Question

I have an objects, tags, and tag_relationships table. Objects and tags each have an id column, and tag_relationships connects the two.

Sample of tag_relationships (used in example below):

+--------+--------+
| obj_id | tag_id |
+--------+--------+
| 1      | 1      |
| 1      | 2      |
| 2      | 1      |
| 2      | 3      |
| 2      | 4      |
+--------+--------+

I want to implement an AND search to allow people to filter tags. Only obj_ids that have a row with the all of the given tag_ids should be returned. For example:

Sample input: 1, 3 (would actually be tag names, but they would converted to ids)

Desired output: 2

It feels like this should be easy, but for the life of me I can’t figure out a simple way to do this. I’m currently thinking of returning a table with all the rows in tag_relationships that have one of the given tag_ids, then sorting through them with PHP. Is there a simpler, SQL-only method?

Answer 1

You can use the HAVING clause in conjunction with GROUP BY to answer questions involving all of something…

SELECT
    obj_id
FROM
    tag_relationships
WHERE
    tag_id IN (1,3)
GROUP BY
    obj_id
HAVING
    COUNT(*) = 2

The 2 in the COUNT(*) represents the number of tags you are searching on. If you wanted objects that satisfied say, three tags… (e.g. 1,3,6), you would up the COUNT(*) to 3.