SQL query: Simulating an “AND” over several rows instead of sub-querying

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

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

Answer 2

I don't know if this is better but it might be more maintainable

select contentid from tags where tagid = 334
intersect
select contentid from tags where tagid = 338
intersect
select contentid from tags where tagid = 342

You'd have to build it dynamically which wouldn't be as bad as your original solution.

Answer 3

What type of SQL? MS SQL Server, Oracle, MySQL?

In SQL Server doesn't this equate to:

select contentid from tags where tagid IN (334,338,342)

Answer 4

The only alternative way i can think of is:

select a.contentid from tags a
inner join tags b on a.contentid = b.contentid and b.tagid=334
inner join tags c on a.contentid = c.contentid and c.tagid=342
where a.tagid=338

Answer 5

Here's a solution that has worked much faster than the for me on a very large database of objects and tags. This is an example for a three-tag intersection. It just chains many joins on the object-tag table (objtags) to indicate the same object and stipulates the tag IDs in the WHERE clause:

SELECT w0.objid

FROM       objtags t0
INNER JOIN objtags t1 ON t1.objid=t0.objid
INNER JOIN objtags t2 ON t2.objid=t1.objid

WHERE t0.tagid=512
  AND t1.tagid=256
  AND t2.tagid=128

I have no idea why this runs faster. It was inspired by the search code in the MusicBrainz server. Doing this in Postgres, I usually get a ~8-10x speedup over the HAVING COUNT(...) solution.