How to update a field with a particular value if it is null in MySQL?

To update a field if it is null, use IS NULL property along with the UPDATE command. Let us first create a table −

mysql> create table DemoTable
(
   StudentScore int
);
Query OK, 0 rows affected (0.47 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable values(89);
Query OK, 1 row affected (0.16 sec)
mysql> insert into DemoTable values(NULL);
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable values(45);
Query OK, 1 row affected (0.09 sec)
mysql> insert into DemoTable values(NULL);
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable values(56);
Query OK, 1 row affected (0.14 sec)

Display all records from the table using select statement −

mysql> select *from DemoTable;

This will produce the following output −

+--------------+
| StudentScore |
+--------------+
|           89 |
|         NULL |
|           45 |
|         NULL |
|           56 |
+--------------+
5 rows in set (0.00 sec)

Following is the query to update a field if it is null in MySQL −

mysql> update DemoTable set StudentScore=30 where StudentScore IS NULL;
Query OK, 2 rows affected (0.34 sec)
Rows matched: 2 Changed: 2 Warnings: 0

Let us check the table records once again.

mysql> select *from DemoTable;

This will produce the following output −

+--------------+
| StudentScore |
+--------------+
|           89 |
|           30 |
|           45 |
|           30 |
|           56 |
+--------------+
5 rows in set (0.00 sec)