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How can we set PRIMARY KEY on multiple columns of a MySQL table?
Actually, MySQL allows us to set PRIMARY KEY on multiple columns. The advantage of doing this is that we can work on multiple columns as a single entity.
Example
We have created the table allotment by defining composite PRIMARY KEY on multiple columns as follows −
mysql> Create table allotment( RollNo Int, Name Varchar(20), RoomNo Int, PRIMARY KEY(RollNo, RoomNo)); Query OK, 0 rows affected (0.23 sec) mysql> Describe allotment; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | RollNo | int(11) | NO | PRI | 0 | | | Name | varchar(20) | YES | | NULL | | | RoomNo | int(11) | NO | PRI | 0 | | +--------+-------------+------+-----+---------+-------+ 3 rows in set (0.10 sec)
Now, when we will try to insert the duplicate composite values in both the columns then MySQL throws an error as follows −
mysql> Insert Into allotment values(1, 'Aarav', 201),(2,'Harshit',201),(1,'Rahul ',201); ERROR 1062 (23000): Duplicate entry '1-201' for key 'PRIMARY'
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