Question

I have this tables: Posts (post_is, title and text), Tags (tag_id, tag), Post_tag_nn (id, tag_id, post_id). I want for a specific post having for example 4 tags all the posts with those tags, then all the posts with any three of those tags, then all posts with any two of those tags and so on. How can i build a SQL query for this purpose (in php it seems like a backtracking problem=all the subsets of a given set).

Answer 1

Have a query to find the tags of the current post, something like

SELECT tag_id
FROM Post_tag_nn
WHERE post_id = $post_id;

Then using those tag id’s, this query should return you the id’s of posts with 4,3,2,… matching tags:

SELECT post_id, COUNT(post_id) AS tag_count
FROM Post_tag_nn
WHERE tag_id IN ($array_of_tag_ids)
GROUP BY post_id
ORDER BY tag_count DESC;