Question

我有一张这样的表：

Id，Comment，LastUpdatedDate

我正在选择该ID的最新评论。该表可以对该ID具有不同日期的许多评论，但我正在尝试获取最新日期。我试过以下但没有成功：

SELECT tt.*
    FROM tagtestresultcomment tt
INNER JOIN
(
SELECT tag_id, MAX(last_update) AS MaxDateTime
FROM tagtestresultcomment
GROUP BY tag_id
) groupedtt ON tt.tag_id = groupedtt.tag_id AND tt.last_update = groupedtt.MaxDateTime
order by tag_id

有没有人对如何实现这一点有任何想法？

谢谢！

Answer 1

试试这个

Select * from tagtestresultcomment where last_update in 
(select max(last_update) from tagtestresultcomment group by tag_id)

Answer 2

您的查询代码过于冗余。第一

 tt.tag_id = groupedtt.tag_id AND tt.last_update = groupedtt.MaxDateTime

这就够了

 tt.tag_id = groupedtt.tag_id

第二，它就够了

SELECT [desired field list extcept last_update and ], 
  tag_id, 
  MAX(last_update) AS MaxDateTime
FROM 
  tagtestresultcomment
group by 
 tag_id, [desired field list extcept last_update and tag_id]

完全实现你的目标

Answer 3

听起来你只想要每个 tag_id的最新评论？在这种情况下，您可以使用SQL 2005中的一种方法：

;WITH CTE AS ( SELECT *, ROW_NUMBER() OVER(PARTITION BY tag_id ORDER BY last_update DESC) AS RowNo FROM TagTestResultComment ) SELECT * FROM CTE WHERE RowNo = 1

Answer 4

我尝试过这样的事情：

declare @tagtestresultcomment table
(
id int
, comment varchar(50)
,LastUpdatedDate datetime
)

--==== Populate table

insert into @tagtestresultcomment(id,comment,LastUpdatedDate)

select 1,'My name is Arthur','2011-06-09 00:00:00' union all

select 2,'My name is DW','2011-06-19 00:00:00' union all

select 2,'Arthur is my brother','2011-06-21 00:00:00' union all

select 1,'I have a sister named DW','2011-06-21 00:00:00' union all

select 3,'I am Muffy','2011-06-14 00:00:00' union all

select 3,'I like sports','2011-06-14 00:00:00'

-- SELECT stmt

select * from @tagtestresultcomment t 
join
(
select id, MAX(lastupdateddate) as LastUpdatedDate from @tagtestresultcomment group by id
) m

on t.id = m.id 

and t.LastUpdatedDate = m.LastUpdatedDate

Answer 5

“ MAX”组功能不适用于我，因此我使用了子查询。我无法将您的注意力集中在单个表的示例上，因此我将一个常见的父子一对多关系与一个博客和评论表作为示例。

SELECT
  b.id,
  b.content,
  c.id,
  c.blog_id,
  c.content,
  c.last_update
FROM blog b
INNER JOIN blog_comment c
ON b.id = c.blog_id AND c.id = (
  SELECT TOP 1 id FROM blog_comment WHERE blog_id = b.id ORDER BY last_update DESC
)

该查询对我的子查询产生了影响，因为它将为博客表中的每个记录调用“ SELECT TOP 1”查询。我希望听到一个更快，更高效的示例。

SQL SERVER：使用最大日期选择最新评论

5 个答案: