select name,Num as 库存量,(select sum(num) from outlibdan as o where o.productID=s.ProductID )  as 出库数量
,(select sum(num) from inlibdan as i where i.productID=s.productID) as 入库数量
from doc_Products as s
left join libs on libs.productID=s.productID
我想加两列，一列是用入库数量－出库数量－库存量，一列是如果前一列那个值结果为０，则平衡，不为０则不平衡
我用,(select sum(num) from inlibdan as i where i.productID=s.productID)－,(select sum(num) from outlibdan as o where o.productID=s.ProductID ) －Num后倒是能到过结果，但是如果出库数量，或其它两个中有一个为null计算的结果就会是ＮＵＬＬ,这怎么解决呢？

select isnull(sum(num),0) from inlibdan as i where i.productID=s.productID)－,(select isnull(sum(num),0) from outlibdan as o where o.productID=s.ProductID

楼上正解
看签名貌似高手....

高手！！出来了，真不错，太感谢了，
看来ＳＱＬ语句还是有很多不会地，得再学学
祝你找到个好工作！

高手，发短消息我这好像这会发不了了，
好像还有点问题
如果全为ＮＵＬＬ时，出现的结果还是ＮＵＬＬ

select isnull(name,''),isnull(Num,'0') as 库存量,(select isnull(sum(num),'0') from outlibdan as o where o.productID=s.ProductID )  as 出库数量
,(select isnull(sum(num),'0') from inlibdan as i where i.productID=s.productID) as 入库数量
from doc_Products as s
left join libs on libs.productID=s.productID


-------
太麻烦了。。。数据库怎么搞的。。先写好数据结构吧。。 设置数据库 not null

select s.productID, name,isnull(Num,0) as LibN,(select isnull(sum(num),0) from outlibdan as o where o.productID=s.ProductID )  as OutLibN
,(select isnull(sum(num),0) from inlibdan as i where i.productID=s.productID) as InLibN,
(select isnull(sum(num),0) from inlibdan as i where i.productID=s.productID)-
(select isnull(sum(num),0) from outlibdan as o where o.productID=s.ProductID)-
(select isnull(num,0) from libs where libs.productID=s.productID)
from doc_Products as s
left join libs on libs.productID=s.productID

我的数据结构应该没有问题，Ｎame不可为空，
出库的不是这里，是后面的减法运算的结果还是会为ＮＵＬＬ

sum(num)
  以前听老师讲过。。
但是 还是不明白其意思。。。
  有谁能说一下吗。。。。

看下结果