How to add not null constraint to existing column in MySQL?

To add not null constraint to an existing column in MySQL, we will use the ALTER command. This is a type of validation to restrict the user from entering null values.

Let us see an example. Firstly, we will create a table. The CREATE command is used to create a table.

mysql> create table AddNotNUlldemo
   - > (
   - > name varchar(100)
   - > );
Query OK, 0 rows affected (0.44 sec)

To insert records.

mysql> insert into AddNotNUlldemo values('John');
Query OK, 1 row affected (0.19 sec)

mysql> insert into AddNotNUlldemo values('Bob');
Query OK, 1 row affected (0.19 sec)

To display all the records.

mysql> select *from AddNotNUlldemo;

Here is the output.

+------+
| name |
+------+
| John |
| Bob  |
+------+
2 rows in set (0.00 sec)

The following is the syntax to add a constraint to an existing column.

ALTER table yourTableName  modify column_name data type constraint;

Let us now implement the above syntax to implement the below query. Here, we are including “not null” constraint.

mysql> ALTER table AddNotNUlldemo modify name varchar(100) not null;
Query OK, 0 rows affected (1.14 sec)
Records: 0  Duplicates: 0  Warnings: 0

Now, we cannot insert a null value into the table “AddNotNUlldemo” because we have set the constraint above as “not null”. If we try to add a null value, we will get an error. The following is an example.

mysql> INSERT into AddNotNUlldemo values(null);
ERROR 1048 (23000): Column 'name' cannot be null

To display all the records.

mysql> SELECT *from AddNotNUlldemo;

Here is the output.

+------+
| name |
+------+
| John |
| Bob  |
+------+
2 rows in set (0.00 sec)