利用union 将多个select 的结果结合起来.union命令将来自不同select命令所生成的数据混合在一起，并将重复的数据删除仅留下一条，也就是数学上的并集关系。
如果你想让重复的数据也留下时，可以使用union all取代原来的union命令

1.select prod_id ,sup_id
from sale_item
where order_no=10001
union
select prod_id,sup_id
from purchase
where pur_no=50001
order by 1,2
2.在sqlserver方面可以用case实现的，但在access里，只能用iif或union
select a.rmpwr,a.buypwr,yd-case when b.dl>0 then a.yd+b.rmpwr-b.bdl

##www.aspxuexi.com##
else a.yd-b.yd end
from (select mid,rmpwr,buypwr,(buypwr-rmpwr)as yd from main where month(dt)=2)a,(select mid,rmpwr,buypwr,(buypwr-rmpwr)as yd,bdl from main where month(dt)=1 b where a.mid=b.mid
在本机数据库中a,b同上
select a.ydl+b.rmpwr-b.ydl as yd from a,b where a.mid=b.mid and b.bdl>0
union all
select a.yd-b.yd from a,b where a.mid=b.mid and isnull(b.bdl,-1)<=0