ADD COLUMN

The ALTER TABLE.. ADD COLUMN statement adds a column to an existing table. This operation is online in TiDB, which means that neither reads or writes to the table are blocked by adding a column.

Synopsis

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 ')' )?

Examples

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 compatibility

• Adding a new column and setting it to the PRIMARY KEY is not supported.
• Adding a new column and setting it to AUTO_INCREMENT is not supported.
• There are limitations on adding generated columns, refer to: generated column limitations.
• Setting a global index by specifying PRIMARY KEY or UNIQUE INDEX as GLOBAL when you add a new column is a TiDB extension for partitioned tables and is not compatible with MySQL.

See also

• ADD INDEX
• CREATE TABLE