CREATE TABLE participates_in (stuid VARCHAR)
CREATE TABLE activity (actid VARCHAR, activity_name VARCHAR)

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'
SELECT "T1"."stuid" FROM "participates_in" AS "T1" JOIN "activity" AS "T2" ON "T2"."actid" = "T2"."actid" AND "T2"."activity_name" = 'Canoeing' INTERSECT SELECT "T1"."stuid" FROM "participates_in" AS "T1" JOIN "activity" AS "T2" ON "T2"."actid" = "T2"."actid" AND "T2"."activity_name" = 'Kayaking'