php - 从下表中按类别检索库存余额

Question

我有如下表结构

id  stock   category    created                 type    
11  10      4           2015-06-05 13:30:56     a
12  5       4           2015-06-05 13:31:30     s
14  12      6           2015-06-05 15:58:41     a
15  10      6           2015-06-05 15:59:07     s   
16  10      4           2015-06-05 16:00:18     a   
17  10      4           2015-06-05 16:00:52     a   
18  1       6           2015-06-05 16:01:23     a   
20  10      6           2015-06-05 16:51:48     a   
21  10      6           2015-06-05 17:00:23     s
22  10      4           2015-06-05 17:02:39     s   
23  45      7           2015-06-05 20:48:31     a   

Type 'a' is for stock added
Type 's' is for sale

我正在执行以下查询以获取库存余额，但我需要获取每个类别的余额。

这是我到目前为止尝试过的查询:

SELECT (
    select SUM(stock)
    from stock
    where type='a'
) - (
    select SUM(stock)
    from stock
    where type='s'
) as balance

Answer 1

我想你可以试试(未测试)

select category
, sum(if(type='a', stock, 0)) - sum(if(type='s', stock, 0)) as balance
from stock
group by category

编辑:

我刚刚测试过它，它可以工作