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，并在该主题上找到1条最新回复。问题是限制也会影响父查询，导致只返回1个线程。

提前感谢，非常感谢帮助。

Answer 1

作为@AgRizzo commented above，您可能需要分组子查询。
此外，内部连接只会产生两个表中匹配连接条件的记录：即，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，您也需要外部连接;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A Visual Explanation of SQL Joins。

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

mysql子查询限制也限制父查询

1 个答案: