怎样用sql语句计算库存结余？

declare @t table(Inv char(3),InOutType varchar(3),Iquantity int)
insert into @t values('001','in',300)
insert into @t values('001','out',200)
insert into @t values('002','in',50)

select a.Inv,isnull(a.入帐,0)-isnull(b.出帐,0) 结余
from
(
  select Inv,sum(Iquantity) 入帐 from @t where InOutType='in' group by Inv
) a
left join
(
  select Inv,sum(Iquantity) 出帐 from @t where InOutType='Out' group by Inv
) b
on a.Inv=b.Inv

/*
Inv  结余         
---- -----------
001  100
002  50

（所影响的行数为 2 行）
*/