sql - 如何在特定条件下找到多对多的总和？

Question

我有一个多对多的关系，我想找到在某个日期创建的一些 A 的所有 W 及其总和（频率）。最好的方法是什么？

结果不应该是：

w.name | sum 
-------+------
 bar   |  2
 bar   |  5  
 baz   |  4
 bim   |  3
 foo   |  2
 foo   |  3  

结果应该是：

w.name | sum 
-------+------
 bar   |  7  
 baz   |  4
 bim   |  3
 foo   |  5 

我的桌子：

表 A：

id  | title | author | created_at 
----+-------+--------+-------------
... |  ...  |  ...   | ...
11  |  abc  |  ccc   |  Thu, 5 Apr 2012 08:01:25 GMT
12  |  bcb  |  ddd   |  Thu, 5 Apr 2012 10:11:15 GMT
13  |  dfg  |  aaa   |  Fri, 6 Apr 2012 07:33:45 GMT

表 AW：

id  | aid | wid | freq
----+-----+-----+------
... | ... | ... | ...
201 | 11  |  2  |  2
202 | 11  |  3  |  4
203 | 11  | 17  |  2
204 | 12  | 17  |  3
205 | 12  | 66  |  1
206 | 12  | 12  |  2
207 | 12  | 24  |  5
208 | 12  |  7  |  3
209 | 13  |  2  |  5
210 | 13  | ... | ...

表 W：

id  | name  |  ...  
----+-------+--------
... |  ...  |  ...   
2   |  bar  |  ...   
3   |  baz  |  ...   
... |  ...  |  ...
7   |  bim  |  ...   
... |  ...  |  ...
17  |  foo  |

Answer 1

对于每个，今天创建W的频率总和：A

select  W.name
,       sum(AW.frequency)
from    A
join    AW
on      AW.aid = a.id
join    W
on      W.id = AW.bid
where   '2012-04-08' < A.created_at and A.created_at < '2012-04-09'
group by
        W.name

Answer 2

SELECT w.name, SUM(aw.frequency)
FROM a INNER JOIN aw ON (a.id = aw.aid)
       INNER JOIN w ON (aw.wid = w.id)
WHERE a.created_at >= INITIAL_DATE and a.created_at <= END_DATE
GROUP BY w.name