Find the id of courses which are registered or attended by student whose id is 121?
create table student_course_attendance (course_id varchar, student_id varchar, PRIMARY KEY (course_id)); create table student_course_registrations (course_id varchar, student_id varchar, PRIMARY KEY (course_id))
select course_id from student_course_registrations where student_id = 121 union select course_id from student_course_attendance where student_id = 121