CASE WHEN Column1 is NULL Then NULL Else Column2 in MySQL

For this, you can use the CASE statement. Let us first create a table−

mysql> create table DemoTable
   -> (
   -> Name varchar(20),
   -> Marks1 int,
   -> Marks2 int
   -> );
Query OK, 0 rows affected (0.72 sec)

Insert some records in the table using insert command−

mysql> insert into DemoTable values('Chris',45,null);
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable values('David',null,78);
Query OK, 1 row affected (0.16 sec)
mysql> insert into DemoTable values('Bob',67,98);
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 −

+-------+--------+--------+
| Name  | Marks1 | Marks2 |
+-------+--------+--------+
| Chris |     45 |   NULL |
| David |   NULL |     78 |
| Bob   |     67 |     98 |
+-------+--------+--------+
3 rows in set (0.00 sec)

Here is the query to implement CASE WHEN −

mysql> select *,
   -> (case when Marks1 is null then null else Marks2 end ) as Value
   -> from DemoTable;

This will produce the following output −

+-------+--------+--------+-------+
| Name  | Marks1 | Marks2 | Value |
+-------+--------+--------+-------+
| Chris |     45 |   NULL |  NULL |
| David |   NULL |     78 |  NULL |
| Bob   |     67 |     98 |    98 |
+-------+--------+--------+-------+
3 rows in set (0.00 sec)