ADD COLUMN

ALTER TABLE.. ADD COLUMN 语句用于在已有表中添加列。在 TiDB 中，ADD COLUMN 为在线操作，不会阻塞表中的数据读写。

语法图

AlterTableStmt

TableName

AddColumnSpec

ColumnType

ColumnOption

AlterTableStmt
         ::= 'ALTER' 'IGNORE'? 'TABLE' TableName AddColumnSpec ( ',' AddColumnSpec )*
TableName ::=
    Identifier ('.' Identifier)?
AddColumnSpec
         ::= 'ADD' 'COLUMN' 'IF NOT EXISTS'? 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 ')' )?

示例

CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT);

Query OK, 0 rows affected (0.11 sec)

INSERT INTO t1 VALUES (NULL);

Query OK, 1 row affected (0.02 sec)

SELECT * FROM t1;

+----+
| id |
+----+
|  1 |
+----+
1 row in set (0.00 sec)

ALTER TABLE t1 ADD COLUMN c1 INT NOT NULL;

Query OK, 0 rows affected (0.28 sec)

SELECT * FROM t1;

+----+----+
| id | c1 |
+----+----+
|  1 |  0 |
+----+----+
1 row in set (0.00 sec)

ALTER TABLE t1 ADD c2 INT NOT NULL AFTER c1;

Query OK, 0 rows affected (0.28 sec)

SELECT * FROM t1;

+----+----+----+
| id | c1 | c2 |
+----+----+----+
|  1 |  0 |  0 |
+----+----+----+
1 row in set (0.00 sec)

MySQL 兼容性

• 不支持将新添加的列设为 PRIMARY KEY。
• 不支持将新添加的列设为 AUTO_INCREMENT。
• 对添加生成列有局限性，具体可参考：生成列局限性。

另请参阅

• ADD INDEX
• CREATE TABLE