how many courses did the student with id 121 register for?
select count ( course_id ) from student_course_registrations where student_id = 121
And how many courses did the student attend?
select count ( * ) from student_course_attendance where student_id = 121
show me the id of courses registered or attended by student whose id is 121
select course_id from student_course_registrations where student_id = 121 union select course_id from student_course_attendance where student_id = 121