Finding posts with tag1 AND tag2? (using a join-table) Exists / Having / subqueries… What to use?

Question

I need to make a query to search and filter on multiple terms.

I have a table with weapons and all can have multiple tags. I want to be able to create a filter o

Answer 1

Turn it around. That is start with tag IN ('blue', 'old') in the innermost subquery. From that, find the tag_ids. From them, find the weapon_ids HAVING COUNT(*) = 2. Last, in the outermost query, get the weapon.name.

No EXISTS(). No LEFT JOINs; I'm not sure you needed LEFT in the first place.

Your original attempt needed to join to tag twice with different aliases. I avoid that by using the IN and HAVING.

More

Yeah, that gets a bit tricky...

SELECT
     FROM  
      ( SELECT  weapon_id
            FROM  weapon_tags
            JOIN  weapon_tag_links USING(tag_id)
            WHERE  tag IN ('blue', 'old')
            GROUP BY  weapon_id
            HAVING  COUNT(*) >= 2 
      ) a
    JOIN  weapons w ON w.id = a.weapon_id;

It would be easier if you had not normalized tags.