Question

Well this one is a headscratcher for me. I’ve got a system of posts with tags. Tags are a many->many relationship with the posts.

The issue is that I’d like to select other posts based on how many tags they have matching with the current one.

A quick visual example:

PostA: TagA, TagB
PostB: TagC
PostC: TagA
PostD: TagA, TagB

So if I inputted PostA, it’d give me PostD, PostC.

I really don’t even know where to start with this one, and I’m hoping somebody smarter than I ran into this issue already and can offer some help.

Answer 1

OK, so supposing your tagged posts table actually looks like this:

PostA   TagA
PostA   TagB
PostB   TagC
PostC   TagA
PostD   TagA
PostD   TagB

then you want

SELECT post FROM tagged_posts
  WHERE post != 'PostA'
  AND tag in (SELECT tag FROM tagged_posts WHERE post = 'PostA')
  GROUP BY post
  ORDER BY COUNT(*) desc