是否根据表B中的值从表A中选择值？

Question

我正在创建一个非常基本的帖子和回复系统，以便更好地理解MySQL和PHP。我有两个表:帖子和评论。

posts(post_id, post_user, timestamp, post_text)
comments(comment_id, post_id, timestamp, comment_text)

我想做的是按最近回复的帖子排序。所以我需要SELECT * from posts ordered by comments.timestamp desc，因为我想按最近的评论排序，而不是按原始帖子的时间戳排序。我找不到一个合适的查询来工作。

Answer 1

你可能在找这个

SELECT  p.* 
FROM  posts p 
INNER JOIN comments c ON c.post_id= p.post_id
ORDER BY c.timestamp desc

Answer 2

SELECT post_id, post_user, timestamp, post_text, 
       most_recent_comment
  FROM posts NATURAL JOIN
       ( SELECT post_id, 
                MAX( timestamp ) AS most_recent_comment
          FROM comments 
         GROUP
            BY post_id ) AS t
UNION
SELECT post_id, post_user, timestamp, post_text,
       NULL AS most_recent_comment
  FROM posts
 WHERE post_id NOT IN ( SELECT post_id FROM comments );

Answer 3

SELECT  A.Post_Id FROM
(SELECT P.Post_Id,C.TimeStamp,ROW_NUMBER() OVER( PARTITION BY S.Post_Id ORDER BY C.TimeStamp DESC) Rnk  FROM POSTS p INNER JOIN  COMMENTS C
ON P.Post_Id=C.Post_Id) A
WHERE A.Rnk=1
ORDER BY A.TimeStamp DESC
This is SQL SERVER version.
So hope you can find Mysql version for it