PERIODON 790 เปิดให้บริการมานานแค่ไหนแล้ว ?ไม่จำเป็นต้องใช้ทุกโต๊ะ
PERIODON 790 has been offered for how long now ?It is not neccessary to use all the tables.
 gsi:  course_offering_id [ int]  student_id [ int] 
SELECT DISTINCT semester.year FROM course, course_offering, semester WHERE course.course_id = course_offering.course_id AND course.department = 'PERIODON' AND course.number = 790 AND semester.semester_id = course_offering.semester ORDER BY semester.year LIMIT 1