MySQL query sorting correct information

Question

I'm currently struggling to find out the correct query for what I want to achieve and was wondering if anyone has some bright ideas. Been at a lost here for quite some days now and just can't find the correct query to get what I need.

Desired result:

I currently have fully working forum, with multiple categories, due to the fact that there's multiple categories I would like to make an overview page basically stating the last thread, last post, by whom and at what time per category.

Here is how my table is structured

Name: forums Fields: id, name

Name: forum_threads Fields: id, f_id, subject, author, lastpost

Name: forum_posts Fields: id, t_id, post, author, time

The actual problem

I have gotten the query thus far that it displays everything as I want except for one thing, it doesn't get the latest thread subject, everything else seems to be displaying correctly (Note I've disabled forums.id 9 and everything above 12, I don't want to show these results.

SELECT forums.id, t.id AS thread_id,
   t.subject,
   t.forum_id,
   t.author, t.lastpost,
   u1.name AS author_name, u1.avatar AS author_avatar,
   u2.name AS lastpost_name, u.avatar AS lastpost_avatar
    COUNT(p.id) AS postscount,  COUNT(DISTINCT (t.id)) AS threadscount,
   forums.name,
   max(p.time) last_post_date
FROM forum_threads t
INNER JOIN users u1 ON u1.id = t.author
INNER JOIN users u2 ON u2.id = t.lastpost
INNER JOIN forum_posts p ON p.t_id = t.id
INNER JOIN forums ON forums.id = t.forum_id
WHERE forums.id != 9 AND forums.id > 12
GROUP BY  forums.id
ORDER BY forums.id ASC, p.id DESC, last_post_date DESC

Anyone has any ideas?

Answer 1

Well, if the thread_id you're retrieving is correct, and the subject column on the forum_threads table contains the subject you're expecting, then there is a problem in your display level code rather than your query. (Since you're pulling thread_id from the same row as t.subject, if one is correct as you expect, the other should be, too.)

Besides that, and this is slightly off topic, I'd caution you against using constructs such as WHERE forums.id != 9 AND forums.id > 12. Instead, you should have a column in the database for disabled, and query against that. Otherwise you have an unmaintainable mess.

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In response to the comments, try adding this block of SQL to your JOIN clauses:

INNER JOIN (SELECT t2.forum_id as forum_id, MAX(lastpost) as lastpost
    FROM forum_threads t2
    GROUP BY t2.forum_id) as latest_post_per_forum
  ON latest_post_per_forum.forum_id = t.forum_id

and this to your WHERE clause:

  AND t.lastpost = latest_post_per_forum.lastpost

Unfortunately I don't have a convenient MySQL instance at this time to test my syntax, but the gist should be obvious: pull the last post ID out on a per-forum basis and then compare each thread's last post ID to that.