这是一个相当老的话题，但我想我要发表我的意见，就像公认的答案对我来说并不是特别有效一样。我在一个大型数据集上尝试了gbn的解决方案，发现它非常慢(在SQL Server 2012中500多万条记录上>45秒)。看一下执行计划，很明显问题在于它需要一个SORT操作，这会大大降低速度。

下面是我从实体框架中提取的一个替代方案，它不需要SORT操作，只进行非聚集索引搜索。这将在前面提到的记录集中将执行时间减少到< 2秒。

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

Now I'm assuming something that isn't entirely specified in the original question, but if your table design is such that your ID column is an auto-increment ID, and the DateCreated is set to the current date with each insert, then even without running with my query above you could actually get a sizable performance boost to gbn's solution (about half the execution time) just from ordering on ID instead of ordering on DateCreated as this will provide an identical sort order and it's a faster sort.

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

SELECT * FROM
DocumentStatusLogs JOIN (
  SELECT DocumentID, MAX(DateCreated) DateCreated
  FROM DocumentStatusLogs
  GROUP BY DocumentID
  ) max_date USING (DocumentID, DateCreated)

什么数据库服务器?这段代码并不是对所有的都有效。

关于你问题的后半部分，我认为应该在一栏中写上“地位”。您可以将DocumentStatusLogs保留为日志，但仍然将最新信息存储在主表中。

顺便说一句，如果你已经在文档表中有DateCreated列，你可以使用它来加入DocumentStatusLogs(只要DateCreated在DocumentStatusLogs中是唯一的)。

编辑:MsSQL不支持USING，因此将其更改为:

ON DocumentStatusLogs.DocumentID = max_date.DocumentID AND DocumentStatusLogs.DateCreated = max_date.DateCreated

SELECT documentid, 
       status, 
       datecreated 
FROM   documentstatuslogs dlogs 
WHERE  status = (SELECT status 
                 FROM   documentstatuslogs 
                 WHERE  documentid = dlogs.documentid 
                 ORDER  BY datecreated DESC 
                 LIMIT  1) 

这是一个相当老的话题，但我想我要发表我的意见，就像公认的答案对我来说并不是特别有效一样。我在一个大型数据集上尝试了gbn的解决方案，发现它非常慢(在SQL Server 2012中500多万条记录上>45秒)。看一下执行计划，很明显问题在于它需要一个SORT操作，这会大大降低速度。

下面是我从实体框架中提取的一个替代方案，它不需要SORT操作，只进行非聚集索引搜索。这将在前面提到的记录集中将执行时间减少到< 2秒。

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

Now I'm assuming something that isn't entirely specified in the original question, but if your table design is such that your ID column is an auto-increment ID, and the DateCreated is set to the current date with each insert, then even without running with my query above you could actually get a sizable performance boost to gbn's solution (about half the execution time) just from ordering on ID instead of ordering on DateCreated as this will provide an identical sort order and it's a faster sort.

这里有3种不同的方法来解决这个问题，以及为每个查询建立索引的最佳选择(请自己尝试索引，并查看逻辑读取、消耗时间和执行计划。我根据自己的经验提供了关于此类查询的建议，但没有针对这个特定问题执行)。

方法1:使用ROW_NUMBER()。如果rowstore索引不能提高性能，对于具有聚合和分组的查询以及始终按不同列排序的表，可以尝试使用非聚集/聚集的columnstore索引，columnstore索引通常是最佳选择。

;WITH CTE AS
    (
       SELECT   *,
                RN = ROW_NUMBER() OVER (PARTITION BY DocumentID ORDER BY DateCreated DESC)
       FROM     DocumentStatusLogs
    )
    SELECT  ID      
        ,DocumentID 
        ,Status     
        ,DateCreated
    FROM    CTE
    WHERE   RN = 1;

方法2:使用FIRST_VALUE。如果rowstore索引不能提高性能，对于具有聚合和分组的查询以及始终按不同列排序的表，可以尝试使用非聚集/聚集的columnstore索引，columnstore索引通常是最佳选择。

SELECT  DISTINCT
    ID      = FIRST_VALUE(ID) OVER (PARTITION BY DocumentID ORDER BY DateCreated DESC)
    ,DocumentID
    ,Status     = FIRST_VALUE(Status) OVER (PARTITION BY DocumentID ORDER BY DateCreated DESC)
    ,DateCreated    = FIRST_VALUE(DateCreated) OVER (PARTITION BY DocumentID ORDER BY DateCreated DESC)
FROM    DocumentStatusLogs;

方法3:使用CROSS APPLY。在DocumentStatusLogs表上创建涵盖查询中使用的列的行存储索引应该足以覆盖查询，而不需要columnstore索引。

SELECT  DISTINCT
    ID      = CA.ID
    ,DocumentID = D.DocumentID
    ,Status     = CA.Status 
    ,DateCreated    = CA.DateCreated
FROM    DocumentStatusLogs D
    CROSS APPLY (
            SELECT  TOP 1 I.*
            FROM    DocumentStatusLogs I
            WHERE   I.DocumentID = D.DocumentID
            ORDER   BY I.DateCreated DESC
            ) CA;