Select parent if all children meet criteria

Question

I have tables set up like so:

Parent
------
id, ...

Child
-----
id, parent_id, x, y

I want to find the Parents, or the distinct parent_id(s),

Answer 1

Should join 2 tables first because the parents does not have children that will satisfy

And should add index for pa_id column

SELECT DISTINCT pa.id 
FROM pa INNER JOIN c ON c.pa_id = pa.id 
WHERE NOT EXISTS ( SELECT 1 FROM c WHERE c.parent_Id = p.id and c.x <> c.y )