DROP COLUMN
DROP COLUMN 语句用于从指定的表中删除列。在 TiDB 中，COLUMN 为在线操作，不会阻塞表中的数据读写。
语法图
- AlterTableStmt
- DropColumnSpec
- ColumnName
AlterTableStmt
::= 'ALTER' 'IGNORE'? 'TABLE' TableName DropColumnSpec ( ',' DropColumnSpec )*
DropColumnSpec
::= 'DROP' 'COLUMN'? 'IF EXISTS'? ColumnName ( 'RESTRICT' | 'CASCADE' )?
ColumnName
::= Identifier ( '.' Identifier ( '.' Identifier )? )?
示例
CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, col1 INT NOT NULL, col2 INT NOT NULL);
Query OK, 0 rows affected (0.12 sec)
INSERT INTO t1 (col1,col2) VALUES (1,1),(2,2),(3,3),(4,4),(5,5);
Query OK, 5 rows affected (0.02 sec)
Records: 5 Duplicates: 0 Warnings: 0
SELECT * FROM t1;
+----+------+------+
| id | col1 | col2 |
+----+------+------+
| 1 | 1 | 1 |
| 2 | 2 | 2 |
| 3 | 3 | 3 |
| 4 | 4 | 4 |
| 5 | 5 | 5 |
+----+------+------+
5 rows in set (0.01 sec)
ALTER TABLE t1 DROP COLUMN col1, DROP COLUMN col2;
ERROR 1105 (HY000): can't run multi schema change
SELECT * FROM t1;
+----+------+------+
| id | col1 | col2 |
+----+------+------+
| 1 | 1 | 1 |
| 2 | 2 | 2 |
| 3 | 3 | 3 |
| 4 | 4 | 4 |
| 5 | 5 | 5 |
+----+------+------+
5 rows in set (0.00 sec)
ALTER TABLE t1 DROP COLUMN col1;
Query OK, 0 rows affected (0.27 sec)
SELECT * FROM t1;
+----+------+
| id | col2 |
+----+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
| 5 | 5 |
+----+------+
5 rows in set (0.00 sec)
MySQL 兼容性
- 目前不支持在一条语句中同时删除多个列。
- 目前不支持删除主键列或组合索引相关列。