求sql？急啊！

Question

select (case when
            sum((isnull(采购数量,0)-isnull(实际收货,0))> then 
            sum((isnull(采购数量,0)-isnull(实际收货,0))  else
            0 end)
       as 未收数 from 采购订单内容

Answer 1

select sum((Case When isnull(采购数量,0)<isnull(实际收货,0) Then 0 
       Else isnull(采购数量,0)-isnull(实际收货,0)
       End) as 未收数 from 采购订单内容

Answer 2

不好意思上面then前少了一个0

Answer 3

select sum((Case When isnull(采购数量,0)>isnull(实际收货,0) then isnull(采购数量,0)-isnull(实际收货,0) else 0 end) as 未收数 from 采购订单内容