如何在MYSQL中减去（从同一表产生）总和？

Question

amount  group
--------------
 100     'a'
  40     'b'
  30     'a'
  50     'b'

查询输出：

diff(a-b)
---------
  40

如何在MYSQL中做到这一点？

Answer 1

您可以简单地：

SELECT (SELECT SUM(amount) FROM t WHERE `group` = 'a') -
       (SELECT SUM(amount) FROM t WHERE `group` = 'b') AS diff

或者：

SELECT SUM(CASE
           WHEN `group` = 'a' THEN  amount
           WHEN `group` = 'b' THEN -amount
       END) AS diff
FROM t