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

This is what you need?

  select id from parent where id not in(
    select parent_id from chile 
    where x<>y
    group by parent_id)