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

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