如何在我的sql数据库中更新此字段？

i have two tables.

我有两张桌子。

BoardPosts
BoardPostId INT PK
ModifiedOn DATETIME NULLABLE

BoardComments
BoardCommentId INT PK
BoardPostId INT
CreatedOn DATETIME

A board post has zero to many comments.

董事会帖子有零到多条评论。

I wish to set the ModifiedOn field to be the most recent comment date, if the board has a comment. Otherwise, just leave it null.

如果董事会有评论,我希望将ModifiedOn字段设置为最近的评论日期。否则,只需将其保留为null。

How do i do this, using TSql ?

我怎么做这个,使用TSql?

something like ...

就像是 ...

UPDATE BoardPosts
SET ModifiedOn = CreatedOn
SELECT TOP(1) CreatedOn
FROM BoardPosts a INNER JOIN BoardComments b ON a.BoardPostId = b.BoardPostId

???

2 个解决方案

#1

I think this will work...

我认为这会奏效......

UPDATE BoardPosts
SET ModifiedOn = (SELECT MAX(CreatedOn) 
                  FROM BoardComments 
                  WHERE BoardComments.BoardPostId = BoardPosts.BoardPostId)

#2

I decided to take into account both commented and uncommented posts:

我决定考虑评论和未评论的帖子:

update p1 set ModifiedOn = (
select
    max(case when c.CreatedOn is null then p.CreatedOn
    else c.CreatedOn end) as ModDate
from 
    boardposts p
    left outer join BoardComments c on
        p.BoardPostId = c.BoardPostId
where
    p.BoardPostId = p1.BoardPostId
group by p.BoardPostId
)
from
    BoardPosts p1

Hope this helps!

希望这可以帮助!

#1