ADD COLUMN
ALTER TABLE.. ADD COLUMN 语句用于向已有表中添加一列。此操作在 TiDB 中是在线的，这意味着在添加列的过程中，表的读写都不会被阻塞。
语法
- 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' )? ( 'GLOBAL' | 'LOCAL' )?
| 'UNIQUE' 'KEY'? ( 'GLOBAL' | 'LOCAL' )?
| '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
mysql> CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT);
Query OK, 0 rows affected (0.11 sec)
mysql> INSERT INTO t1 VALUES (NULL);
Query OK, 1 row affected (0.02 sec)
mysql> SELECT * FROM t1;
+----+
| id |
+----+
| 1 |
+----+
1 row in set (0.00 sec)
mysql> ALTER TABLE t1 ADD COLUMN c1 INT NOT NULL;
Query OK, 0 rows affected (0.28 sec)
mysql> SELECT * FROM t1;
+----+----+
| id | c1 |
+----+----+
| 1 | 0 |
+----+----+
1 row in set (0.00 sec)
mysql> ALTER TABLE t1 ADD c2 INT NOT NULL AFTER c1;
Query OK, 0 rows affected (0.28 sec)
mysql> SELECT * FROM t1;
+----+----+----+
| id | c1 | c2 |
+----+----+----+
| 1 | 0 | 0 |
+----+----+----+
1 row in set (0.00 sec)
MySQL 兼容性
- 不支持添加新列并将其设置为
PRIMARY KEY。 - 不支持添加新列并将其设置为
AUTO_INCREMENT。 - 添加生成列存在限制，详见：generated column limitations。
- 在添加新列时，通过指定
PRIMARY KEY或UNIQUE INDEX为GLOBAL来设置 全局索引 是 TiDB 针对 分区表 的扩展功能，与 MySQL 不兼容。