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 TIME 类型、BIT、SET、ENUM、JSON 等）向某些类型的变更，因为 TiDB 的
CAST函数与 MySQL 的行为存在兼容性问题。