请达人，帮我举一些union的例子，我觉得没太会用

[ProjectDD]

比如：

if(object_id('a') is not null) drop table a
go

create table a (col1 smalldatetime,col2 smalldatetime, col3 char(10))
go

select col1 from a order by col1
union
select col2 from a order by col2

两个order by 通不过，

顺带还有一个问题：

select top1 col1 from a
union
select top1 col1 from a order by col1 asc
--和
select top1 col1 from a
union
select top1 col1 from a order by col1 desc

这两个结果好象都是一样的 为什么呢？但 asc和 desc的结果集是不一样的得。这是怎么回事呢？

[feixianxxx]

小技巧：这时候如果你想对2输入表分别进行排序,比如上面的#a的a列升序,#b的d列降序,可以这样

select a,b
from (
select *,flag=1 from #a
union all
select *,2 from #b
) l
order by case when flag=1 then a  end ,case when flag=2 then b end desc 
/*
a           b
----------- -----------
5           9
4           9
4           2
3           2
1           2
3           9
4           8
*/

[feiyanglove]

asc
desc
本身就不一样
既然按照这个排序
那又怎么会一样呢

[永生天地]

union子查询不能加order
可以外套个select
select col1 from(
select col1 from a order by col1 ) t1
union
select col2 from(
select col2 from a order by col2 ) t2

[feiyanglove]

[Quote=引用 2 楼 xys_777 的回复:]
union子查询不能加order
可以外套个select
select col1 from(
select col1 from a order by col1 ) t1
union
select col2 from(
select col2 from a order by col2 ) t2
[/Quote]
你确定这样可以？

[feiyanglove]

select * from (select top 100 percent col1 from a order by col1)a
union all
select * from (select top 100 percent col2 from a order by col2)b