Find foreign key matching multiple row values

Question

I have a table design like so

person_id   |   department
1           |   a
1           |   b
2           |   a
2           |   c
3           |   b
3           |          


        

Answer 1

SELECT
  person_id
FROM
  yourTable
WHERE
     department = 'a'
  OR department = 'b'
GROUP BY
  person_id
HAVING
  COUNT(DISTINCT department) = 2

Note: The DISTINCT is only needed if a person can be a member of the same department more than once.

Answer 2

You can also achieve this with a subquery:

SELECT person_id 
FROM table t1 
WHERE t1.department = 'a' 
    AND EXISTS (SELECT 1 
            FROM table t2 
            WHERE t2.department = 'b' 
                AND t1.person_id = t2.person_id`)