Set NOT NULL attribute to an existing column in MySQL

To set NOT NULL attribute to an existing column, use ALTER TABLE command. Let us first create a table −

mysql> create table DemoTable1949
   (
   UserId int,
   UserName varchar(20)
   );
Query OK, 0 rows affected (0.00 sec)

Here is the query to set NOT NULL attribute to an existing column −

mysql> alter table DemoTable1949 modify UserName varchar(20) not null;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

Let us check the description of table −

mysql> desc DemoTable1949;

This will produce the following output −

+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| UserId   | int(11)     | YES  |     | NULL    |       |
| UserName | varchar(20) | NO   |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable1949 values(101,NULL);
ERROR 1048 (23000): Column 'UserName' cannot be null
mysql> insert into DemoTable1949 values(101,'Chris');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1949 values(102,'Bob');
Query OK, 1 row affected (0.00 sec)

Display all records from the table using select statement −

mysql> select * from DemoTable1949;

This will produce the following output −

+--------+----------+
| UserId | UserName |
+--------+----------+
|    101 | Chris    |
|    102 | Bob      |
+--------+----------+
2 rows in set (0.00 sec)