BoardPosts
BoardPostId INT PK
ModifiedOn DATETIME NULLABLE
BoardComments
BoardCommentId INT PK
BoardPostId INT
CreatedOn DATETIME
董事会帖子有零到多条评论。
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,只需将其保留为空。
我如何使用TSql执行此操作?
类似......
UPDATE BoardPosts
SET ModifiedOn = CreatedOn
SELECT TOP(1) CreatedOn
FROM BoardPosts a INNER JOIN BoardComments b ON a.BoardPostId = b.BoardPostId
???
答案 0 :(得分:2)
我认为这会奏效......
UPDATE BoardPosts
SET ModifiedOn = (SELECT MAX(CreatedOn)
FROM BoardComments
WHERE BoardComments.BoardPostId = BoardPosts.BoardPostId)
答案 1 :(得分:0)
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
希望这有帮助!