CREATE TABLE activity (actid DECIMAL, activity_name TEXT)
CREATE TABLE student (stuid DECIMAL, lname TEXT, fname TEXT, age DECIMAL, sex TEXT, major DECIMAL, advisor DECIMAL, city_code TEXT)
CREATE TABLE faculty (facid DECIMAL, lname TEXT, fname TEXT, rank TEXT, sex TEXT, phone DECIMAL, room TEXT, building TEXT)
CREATE TABLE participates_in (stuid DECIMAL, actid DECIMAL)
CREATE TABLE faculty_participates_in (facid DECIMAL, actid DECIMAL)

What are the ids of the students who are under 20 years old and are involved in at least one activity.
SELECT stuid FROM participates_in INTERSECT SELECT stuid FROM student WHERE age < 20
SELECT "stuid" FROM "participates_in" INTERSECT SELECT "stuid" FROM "student" WHERE "age" < 20