SQL db design constraints for co-belonging - sql

Question

Here is my database structure, it contains 3 tables : School, Student, Course
Each student must belong to a school (so School ID will be a foreign key in the student table referring to the School table).
Each course also must belong to a school (so School ID will be a foreign key in the course table referring to the School table).
Finally, each student may sign up for one or more courses (pivot table with a foreign key for student and a foreign key for school), but with the restriction that the student and the course must both belong to the same school. A student cannot be allowed to sign up for a course which belongs to a different school from the student.
Is there a way to add this constraint in the pivot table, or some other way model it?
Or should the constraint be enforced in the code before each insert on the pivot table?

Answer 1

You can add the SchoolID as a part Primary keys in both Student and Course.
This will force foreign keys to those tables to also specify the SchoolID.
We use this fact in our StudentInCourse-table to force both the student and the course to belong to the same school.
CREATE TABLE School(id INT PRIMARY KEY);
CREATE TABLE Course(id int, schoolID INT
FOREIGN KEY REFERENCES dbo.School(id),
PRIMARY KEY(id, schoolID));
CREATE TABLE Student(id INT, schoolID INT
FOREIGN KEY REFERENCES dbo.School(id),
PRIMARY KEY(id, SchoolID));
CREATE TABLE StudentInCourse(StudentId INT, schoolId INT, CourseID INT,
CONSTRAINT [fk_student]
FOREIGN KEY (StudentId,schoolID) REFERENCES student(id, SchoolID),
CONSTRAINT [fk_course]
FOREIGN KEY (CourseID,schoolID) REFERENCES Course(id, SchoolID),
);
INSERT dbo.School ( id )
VALUES ( 1 ), ( 2 );
INSERT dbo.Student
( id, schoolID )
VALUES ( 19950516, 1 );
INSERT dbo.Course
( id, schoolID )
VALUES ( 77666, 1 ),
( 99988, 2 );
-- Student in same school as course is OK
INSERT dbo.StudentInCource
( StudentId, schoolId, CourseID )
VALUES ( 19950516, 1, 77666 );
-- Student in other school as course is FAIL
INSERT dbo.StudentInCourse
( StudentId, schoolId, CourseID )
VALUES ( 19950516, 2, 99988 );
INSERT dbo.StudentInCourse
( StudentId, schoolId, CourseID )
VALUES ( 19950516, 1, 99988 );

Answer 2

IT is better to have 4 tables one each for School, Student, Course and forth table for Mapping Student to a particular Course.
Mapping student courses in separate table will give you ability to Map single student with Multiple courses.
This is the simplest of thing you can do.
If you need to map same student with different schools then you can add Map students & Courses either in same table of separate table

Answer 3

I recommend to make :
The three fields Pivot(StudentID,CourseID,SchoolID) a Primary Key
The two fields Pivot(StudentID,SchoolID) a double Foreign Key to the Student table
The two fields Pivot(CourseID,SchoolID) a double Foreign Key to the Course table
So the model will be in this way:
School Table:
+----------+
| SchoolID |
+----------+
| 1 |
| 2 |
| 3 |
+----------+
where SchoolID is a Primary Key
Course Table:
+----------+----------+
| CourseID | SchoolID |
+----------+----------+
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
+----------+----------+
where SchoolID references School(SchoolID) and (CourseID,SchoolID) a Primary Key
Student Table:
+-----------+----------+
| StudentID | SchoolID |
+-----------+----------+
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
+-----------+----------+
where SchoolID references School(SchoolID) and (StudentID ,SchoolID) a Primary Key
Pivot Table:
+-----------+----------+----------+
| StudentID | CourseID | SchoolID |
+-----------+----------+----------+
| 1 | 1 | 1 |
| 2 | 2 | 1 |
| 3 | 3 | 1 |
+-----------+----------+----------+
where Pivot (StudentID,SchoolID) references Student (StudentID,SchoolID)
and Pivot (CourseID,SchoolID) references Course (CourseID,SchoolID)