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

You can use NOT EXISTS

SELECT id 
FROM Parent p
WHERE NOT EXISTS
(
   SELECT 1 FROM Child c
   WHERE c.parent_Id = p.id
   AND c.x <> c.y
)

Edit: Here's the sql-fiddle: http://sqlfiddle.com/#!3/20128/1/0