Question

Suppose I have a ‘tags’ table with two columns: tagid and contentid. Each row represents a tag assigned to a piece of content. I want a query that will give me the contentid of every piece of content which is tagged with tagids 334, 338, and 342.

The ‘easy’ way to do this would be (pseudocode):

select contentid from tags where tagid = 334 and contentid in (     select contentid from tags where tagid = 338 and contentid in (         select contentid from tags where tagid = 342     ) ) 

However, my gut tells me that there’s a better, faster, more extensible way to do this. For example, what if I needed to find the intersection of 12 tags? This could quickly get horrendous. Any ideas?

EDIT: Turns out that this is also covered in this excellent blog post.

Answer 1

SELECT contentID FROM tags WHERE tagID in (334, 338, 342) GROUP BY contentID HAVING COUNT(DISTINCT tagID) = 3   --In general SELECT contentID FROM tags WHERE tagID in (...) --taglist GROUP BY contentID HAVING COUNT(DISTINCT tagID) = ... --tagcount