我有一个表,我想获得每组的最新条目。下面是表格:

DocumentStatusLogs表

|ID| DocumentID | Status | DateCreated |
| 2| 1          | S1     | 7/29/2011   |
| 3| 1          | S2     | 7/30/2011   |
| 6| 1          | S1     | 8/02/2011   |
| 1| 2          | S1     | 7/28/2011   |
| 4| 2          | S2     | 7/30/2011   |
| 5| 2          | S3     | 8/01/2011   |
| 6| 3          | S1     | 8/02/2011   |

该表将按documententid分组,并按DateCreated降序排序。对于每个documententid,我希望获得最新的状态。

我的首选输出:

| DocumentID | Status | DateCreated |
| 1          | S1     | 8/02/2011   |
| 2          | S3     | 8/01/2011   |
| 3          | S1     | 8/02/2011   |

Is there any aggregate function to get only the top from each group? See pseudo-code GetOnlyTheTop below: SELECT DocumentID, GetOnlyTheTop(Status), GetOnlyTheTop(DateCreated) FROM DocumentStatusLogs GROUP BY DocumentID ORDER BY DateCreated DESC If such function doesn't exist, is there any way I can achieve the output I want? Or at the first place, could this be caused by unnormalized database? I'm thinking, since what I'm looking for is just one row, should that status also be located in the parent table?

更多信息请参见父表:

当前文档表

| DocumentID | Title  | Content  | DateCreated |
| 1          | TitleA | ...      | ...         |
| 2          | TitleB | ...      | ...         |
| 3          | TitleC | ...      | ...         |

父表应该是这样的,以便我可以轻松地访问它的状态吗?

| DocumentID | Title  | Content  | DateCreated | CurrentStatus |
| 1          | TitleA | ...      | ...         | s1            |
| 2          | TitleB | ...      | ...         | s3            |
| 3          | TitleC | ...      | ...         | s1            |

更新 我刚刚学会了如何使用“apply”,它可以更容易地解决这类问题。


当前回答

CROSS APPLY是我在解决方案中使用的方法,因为它对我和客户的需求都有效。从我所读到的,应该提供最好的整体性能,如果他们的数据库大幅增长。

其他回答

SELECT o.*
FROM `DocumentStatusLogs` o                   
  LEFT JOIN `DocumentStatusLogs` b                   
  ON o.DocumentID = b.DocumentID AND o.DateCreated < b.DateCreated
 WHERE b.DocumentID is NULL ;

如果您只想按DateCreated返回最近的文档顺序,它将只按documententid返回前1个文档

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

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
)

试试这个:

SELECT [DocumentID]
    ,[tmpRez].value('/x[2]', 'varchar(20)') AS [Status]
    ,[tmpRez].value('/x[3]', 'datetime') AS [DateCreated]
FROM (
    SELECT [DocumentID]
        ,cast('<x>' + max(cast([ID] AS VARCHAR(10)) + '</x><x>' + [Status] + '</x><x>' + cast([DateCreated] AS VARCHAR(20))) + '</x>' AS XML) AS [tmpRez]
    FROM DocumentStatusLogs
    GROUP BY DocumentID
    ) AS [tmpQry]

一些数据库引擎*开始支持允许过滤窗口函数结果的qualifier子句(接受的答案使用该子句)。

所以公认的答案可以变成

SELECT *, ROW_NUMBER() OVER (PARTITION BY DocumentID ORDER BY DateCreated DESC) AS rn
FROM DocumentStatusLogs
QUALIFY rn = 1

查看这篇文章以获得更深入的解释:https://jrandrews.net/the-joy-of-qualify

您可以使用此工具查看哪个数据库支持此子句:https://www.jooq.org/translate/ 当目标方言不支持qualifier子句时,可以选择转换它。

*Teradata, BigQuery, H2, Snowflake…