As we know that a PRIMARY KEY column must have unique values and cannot have null values hence if we will define a column with UNIQUE and NOT NULL constraint both then that column would become PRIMARY KEY column.
In this example, we have created a table ‘Student123’ by defining column ‘RollNo’ with UNIQUE and NOT NULL constraints. Now, by describing the table we can see that ‘RollNo’ is the PRIMARY KEY column.
mysql> Create table Student123(RollNo INT UNIQUE NOT NULL, Name varchar(20)); Query OK, 0 rows affected (0.25 sec) mysql> DESCRIBE Student123; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | RollNo | int(11) | NO | PRI | NULL | | | Name | varchar(20) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 2 rows in set (0.04 sec)
