ysai请看！刚才您回答的问题却是存在空值问题！在线等待！(30分)

http://www.delphibbs.com/delphibbs/dispq.asp?lid=1440756
得到的结果中
是这样的：
bh(客户编号) ys(应收) yf(已付) qk(欠款)
001 300 150 150
002 200
请问如何用iff判断一下空值的问题！

sqlserver:
select a.bh,ys,yf,ys-isnull(yf,0) as qk from (
select bh,sum(ys) as ys from a group by bh
) as a left join (
select bh,sum(yf) as yf from b group by bh)
) as b on a.bh=b.bh
access:
select a.bh,ys,yf,ys-iif(yf is null,0,yf) as qk from (
select bh,sum(ys) as ys from a group by bh
) as a left join (
select bh,sum(yf) as yf from b group by bh)
) as b on a.bh=b.bh

谢谢了！