find the id of users who are followed by mary and susan.
SELECT T2.f1 FROM user_profiles AS T1 JOIN follows AS T2 ON T1.uid   =   T2.f2 WHERE T1.name   =   "Mary" INTERSECT SELECT T2.f1 FROM user_profiles AS T1 JOIN follows AS T2 ON T1.uid   =   T2.f2 WHERE T1.name   =   "Susan"
users followed by both Mary and Susan requires an INTERSECT of two SELECT queries; T1 refers to user_profiles table, T2 refers to follows table; f1 in T2 represents the user ID of the followed user; f2 in T2 represents the user ID of the follower; uid in T1 represents the user ID;