How to change a primary key in MySQL to auto_increment?

To change a primary key to auto_increment, you can use MODIFY command. Let us first create a table.

mysql> create table changePrimaryKeyInAutoIncrement
   -> (
   -> StudentId int not null primary key,
   -> StudentName varchar(100),
   -> StudentAge int,
   -> StudentAddress varchar(100)
   -> );
Query OK, 0 rows affected (0.63 sec)

Let us now check the description of table using desc command:

mysql> desc changePrimaryKeyInAutoIncrement;

This will produce the following output

+----------------+--------------+------+-----+---------+-------+
| Field          | Type         | Null | Key | Default | Extra |
+----------------+--------------+------+-----+---------+-------+
| StudentId      | int(11)      | NO   | PRI | NULL    |       |
| StudentName    | varchar(100) | YES  |     | NULL    |       |
| StudentAge     | int(11)      | YES  |     | NULL    |       |
| StudentAddress | varchar(100) | YES  |     | NULL    |       |
+----------------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

Look at the above sample output, StudentId column is a primary key. Now let us change the primary key to auto_increment:

mysql> alter table changePrimaryKeyInAutoIncrement MODIFY StudentId INT AUTO_INCREMENT;
Query OK, 0 rows affected (1.48 sec)
Records: 0 Duplicates: 0 Warnings: 0

Let us check the table description of once again:

mysql> desc changePrimaryKeyInAutoIncrement;

This will produce the following output

+----------------+--------------+------+-----+---------+----------------+
| Field          | Type         | Null | Key | Default | Extra          |
+----------------+--------------+------+-----+---------+----------------+
| StudentId      | int(11)      | NO   | PRI | NULL    | auto_increment |
| StudentName    | varchar(100) | YES  |     | NULL    |                |
| StudentAge     | int(11)      | YES  |     | NULL    |                |
| StudentAddress | varchar(100) | YES  |     | NULL    |                |
+----------------+--------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

Look at the above sample output, StudentId column has been changed to auto_increment.