SQL: Limit rows linked to each joined row
- by SolidSnakeGTI
Hello,
I've certain situation that requires certain result set from MySQL query, let's see the current query first & then ask my question:
SELECT thread.dateline AS tdateline, post.dateline AS pdateline, MIN(post.dateline)
FROM thread AS thread
LEFT JOIN post AS post ON(thread.threadid = post.threadid)
LEFT JOIN forum AS forum ON(thread.forumid = forum.forumid)
WHERE post.postid != thread.firstpostid
AND thread.open = 1
AND thread.visible = 1
AND thread.replycount >= 1
AND post.visible = 1
AND (forum.options & 1)
AND (forum.options & 2)
AND (forum.options & 4)
AND forum.forumid IN(1,2,3)
GROUP BY post.threadid
ORDER BY tdateline DESC, pdateline ASC
As you can see, mainly I need to select dateline of threads from 'thread' table, in addition to dateline of the second post of each thread, that's all under the conditions you see in the WHERE CLAUSE. Since each thread has many posts, and I need only one result per thread, I've used GROUP BY CLAUSE for that purpose.
This query will return only one post's dateline with it's related unique thread.
My questions are:
How to limit returned threads per
each forum!? Suppose I need only 5
threads -as a maximum- to be
returned for each forum declared in
the WHERE CLAUSE 'forum.forumid
IN(1,2,3)', how can this be
achieved.
Is there any recommendations for
optimizing this query (of course
after solving the first point)?
Notes:
I prefer not to use sub-queries, but
if it's the only solution available
I'll accept it. Double queries not
recommended. I'm sure there's a smart
solution for this situation.
I'm using MySQL 4.1+, but if you know
the answer for another engine, just
share.
Appreciated advice in advance :)
