如何更新我的SQL数据库这个领域？

Question

我有两个表。

BoardPosts
BoardPostId INT PK
ModifiedOn DATETIME NULLABLE

BoardComments
BoardCommentId INT PK
BoardPostId INT
CreatedOn DATETIME

一个基板柱具有零到许多意见。

我要设置的ModifiedOn领域是最近的评论日期，如果董事会有一个注释。否则，就让它空。

我如何做到这一点，使用TSQL？

像...

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

???

Answer 1

我想这会工作...

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

Answer 2

我决定兼顾评论和未注释的帖子：

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

希望这有助于！