Multiple tags search query

Question

I working on a tag based search. I have three tables tag(id,name), tagXmedia(id,tag_id,media_id), and media(id,...). tagXmedia is the mapping table between the tag and media

Answer 1

@kba's answer is correct but you can also do this with a JOIN which is probably more efficient.

SELECT media_id
  FROM tagXmedia
  LEFT JOIN tag ON tag_id = tag.id
  WHERE tag.name IN ('home', 'hawaii')
  GROUP BY media_id
  HAVING COUNT(tag_id) = 2;

I had a similar problem where I wanted to get not just the media_id but the actual object and wanted to pass in arbitrary comma separated lists of tags. Here's my complete solution using a stored procedure:

CREATE PROCEDURE FindByTag(IN _tags VARCHAR(256))
BEGIN
  DECLARE _length INT;

  -- Get the length of the list
  SET _tags =  TRIM(BOTH ',' FROM _tags);
  SET _length = LENGTH(_tags) - LENGTH(REPLACE(_tags, ',', '')) + 1;

  -- Find media
  SELECT * FROM media
    WHERE id IN (
      SELECT media_id FROM tagXmedia
        LEFT JOIN tag ON tag_id = tag.id
        WHERE FIND_IN_SET(tag.name, _tags)
        GROUP BY media_id
        HAVING COUNT(tag_id) = _length
    )
END