SQL查询 - 结合DISTINCT和TOP？

Question

我有以下要查询的查询:

SELECT DISTINCT TOP(5) fp.PostId FROM dbForumPosts fp
LEFT JOIN dbForumEntry fe ON fp.PostId = fe.PostId
Order by fe.Datemade DESC

但是,当我开火时,我收到错误:

Msg 145, Level 15, State 1, Line 1
ORDER BY items must appear in the select list if SELECT DISTINCT is specified.

我试图更改查询,所以它使用GROUP BY,但后来我有以下问题:

Msg 8127, Level 16, State 1, Line 4
Column "dbForumEntry.Datemade" is invalid in the ORDER BY clause because it is not contained in either an aggregate function or the GROUP BY clause.

我想要什么:

把它想象成一个论坛.有帖子(dbForumPosts)和条目(dbForumEntry).pr post有0个条目.

我想要的是获取最近活动的帖子(包含最新更新条目的帖子).

Answer 1

你可以找到最近的Datemade每PostId用row_number.然后你可以搜索最近的5个帖子:

select  top 5 PostId
from    (
        select  PostId
        ,       Datemade
        ,       row_number() over (partition by PostId
                    order by Datemade) as rn
        from    dbForumEntry
        ) SubQueryAlias
where   rn = 1 -- Most recent row per PostId
order by
        Datemade desc

或者,您可以使用group by子查询实现相同的功能:

select  top 5 PostId
from    (
        select  PostId
        ,       max(Datemade) as LastDate
        from    dbForumEntry
        group by
                PostId
        ) SubQueryAlias
order by
        LastDate desc

如果dbForumEntry有一个ID列(比方说ForumEntryId),这样的查询可能会表现得更好.该数据库可以在不编译运行这个row_number或max(Datemade)整个表格.

select  top 5 PostId
from    dbForumPosts fp
where   not exists -- No later entry for the same post exists
        (
        select  *
        from    dbForumPosts fp2
        where   fp2.PostId = fp.PostId
                and fp2.ForumEntryId > fp.ForumEntryId
        )
order by
        Datemade desc