Only update the MySQL field if the field contains null or 0?

For this, set conditions using MySQL IF(). Let us first create a table −

mysql> create table DemoTable1592
   -> (
   -> StudentMarks int
   -> );
Query OK, 0 rows affected (0.76 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable1592 values(56);
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable1592 values(NULL);
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable1592 values(98);
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable1592 values(0);
Query OK, 1 row affected (0.10 sec)
mysql> insert into DemoTable1592 values(75);
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable1592 values(NULL);
Query OK, 1 row affected (0.11 sec)

Display all records from the table using select statement −

mysql> select * from DemoTable1592;

This will produce the following output −

+--------------+
| StudentMarks |
+--------------+
|           56 |
|         NULL |
|           98 |
|            0 |
|           75 |
|         NULL |
+--------------+
6 rows in set (0.00 sec)

Here is the query to only update the field if it contains null or 0 −

mysql> update DemoTable1592 set StudentMarks=if(StudentMarks IS NULL or StudentMarks=0,33,StudentMarks);
Query OK, 3 rows affected (0.17 sec)
Rows matched: 6  Changed: 3 Warnings: 0

Let us check the table records once again −

mysql> select * from DemoTable1592;

This will produce the following output −

+--------------+
| StudentMarks |
+--------------+
|           56 |
|           33 |
|           98 |
|           33 |
|           75 |
|           33 |
+--------------+
6 rows in set (0.00 sec)