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.

Example

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)