Query Posts relacionados por TAG

post_tag.sql

SELECT c.post_id, c.post_c FROM (
SELECT b.post_id, COUNT(b.post_id) AS post_c FROM(
SELECT post_id, tag_id FROM post_tag
WHERE tag_id IN (SELECT tag_id FROM post_tag WHERE post_id = 1)) AS b
GROUP BY b.post_id
) AS c
WHERE c.post_c >= 2


-- c.post_c >=2   -> cantidad de TAGS en común
-- post_id        -> id del POST 

Directo a WP!