Find the id of courses which are 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
courses registered or attended by a student refers to information from both student_course_registrations and student_course_attendance tables; student whose id is 121 refers to student_id = 121; course id refers to course_id;