How to add NOT NULL constraint to an already created MySQL column?

Achieve this using ALTER TABLE. Let us first create a table −

mysql> create table DemoTable
   -> (
   -> StudentId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   -> StudentName varchar(100)
   -> );
Query OK, 0 rows affected (0.86 sec)

Let us check the description of the table −

mysql> desc DemoTable;

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    |                |
+-------------+--------------+------+-----+---------+----------------+
2 rows in set (0.01 sec)

Here is the query to add a NOT NULL constraint to the other column “StudentName”, which wasn’t set NOT NULL initially −

mysql> alter table DemoTable modify StudentName varchar(100) NOT NULL;
Query OK, 0 rows affected (1.57 sec)
Records: 0 Duplicates: 0 Warnings: 0

Let us check the description of the table once again −

mysql> desc DemoTable;

This will produce the following output −

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