Question

How can I write an SQL query that returns a record only if ALL of the associated records in a joined table satisfy some condition.

For example, if A has many B, I want to SELECT * FROM A WHERE all related B’s for a given A have B.some_val > value

I know this is probably a pretty basic question, so thanks for any help. Also, if it makes a difference, I’m using postgres.

Sam

Answer 1

Assuming no need for correlation, use:

SELECT a.*
  FROM A a
 WHERE EXISTS(SELECT NULL
                FROM B b
              HAVING MIN(b.some_val) > a.val)

If you do need correlation:

SELECT a.*
  FROM A a
 WHERE EXISTS(SELECT NULL
                FROM B b
               WHERE b.id = a.id
              HAVING MIN(b.some_val) > a.val)

Explanation

The EXISTS evaluates on a boolean, based on the first match – this makes it faster than say using IN, and — unlike using a JOIN — will not duplicate rows. The SELECT portion doesn’t matter – you can change it to EXISTS SELECT 1/0 ... and the query will still work though there’s an obvious division by zero error.

The subquery within the EXISTS uses the aggregate function MIN to get the smallest B.some_val – if that value is larger than the a.val value, the a.val is smaller than all of the b values. The only need for a WHERE clause is for correlation – aggregate functions can only be used in the HAVING clause.