Select rows from table with tag a or b and c

I have a table with objects, a table with tags and a linking table.

The statement without the second condition works, but the out commented one yields no results, this probably because objects_tags.id_tag cannot be two different numbers at the same time.

SELECT objects.id
    FROM objects
    JOIN objects_tags ON objects.id = objects_tags.id_object
    JOIN tags ON tags.id = objects_tags.id_tag
WHERE objects_tags.id_ IN (3,4);
#WHERE (objects_tags.id_tag IN (3,4)) AND (objects_tags.id_tag = 6);

Is there a way to select all the entries in objects which are linked to tags with ID 3 or 4 and 6?

Answer

I would recommend group by and having:

SELECT ot.id_object
FROM objects_tags ot 
GROUP BY ot.id_object
HAVING SUM(ot.id_ IN (3, 4)) > 0 AND   -- has either 3 or 4
       SUM(ot.id_tag = 6) > 0;         -- also has 6

Note that you only need the object_tags table for this query, because the query is only using ids. If you want additional information, then joins may be necessary.