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

CHANGE COLUMN - 图1

ChangeColumnSpec

CHANGE COLUMN - 图2

ColumnType

CHANGE COLUMN - 图3

ColumnOption

CHANGE COLUMN - 图4

ColumnName

CHANGE COLUMN - 图5

  1. AlterTableStmt
  2.          ::= 'ALTER' 'IGNORE'? 'TABLE' TableName ChangeColumnSpec ( ',' ChangeColumnSpec )*
  4. ChangeColumnSpec
  5.          ::= 'CHANGE' ColumnKeywordOpt 'IF EXISTS' ColumnName 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

  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 CHANGE col1 col2 INT;
  1. Query OK, 0 rows affected (0.09 sec)
  1. ALTER TABLE t1 CHANGE col2 col3 BIGINT, ALGORITHM=INSTANT;
  1. Query OK, 0 rows affected (0.08 sec)
  1. ALTER TABLE t1 CHANGE col3 col4 BIGINT, CHANGE id id2 INT NOT NULL;
  1. ERROR 1105 (HY000): can't run multi schema change
  1. CREATE TABLE t (a int primary key);
  2. ALTER TABLE t CHANGE COLUMN a a VARCHAR(10);
  1. ERROR 8200 (HY000): Unsupported modify column: column has primary key flag
  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 CHANGE COLUMN c1 c1 DATETIME;
  1. ERROR 8200 (HY000): Unsupported modify column: table is partition table
  1. CREATE TABLE t (a INT, b INT as (a+1));
  2. ALTER TABLE t CHANGE COLUMN b b VARCHAR(10);
  1. ERROR 8200 (HY000): Unsupported modify column: column is generated
  1. CREATE TABLE t (a DECIMAL(13, 7));
  2. ALTER TABLE t CHANGE COLUMN a a DATETIME;
  1. 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