SQL Sum（）查询错误

Question

实际数据

使用这些查询：

Select P.productid, Sum(pr.quantity) AS PQuantity from Products p
join purchases pr on pr.Productid = p.productid
group by p.productid;

Select P.productid, Sum(s.quantity) AS SQuantity from Products p
join Sales s on s.Productid = p.productid
group by p.productid;

这是我的表格（购买和销售）中的内容，右边的是带分组的总和：

问题：

select p.productid, sum(pr.quantity) as PQuantity, sum(s.quantity) as SQuantity 
from products p
join sales s on p.productid = s.productid
join purchases pr on p.productid = pr.productid
group by p.productid

但是当我加入三张桌子时会出现问题：

1. 产品
2. 购买
3. 销售

Answer 1

你必须在加入前总结这些数字，如下所示：

select isnull(P.productid, S.productid), P.PQuantity, S.SQuantity
from
(
  Select P.productid, Sum(pr.quantity) AS PQuantity from Products p
  join purchases pr on pr.Productid = p.productid
  group by p.productid
) P
full outer join
(
  Select P.productid, Sum(s.quantity) AS SQuantity from Products p
  join Sales s on s.Productid = p.productid
  group by p.productid
) S on S.productid = P.productid