数据
username month salary
A,2015-01,5
A,2015-01,15
B,2015-01,5
A,2015-01,8
B,2015-01,25
A,2015-01,5
A,2015-02,4
A,2015-02,6
B,2015-02,10
B,2015-02,5
求结果
A 2015-01 33 33
A 2015-02 10 43
B 2015-01 30 30
B 2015-02 15 45
即级联查询，比如
a一月份 salary为33 二月份为10 最后一个字段为salary(1+2) 43
首先遇到sql复杂问题一步一步拆分来解决
思路通过一个表自身内链接，来进行错位级联
1查询出每个用户每个月每个group by(username,month)的总salary
select username,month,sum(salary) as salary from time group by username,month;
+-----------+----------+---------+--+
| username | month | salary |
+-----------+----------+---------+--+
| A | 2015-01 | 33 |
| A | 2015-02 | 10 |
| B | 2015-01 | 30 |
| B | 2015-02 | 15 |
+-----------+----------+---------+--+
2
将表自身链接,左右内都可，自身链接
条件是username相等则，A表的1月会和B 表的1和2月相链接
同理
select A.*,B.*
from
(select username,month,sum(salary) as salary from time group by username,month) A
inner join
(select username,month,sum(salary) as salary from time group by username,month) B
on
A.username=B.username;
+-------------+----------+-----------+-------------+----------+-----------+--+
| a.username | a.month | a.salary | b.username | b.month | b.salary |
+-------------+----------+-----------+-------------+----------+-----------+--+
| A | 2015-01 | 33 | A | 2015-01 | 33 |
| A | 2015-01 | 33 | A | 2015-02 | 10 |
| A | 2015-02 | 10 | A | 2015-01 | 33 |
| A | 2015-02 | 10 | A | 2015-02 | 10 |
| B | 2015-01 | 30 | B | 2015-01 | 30 |
| B | 2015-01 | 30 | B | 2015-02 | 15 |
| B | 2015-02 | 15 | B | 2015-01 | 30 |
| B | 2015-02 | 15 | B | 2015-02 | 15 |
+-------------+----------+-----------+-------------+----------+-----------+--+
3 将2作为一个表，查找，先按照A表用户名然后月份分组则变成
要取salary则从下面中取，两个相同的所以max就行从一组中
A | 2015-01 | 33 | A | 2015-01 | 33 |
| A | 2015-01 | 33 | A | 2015-02 | 10 |
where限定，取小于当前月份的
select A.username ,A.month ,max(salary) as salary ,sum(B.salary) as result
from
(select username,month,sum(salary) as salary from time group by username,month) A
inner join
(select username,month,sum(salary) as salary from time group by username,month) B
on
A.username=B.username
where B.month<=A.month
group by A.username,A.month;