如何使用MySQL Join通过上次回复对论坛帖子进行排序？

Question

我在为此编写查询时遇到了一些麻烦.我想我有基本的逻辑,虽然我可能没有.我想要做的是根据用户正在查看的主板获取所有线程,然后根据上次回复的时间对这些线程进行排序.查询不返回任何错误,它只获取最近更新的线程.

这是我的查询:

$query = "
            SELECT
            t.child_id, t.thread_id,
            m.thread_id, m.message_id, m.date_posted
            FROM forum_threads AS t
            LEFT JOIN forum_messages AS m ON t.thread_id = m.thread_id
            WHERE t.child_id = ".$board_id."
            ORDER BY m.date_posted DESC
            LIMIT ".$starting.", ".$this->user['results_per_page'];

这是要返回的查询:

            SELECT t.child_id, t.thread_id, m.thread_id, m.message_id, m.date_posted   
            FROM forum_threads AS t   
            LEFT JOIN forum_messages AS m   
            ON t.thread_id = m.thread_id   
            WHERE t.child_id = 2   
            ORDER BY m.date_posted DESC LIMIT 0, 15 

更新

我试图采用ethrbunny建议的想法,尽管我完全迷失了导致它返回错误的原因.

$query = "
            SELECT
            t.board_id, t.thread_id
            FROM forum_threads AS t
            LEFT JOIN (
            SELECT m.thread_id, m.message_id
            FROM forum_messages AS m
            WHERE m.thread_id = t.thread_id
            ORDER BY m.message_id DESC
            LIMIT 1
            ) AS q
            WHERE t.board_id = ".$board_id."
            ORDER BY q.date_posted DESC
            LIMIT ".$starting.", ".$this->user['results_per_page'];

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE t.board_id = 4 ORDER BY q.date_posted DESC LIMIT' at line 11

Answer 1

SELECT ... , MAX(m.date_posted) AS latest_reply ...

GROUP BY t.thread_id
ORDER BY latest_reply DESC ...

但是为什么date_posted是一个TEXT？不应该是日期时间,或者可能是int(如果是时间戳)

因为永远无法在文本列上优化运行MAX,所以建议使用

MAX(m.message_id) AS latest_reply

相反,作为消息可能以日期顺序插入,应该是等效的.

编辑添加:查询完整的查询...

$query = "
SELECT t.thread_id, title, 
    MAX(m.message_id) AS latest_reply 
FROM forum_threads AS t
    LEFT JOIN forum_messages AS m ON t.thread_id = m.thread_id
WHERE t.child_id = ".$board_id."
GROUP BY t.thread_id
ORDER BY latest_reply DESC 
LIMIT ".$starting.", ".$this->user['results_per_page'];