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