mysql子查询限制也限制父查询

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 

我想从论坛检索所有记录，其中包含该主题的 1 个最新回复。 问题是该限制还会影响父查询，导致仅返回 1 个线程。

非常感谢您的帮助，提前致谢。

Answer 1

1. 如@AgRizzo commented above ，您可能需要对子查询进行分组。

2. 此外，内部联接只会产生联接条件在两个表中都匹配的记录:也就是说，没有回复的帖子将被排除。 p>

   如果您希望保留一个表中的记录，即使连接条件不匹配，您也需要一个外部连接；在本例中，是左外部联接(以便来自联接左操作数的记录始终包含在结果集中)。

   参见A Visual Explanation of SQL Joins了解更多信息。

因此:

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，在连接之后执行分组:

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