SQL: Select records where ALL joined records satisfy some condition

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.

Answer 2

select * from A
 where -- at least one record in B is greater than some_val
       exists (select null from B
                where B.some_val > :value
                  and A.join_column = B.join_column)
   and -- no records in B are not greater than some_val
       not exists (select null from B
                    where B.some_val <= :value
                      and A.join_column = B.join_column)