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.