthNum ')' )?

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

示例

Meta-Only Change

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

Query OK, 0 rows affected (0.11 sec)

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

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

ALTER TABLE t1 MODIFY col1 BIGINT;

Query OK, 0 rows affected (0.09 sec)

SHOW CREATE TABLE t1\G;

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

Reorg-Data Change

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

Query OK, 0 rows affected (0.11 sec)

INSERT INTO t1 (col1) VALUES (12345),(67890);

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

ALTER TABLE t1 MODIFY col1 VARCHAR(5);

Query OK, 0 rows affected (2.52 sec)

SHOW CREATE TABLE t1\G;

*************************** 1. row ***************************
       Table: t1
CREATE TABLE `t1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `col1` varchar(5) DEFAULT NULL,
  PRIMARY KEY (`id`) /*T![clustered_index] CLUSTERED */
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin AUTO_INCREMENT=30001
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：
alter table t1 modify column col1 varchar(4);
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：
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：

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

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

CREATE TABLE t (a int primary key);
ALTER TABLE t MODIFY COLUMN a bigint;
Query OK, 0 rows affected (0.01 sec)

不支持修改生成列的类型。例如：

CREATE TABLE t (a INT, b INT as (a+1));
ALTER TABLE t MODIFY COLUMN b VARCHAR(10);
ERROR 8200 (HY000): Unsupported modify column: column is generated

不支持修改分区表上的列类型。例如：

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));
ALTER TABLE t MODIFY COLUMN c1 DATETIME;
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：

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

另请参阅

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