[Opserty]

Hmm that is more complicated, I don't know if you can use a COUNT in the WHERE clause of MySQL?

You'd have fetch the threads active in the last 24 hours, count the number of posts since then and order it by that...

[sql]
SELECT thread . * , post . *
FROM thread
LEFT JOIN post ON (thread.threadid = post.threadid )
WHERE post.dateline > ( UNIX_TIMESTAMP( ) -86400 )
GROUP BY thread.threadid
ORDER BY COUNT(post.postid) DESC
LIMIT 5
[/sql]

Not really a MySQL expert but that should do the trick I think. You will probably need to optimise/tweak it a bit.