Here is some PHP code you can use in wordpress to get the tags of your post ID.

 $tags = wp_get_post_tags( $post->ID );

Let’s assume the result is one category (264) and one tag (41).

1. If you are not using WordPress Mu, you can simply select from wp_term_relationships and wp_posts
2. If you are using WordPress Mu, you will need to specify the tables as wp_MU-BLOG-ID#_term_relationships and wp_MU-BLOG-ID#_posts. Let’s assume we are using WordPress Mu and our blog ID is 1. The tables we are working with are wp_1_term_relationships and wp_1_posts

SELECT p.ID, COUNT(tr.object_id) AS cnt
FROM wp_1_term_relationships AS tr, wp_1_posts AS p
WHERE tr.object_id = p.id
AND tr.term_taxonomy_id IN(264,41)
AND p.id!=3987
AND p.post_status='publish'
GROUP BY tr.object_id ORDER BY cnt DESC, p.post_date_gmt DESC LIMIT 5;

Notice that we have limited our results to the 5 most relevant posts. It is recommended that you limit this query because it can be a difficult query to process if you are running it on a blog that has thousands of posts. You can change LIMIT 5 to LIMIT 25 or so on blogs that are hosting less than 5,000 posts without seeing much of a difference in performance. On larger, more frequently updated blogs or WordPress MU sites which are hosting a lot of blogs on the same server, it is recommended that you cache the results of this query and only use it once an hour or once every time a blog entry has been updated.

Why is this useful for WordPress?

Well sometimes you want to update a code snippet or something pasted across multiple posts.