If we have the id for a post in postid and a limit of num and we want to find posts which share as many tags as possible with postid's post, the following SQL will get you there.
SELECT p.*, COUNT(pt2.post_id) AS count FROM posts p, posts_tags pt, tags t, posts_tags pt2 WHERE pt.post_id=#{postid} AND t.id = pt.tag_id AND pt2.post_id != pt.post_id AND pt2.tag_id=pt.tag_id AND p.id = pt2.post_id GROUP BY pt2.post_id ORDER BY count DESC LIMIT #{num};")
