;WITH cte AS
(
   SELECT *,
         ROW_NUMBER() OVER (PARTITION BY DocumentID ORDER BY DateCreated DESC) AS rn
   FROM DocumentStatusLogs
)
SELECT *
FROM cte
WHERE rn = 1

如果您希望每天有2个条目，那么这将任意选择一个。要获得一天的两个条目，请使用DENSE_RANK代替

至于是否正常化，这取决于你是否想:

在两个地方保持状态 保存状态历史 .．.

目前，您保留了状态历史。如果你也想在父表中保持最新状态(这是非规范化的)，你需要一个触发器来维持父表中的“状态”。或者删除这个状态历史表。

从上面验证克林特的正确答案:

下面两个查询之间的性能非常有趣。52%是最高的。48%是第二个。使用DISTINCT而不是ORDER BY提高了4%的性能。但是ORDER BY具有按多列排序的优势。

IF (OBJECT_ID('tempdb..#DocumentStatusLogs') IS NOT NULL) BEGIN DROP TABLE #DocumentStatusLogs END

CREATE TABLE #DocumentStatusLogs (
    [ID] int NOT NULL,
    [DocumentID] int NOT NULL,
    [Status] varchar(20),
    [DateCreated] datetime
)

INSERT INTO #DocumentStatusLogs([ID], [DocumentID], [Status], [DateCreated]) VALUES (2, 1, 'S1', '7/29/2011 1:00:00')
INSERT INTO #DocumentStatusLogs([ID], [DocumentID], [Status], [DateCreated]) VALUES (3, 1, 'S2', '7/30/2011 2:00:00')
INSERT INTO #DocumentStatusLogs([ID], [DocumentID], [Status], [DateCreated]) VALUES (6, 1, 'S1', '8/02/2011 3:00:00')
INSERT INTO #DocumentStatusLogs([ID], [DocumentID], [Status], [DateCreated]) VALUES (1, 2, 'S1', '7/28/2011 4:00:00')
INSERT INTO #DocumentStatusLogs([ID], [DocumentID], [Status], [DateCreated]) VALUES (4, 2, 'S2', '7/30/2011 5:00:00')
INSERT INTO #DocumentStatusLogs([ID], [DocumentID], [Status], [DateCreated]) VALUES (5, 2, 'S3', '8/01/2011 6:00:00')
INSERT INTO #DocumentStatusLogs([ID], [DocumentID], [Status], [DateCreated]) VALUES (6, 3, 'S1', '8/02/2011 7:00:00')

选项1:

    SELECT
    [Extent1].[ID], 
    [Extent1].[DocumentID],
    [Extent1].[Status], 
    [Extent1].[DateCreated]
FROM #DocumentStatusLogs AS [Extent1]
    OUTER APPLY (
        SELECT TOP 1
            [Extent2].[ID], 
            [Extent2].[DocumentID],
            [Extent2].[Status], 
            [Extent2].[DateCreated]
        FROM #DocumentStatusLogs AS [Extent2]
        WHERE [Extent1].[DocumentID] = [Extent2].[DocumentID]
        ORDER BY [Extent2].[DateCreated] DESC, [Extent2].[ID] DESC
    ) AS [Project2]
WHERE ([Project2].[ID] IS NULL OR [Project2].[ID] = [Extent1].[ID])

选项2:

SELECT 
    [Limit1].[DocumentID] AS [ID], 
    [Limit1].[DocumentID] AS [DocumentID], 
    [Limit1].[Status] AS [Status], 
    [Limit1].[DateCreated] AS [DateCreated]
FROM (
    SELECT DISTINCT [Extent1].[DocumentID] AS [DocumentID] FROM #DocumentStatusLogs AS [Extent1]
) AS [Distinct1]
    OUTER APPLY  (
        SELECT TOP (1) [Project2].[ID] AS [ID], [Project2].[DocumentID] AS [DocumentID], [Project2].[Status] AS [Status], [Project2].[DateCreated] AS [DateCreated]
        FROM (
            SELECT 
                [Extent2].[ID] AS [ID], 
                [Extent2].[DocumentID] AS [DocumentID], 
                [Extent2].[Status] AS [Status], 
                [Extent2].[DateCreated] AS [DateCreated]
            FROM #DocumentStatusLogs AS [Extent2]
            WHERE [Distinct1].[DocumentID] = [Extent2].[DocumentID]
        )  AS [Project2]
        ORDER BY [Project2].[ID] DESC
    ) AS [Limit1]

在Microsoft SQL Server Management Studio中:在突出显示并运行第一个块后，突出显示选项1和选项2，右键单击->[显示估计的执行计划]。然后运行整个程序以查看结果。

选项1结果:

ID  DocumentID  Status  DateCreated
6   1   S1  8/2/11 3:00
5   2   S3  8/1/11 6:00
6   3   S1  8/2/11 7:00

选项2结果:

ID  DocumentID  Status  DateCreated
6   1   S1  8/2/11 3:00
5   2   S3  8/1/11 6:00
6   3   S1  8/2/11 7:00

注意:

当我想要一个连接为1到-(多个连接中的1个)时，我倾向于使用APPLY。 如果我想要连接为1对多或多对多，则使用JOIN。 我使用ROW_NUMBER()避免CTE，除非我需要做一些高级的事情，并且可以接受窗口性能损失。

我还避免在WHERE或ON子句中使用EXISTS / IN子查询，因为我曾经经历过这样会导致一些糟糕的执行计划。但具体里程各不相同。在需要的时候随时随地检查执行计划和概要文件的性能!

我的代码从每组中选择top 1

select a.* from #DocumentStatusLogs a where 
 datecreated in( select top 1 datecreated from #DocumentStatusLogs b
where 
a.documentid = b.documentid
order by datecreated desc
)

在你想避免使用row_count()的场景中，你也可以使用左连接:

select ds.DocumentID, ds.Status, ds.DateCreated 
from DocumentStatusLogs ds
left join DocumentStatusLogs filter 
    ON ds.DocumentID = filter.DocumentID
    -- Match any row that has another row that was created after it.
    AND ds.DateCreated < filter.DateCreated
-- then filter out any rows that matched 
where filter.DocumentID is null 

对于示例模式，你也可以使用"not in subquery"，它通常编译到与左连接相同的输出:

select ds.DocumentID, ds.Status, ds.DateCreated 
from DocumentStatusLogs ds
WHERE ds.ID NOT IN (
    SELECT filter.ID 
    FROM DocumentStatusLogs filter
    WHERE ds.DocumentID = filter.DocumentID
        AND ds.DateCreated < filter.DateCreated)

注意，如果表没有至少一个单列唯一键/约束/索引(在本例中是主键“Id”)，那么子查询模式将无法工作。

这两个查询往往比row_count()查询(由query Analyzer衡量)更“昂贵”。但是，您可能会遇到它们更快地返回结果或启用其他优化的情况。

此解决方案可用于获取每个分区的TOP N最近行(在本例中，WHERE语句中的N为1,partition为doc_id):

SELECT T.doc_id, T.status, T.date_created FROM 
(
    SELECT a.*, ROW_NUMBER() OVER (PARTITION BY doc_id ORDER BY date_created DESC) AS rnk FROM doc a
) T
WHERE T.rnk = 1;

我刚学会如何使用交叉应用。下面是如何在这种情况下使用它:

 select d.DocumentID, ds.Status, ds.DateCreated 
 from Documents as d 
 cross apply 
     (select top 1 Status, DateCreated
      from DocumentStatusLogs 
      where DocumentID = d.DocumentId
      order by DateCreated desc) as ds