Find the course ids of the courses registered by the student with id 171.
select course_id from student_course_registrations where student_id = 171
Find the same for the student with id 121.
select course_id from student_course_registrations where student_id = 121
Give also the id of courses which are attended by that student.
select course_id from student_course_registrations where student_id = 121 union select course_id from student_course_attendance where student_id = 121