How do I modify a MySQL column to allow NULL?

For our example, let us create a table with NOT NULL constraint. After that, we will modify a column to allow NULL.

The following is the query to create a table with NOT NULL constraint.

mysql> create table AllowNullDemo
   -> (
   -> id int not null
   -> );
Query OK, 0 rows affected (0.48 sec)=

Insert records with the help of INSERT command. The query is as follows.

mysql> insert into AllowNullDemo values();
Query OK, 1 row affected, 1 warning (0.19 sec)

mysql> insert into AllowNullDemo values();
Query OK, 1 row affected, 1 warning (0.15 sec)

The query to display records.

mysql> select *from AllowNullDemo;

Here is the output. The value 0 is displayed, since we haven’t added any value while using INSERT command above.

+----+
| id |
+----+
|  0 |
|  0 |
+----+
2 rows in set (0.00 sec)

Here is the syntax to allow NULL value.

alter table yourTableName  modify column yourColumnName datatype;

Apply the above syntax to modify the column to allow NULL. The query is as follows.

mysql> alter table AllowNullDemo modify column id int;
Query OK, 0 rows affected (1.59 sec)
Records: 0  Duplicates: 0  Warnings: 0

After executing the above query, you can insert NULL value to that column because the column is modified successfully above.

mysql>  insert into AllowNullDemo values();
Query OK, 1 row affected (0.15 sec)

Display records to check whether the last value inserted is NULL or not.

mysql> select *from AllowNullDemo;

The following is the output wherein NULL value is visible now.

+------+
| id   |
+------+
|    0 |
|    0 |
| NULL |
+------+
3 rows in set (0.00 sec)

Using the above method, we can easily modify a MySQL column to allow NULL.