sql-server - SQL : Using Top 1 in UNION query with Order By

Question

我有一个表格如下

Rate Effective_Date
---- --------------
5.6  02/02/2009
5.8  05/01/2009
5.4  06/01/2009
5.8  12/01/2009
6.0  03/15/2009

我应该找到当前日期及之后有效的所有费率。因此，为了获得当前的有效率，我使用

SELECT TOP 1 * from table 
where effective_date < '05/05/2009' 
order by effective date desc

对于当前日期之后的费率，查询为

SELECT * from table 
where effective_date > '05/05/2009'

为了结合这两个结果，我使用联合作为

SELECT TOP 1 * from table 
where effective_date < '05/05/2009' 
order by effective date desc

UNION

SELECT * from table 
where effective_date > '05/05/2009'

预期结果是

Rate Effective Date
---- --------------
5.8  05/01/2009
5.4  06/01/2009
5.8  12/01/2009
6.0  03/15/2009

但我得到的实际结果是

Rate Effective Date
---- --------------
5.6  02/02/2009
5.4  06/01/2009
5.8  12/01/2009
6.0  03/15/2009

我不知道为什么会发生这种情况？有什么建议吗？

Answer 1

它的工作原理是这样的:

select *
from (
    select top 1 *
    from table
    where effective_date <= '05/05/2009'
    order by effective_date desc
) as current_rate

union all

select *
from table
where effective_date > '05/05/2009'