SQL:在 UNION 查询中使用 Top 1 和 Order By [英] SQL: Using Top 1 in UNION query with Order By

问题描述

我有一张如下表

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

我应该找到在当前日期和之后有效的所有费率.所以为了获得当前的有效率，我使用

I am supposed to find the all rates that are effective for current date and after it. So to get the current effective rate, i use

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

对于查询是当前日期之后的费率

for the rates after the current date the query is

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

To combine these two result i use a union as

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'

预期的结果是

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为

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?

I don't have a clue as to why this happens? Any suggestions?

推荐答案

它是这样工作的:

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'

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