Database-Relationship.md

-## Appendix: Relationships between elements in the database
 **Sprint3**
 
 All information is classified as 6 tables: s_admin, s_selected_course, s_course, s_clazz, s_student, s_teacher. 
@@ -13,5 +12,5 @@ Their elements are listed as follow:
 **Sprint4**
 
 Set up a new table: s_attendance.
-s_attendance: uses **id** as primary key, and stores course_id, type, student_id and date at the same time.
+s_attendance: uses **id** as primary key, and stores course_id, type, student_id and date at the same time. Besides, course_id and student_id **must relatively stay the same as** id in s_course and id in s_student.
 ![Relationship-sp4](uploads/62be47df78650e57c93c517117ad524b/Relationship-sp4.png)
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