Database: conference (difficulty: simple)
Question: What are the names of the staff members who have been both a Participant and a sponsor at some conference?
SQL: SELECT T1.name FROM staff AS T1 JOIN conference_participation AS T2 ON T1.staff_id  =  T2.staff_id WHERE T2.role  =  'Participant' INTERSECT SELECT T1.name FROM staff AS T1 JOIN conference_participation AS T2 ON T1.staff_id  =  T2.staff_id WHERE T2.role  =  'Sponsor'