MySQL 案例与分组依据

Question

我有一张像这样的表（名称=费用）：

id  amount  date
1   -1687   2014-01-02 00:00:00.0
2   11000   2014-01-02 00:00:00.0
3   1500    2014-01-03 00:00:00.0
4   -3800   2014-01-03 00:00:00.0
5   119500  2014-01-01 00:00:00.0
6   -2999   2014-01-04 00:00:00.0
7   5972    2014-01-04 00:00:00.0
..
8   7208    2014-12-31 00:00:00.0

我能够检索 收入 并按月分组，如下所示：

SELECT 
      SUM(amount), 
      date 
 FROM expense 
WHERE YEAR(now()) = YEAR(date) AND amount>0 
GROUP BY MONTH(date);

我可以检索 expense 并按月份对其进行分组，如下所示：

SELECT
      SUM(amount), 
      date 
 FROM expense 
WHERE YEAR(now()) = YEAR(date) AND amount<=0 
GROUP BY MONTH(date);

我想知道是否有办法将这两个查询结合起来，同时检索 income 和 expense 并在一个单个查询中按月分组。

Answer 1

是的，你需要一个案例陈述：

SELECT
   sum(case when amount> 0 then amount end) as income, 
   sum(case when amount<=0 then amount end) as expense, 
   date 
FROM expense 
WHERE YEAR(now()) = YEAR(date) 
GROUP BY MONTH(date);