SELECT user.username AS username, COUNT(*) AS posts
FROM xf_post AS post
LEFT JOIN xf_user AS user ON (user.user_id = post.user_id)
WHERE post.post_date BETWEEN UNIX_TIMESTAMP('2008-01-15 00:00:00') AND UNIX_TIMESTAMP('2010-02-15 00:00:00')
GROUP BY post.user_id
ORDER BY posts
DESC