CHANGE COLUMN
ALTER TABLE.. CHANGE COLUMN
语句用于修改现有表中的列。该修改可以包括重命名列以及将数据类型更改为兼容的类型。
自 v5.1.0 版本起,TiDB 已支持更改 Reorg 数据类型,包括但不限于:
- 将
VARCHAR
改为BIGINT
- 修改
DECIMAL
的精度 - 将
VARCHAR(10)
压缩为VARCHAR(5)
概要
- 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
- 不支持对 Reorg-Data 类型的主键列进行更改。
- 不支持对分区表的列类型进行更改。
- 不支持对生成列的列类型进行更改。
- 由于 TiDB 和 MySQL 之间的
CAST
函数行为存在兼容性问题,不支持将某些数据类型(例如 TIME、BIT、SET、ENUM 和 JSON 类型)更改为其他类型。