CREATE TABLE student_course_attendance (course_id VARCHAR(255), student_id VARCHAR(255)); CREATE TABLE student_course_registrations (course_id VARCHAR(255), student_id VARCHAR(255))
Find the id of courses which are registered or attended by student whose id is 121?
    
SQL: SELECT course_id FROM student_course_registrations WHERE student_id = 121 UNION SELECT course_id FROM student_course_attendance WHERE student_id = 121