DROP COLUMN
This statement drops a column from a specified table. DROP COLUMN is online in TiDB, which means that it does not block read or write operations.
Synopsis
- AlterTableStmt
- DropColumnSpec
- ColumnName
AlterTableStmt
::= 'ALTER' 'IGNORE'? 'TABLE' TableName DropColumnSpec ( ',' DropColumnSpec )*
DropColumnSpec
::= 'DROP' 'COLUMN'? 'IF EXISTS'? ColumnName ( 'RESTRICT' | 'CASCADE' )?
ColumnName
::= Identifier ( '.' Identifier ( '.' Identifier )? )?
Examples
mysql> 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)
mysql> 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
mysql> 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)
mysql> ALTER TABLE t1 DROP COLUMN col1, DROP COLUMN col2;
ERROR 1105 (HY000): can't run multi schema change
mysql> 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)
mysql> ALTER TABLE t1 DROP COLUMN col1;
Query OK, 0 rows affected (0.27 sec)
mysql> SELECT * FROM t1;
+----+------+
| id | col2 |
+----+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
| 5 | 5 |
+----+------+
5 rows in set (0.00 sec)
MySQL compatibility
- Dropping multiple columns in the same statement is not supported.
- Dropping primary key columns or columns covered by the composite index is not supported.