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。

如果你担心性能问题，你也可以用MAX()这样做:

SELECT *
FROM DocumentStatusLogs D
WHERE DateCreated = (SELECT MAX(DateCreated) FROM DocumentStatusLogs WHERE ID = D.ID)

ROW_NUMBER()要求对SELECT语句中的所有行进行排序，而MAX则不需要。应该会大大加快你的查询速度。

I've done some timings over the various recommendations here, and the results really depend on the size of the table involved, but the most consistent solution is using the CROSS APPLY These tests were run against SQL Server 2008-R2, using a table with 6,500 records, and another (identical schema) with 137 million records. The columns being queried are part of the primary key on the table, and the table width is very small (about 30 bytes). The times are reported by SQL Server from the actual execution plan.

Query                                  Time for 6500 (ms)    Time for 137M(ms)

CROSS APPLY                                    17.9                17.9
SELECT WHERE col = (SELECT MAX(COL)…)           6.6               854.4
DENSE_RANK() OVER PARTITION                     6.6               907.1

我认为真正令人惊讶的是，无论涉及的行数是多少，CROSS APPLY的时间都是如此一致。

这是这个话题中最容易找到的问题之一，所以我想给出一个现代的答案(既供我参考，也帮助其他人)。通过使用first_value和over，你可以简化上面的查询:

Select distinct DocumentID
  , first_value(status) over (partition by DocumentID order by DateCreated Desc) as Status
  , first_value(DateCreated) over (partition by DocumentID order by DateCreated Desc) as DateCreated
From DocumentStatusLogs

这应该工作在Sql Server 2008及更高。First_value可以被认为是在使用over子句时实现Select Top 1的一种方法。Over允许在选择列表中分组，而不是编写嵌套的子查询(像许多现有的答案那样)，这以一种更可读的方式进行。希望这能有所帮助。

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。

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