sql - 选择具有最高(相对)时间戳的所有行

Question

我在 SQL 2000 数据库中有以下简化的表结构:

ID  AppName  Key    Value   EffectiveDate
--  -------  -----  ------- -------------
1   App1     One    Past    1/1/1900
2   App1     One    Present 1/1/2010
3   App1     One    Future  1/1/9999
4   App1     Two    Past    1/1/1900
5   App1     Two    Present 1/1/2010
6   App1     Two    Future  1/1/9999
7   App2     One    Present 1/1/2010
8   App2     Two    Present 1/1/2010

我需要能够提出问题:

给定一个特定的 AppName , 显示所有 ONLY THE MOST RECENT 键/值对 EffectiveDate <= GetDate()

所以如果我用 AppName = 'App1' 调用我的神秘查询那么我的结果将是:

ID  AppName  Key    Value   EffectiveDate
--  -------  -----  ------- -------------
2   App1     One    Present 1/1/2010
5   App1     Two    Present 1/1/2010

编辑:

值可以是任何东西。 ('Past','Present','Future') 只是用来使示例更清楚。他们很可能是 (45,'Bob','%$#%@#$')。

Answer 1

我认为你需要使用这样的东西:

SELECT T3.*
FROM your_table T4
JOIN
(
    SELECT T2.[Key], T2.EffectiveDate, MAX(T2.ID) AS ID
    FROM your_table T2
    JOIN 
    (
        SELECT [Key], MAX(EffectiveDate) AS EffectiveDate
        FROM your_table
        WHERE AppName = 'App1'
        AND EffectiveDate <= GetDate()
        GROUP BY [Key]
    ) T1
    ON T1.[Key] = T2.[Key] AND T1.EffectiveDate = T2.EffectiveDate
    WHERE T2.AppName = 'App1'
    GROUP BY T2.[Key], T2.EffectiveDate
) T3
ON T3.ID = T4.ID