Set an alternative of WHERE clause for each SELECT field in MySQL

You can use CASE statement −

mysql> create table DemoTable1988
   (
   Value1 int,
   Value2 int,
   Price int
   );
Query OK, 0 rows affected (0.64 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable1988 values(10,7,500);
Query OK, 1 row affected (0.21 sec)
mysql> insert into DemoTable1988 values(7,9,400);
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable1988 values(8,7,200);
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable1988 values(7,4,300);
Query OK, 1 row affected (0.16 sec)

Display all records from the table using select statement −

mysql> select * from DemoTable1988;

This will produce the following output −

+--------+--------+-------+
| Value1 | Value2 | Price |
+--------+--------+-------+
|     10 |      7 |   500 |
|      7 |      9 |   400 |
|      8 |      7 |   200 |
|      7 |      4 |   300 |
+--------+--------+-------+
4 rows in set (0.00 sec)

Here is the query to WHERE clause for each SELECT field:

mysql> select sum( case when Value2=7 then Price else 0 end)-
   sum( case when Value1=7 then Price else 0 end) as Result
   from DemoTable1988;

This will produce the following output −

+--------+
| Result |
+--------+
|      0 |
+--------+
1 row in set (0.23 sec)