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), if all of the rows in Child containing a given parent_id meet a criteria involving x and y(in my case x = y).

For example:

Parent
------
id
 1
 2
 3

Child
id, parent_id, x, y
1,      1,     2, 3
2,      1,     3, 4
3,      2,     5, 5
4,      2,     6, 7
5,      3,     8, 8
6,      3,     9, 9

would result in 3. Currently, I have a query that finds parent_ids that any of the children meet the criteria. I then use that to retrieve those records and check them in code if all the children meet the criteria. With the example data, I get parent_id 2 and 3, get the two parent records with all children, and evaluate. I want to do this with a single query, if possible.

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