Database: conference
Question: What are the names of the staff members who have been both a speaker 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  =  'Speaker' INTERSECT SELECT T1.name FROM staff AS T1 JOIN conference_participation AS T2 ON T1.staff_id  =  T2.staff_id WHERE T2.role  =  'Sponsor'