Hello there!  Which roll code on this list is the most common?
select role_code from project_staff group by role_code order by count ( * ) desc limit 1
Great!  Can you tell me how many instances of role code "researcher" there are?
select count ( * ) from project_staff where role_code = 'researcher'
Excellent!  Can you now list for me all of the staff IDs that have researcher as their role code?
select staff_id from project_staff where role_code = 'researcher'