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
course id of courses registered or attended by a specific student refers to selecting course_id from both student_course_registrations and student_course_attendance where the student_id matches; UNION is used to combine results from two tables without duplicates.