mysql - mysql子查询限制也在限制父查询

Question

SELECT * FROM forum_posts AS post 

INNER JOIN( SELECT parent AS rparent, author AS rauthor, MAX(created_at( AS rdate FROM forum_replies) AS reply
ON post.id = reply.rparent 

我想从论坛中检索所有记录，并在该线程上有 1 个最新回复。问题是限制也会影响父查询，导致只返回 1 个线程。

帮助将不胜感激，在此先感谢。

Answer 1

1. 正如@AgRizzo 上面评论的那样，您可能需要对子查询进行分组。

2. 此外，内连接只会导致两个表中连接条件都匹配的记录：也就是说，将排除没有回复的帖子。

   如果您希望保留一个表中的记录，即使连接条件不匹配，您将需要外连接；在这种情况下，左外连接（以便连接的左操作数中的记录始终包含在结果集中）。

   有关详细信息，请参阅SQL 连接的可视化解释。

所以：

SELECT * FROM forum_posts AS post LEFT JOIN (
  SELECT   parent          AS rparent
    ,      author          AS rauthor
    ,      MAX(created_at) AS rdate
  FROM     forum_replies
  GROUP BY parent
) AS reply ON post.id = reply.rparent

或者，在加入后执行分组：

SELECT      post.*
  ,         reply.parent          AS rparent
  ,         reply.author          AS rauthor
  ,         MAX(reply.created_at) AS rdate
FROM        forum_posts           AS post
  LEFT JOIN forum_replies         AS reply ON reply.parent = post.id
GROUP BY    post.id