Find the ids of the students who participate in Canoeing and Kayaking.
SELECT T1.stuid FROM participates_in AS T1 JOIN activity AS T2 ON T2.actid   =   T2.actid WHERE T2.activity_name   =   'Canoeing' INTERSECT SELECT T1.stuid FROM participates_in AS T1 JOIN activity AS T2 ON T2.actid   =   T2.actid WHERE T2.activity_name   =   'Kayaking'
students who participate in both Canoeing and Kayaking requires an INTERSECT operation; stuid refers to student ids; participates_in is a table that relates students to activities; activity is a table that contains activity names and ids;