Product(pid,name,amount,nowAmount)：标识,名称,已有数量，当前数量
Trade(id,pid,operType,number)：标识，产品标识，操作类型(入库：1，出库：0)，数量
Product表中测试数据：
Trade表中测试数据:
| 1 |
1 |
1 |
432 |
| 2 |
1 |
0 |
50 |
| 3 |
2 |
1 |
20 |
| 4 |
2 |
0 |
40 |
| 5 |
1 |
1 |
30 |
| 6 |
2 |
0 |
20 |
现要求一条SQL语句更新Product表中nowMount值
语句如下：
update p set p.nowAmount = p.amount+t.number
from Product as p ,(select pid,sum(case operType when '0' then number* (-1) else number end) number
from Trade group by pid) as t where t.pid=s.pid
或者
update p
set p.nowAmount= t.number+p.amount
from Product as p
inner join (select pid ,sum(case operType when '0' then number* (-1) else number end) as number from Trade group by pid) as t
on p.pid=t.pid
两者一样执行后Product表：