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

AlterTableStmt
ChangeColumnSpec
ColumnType
ColumnOption
ColumnName

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

ChangeColumnSpec
         ::= 'CHANGE' ColumnKeywordOpt 'IF EXISTS' ColumnName 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

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 CHANGE col1 col2 INT;

Query OK, 0 rows affected (0.09 sec)

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

Query OK, 0 rows affected (0.08 sec)

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

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

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

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

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 CHANGE COLUMN c1 c1 DATETIME;

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

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

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

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

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

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