Sử dụng CASE statement. Tạo bảng:
mysql> create table DemoTable1988 ( Value1 int, Value2 int, Price int ); Query OK, 0 rows affected (0.64 sec)
Thêm dữ liệu:
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)
Hiển thị dữ liệu:
mysql> select * from DemoTable1988;
+--------+--------+-------+ | Value1 | Value2 | Price | +--------+--------+-------+ | 10 | 7 | 500 | | 7 | 9 | 400 | | 8 | 7 | 200 | | 7 | 4 | 300 | +--------+--------+-------+ 4 rows in set (0.00 sec)
Truy vấn dùng CASE để tổng Price khi Value2=7 trừ tổng Price khi Value1=7:
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;
Kết quả:
+--------+ | Result | +--------+ | 0 | +--------+ 1 row in set (0.23 sec)