recursive associations many to many and primary keys and foreign keys - sql

Question

Hi i have ER scheme with an recursive associations many to many with primary keys for example:
______(0,N)_________
ID | base \
\course preparatory
|___advanced_________/
(1,N)
where course is entity and preparatory is association nad course have ID primary key.
My question is:
course have ID primary key but, preparatory association?
I suppose that preparatory association have base and advanced primary keys and ID_course foreign key:
course primary key:ID
preparatory primary key:base,advanced,ID_course foreign key:ID_course
is it right?

Answer 1

In your model, I assume that preparatory represents all the courses that are required to be completed because you can start the principal course.
In that case you are correct, you will need a separate table to manage the multiple IDs, I would suggest something like this:
CREATE TABLE Course(
ID INT IDENTITY(1,1) NOT NULL,
CONSTRAINT Course_PK PRIMARY KEY (ID)
);
CREATE TABLE Preparatory(
ID INT IDENTITY(1,1) NOT NULL,
Advanced_CourseID INT NOT NULL,
Required_CourseID INT NOT NULL,
CONSTRAINT Preparatory_PK PRIMARY KEY (ID),
CONSTRAINT Preparatory_Advanced_Course_ID_FK
FOREIGN KEY (Advanced_CourseID)
REFERENCES Course (ID),
CONSTRAINT Preparatory_Required_Course_ID_FK
FOREIGN KEY (Required_CourseID)
REFERENCES Course (ID)
);
The expectation is that Advanced_CourseID represents the princial singular reference and there will be unique Required_CourseID for each Advanced_CourseID.
To further describe this I would suggest declaring a unique index on this relationship to enforce the desired structural integrity:
CREATE UNIQUE INDEX CONCURRENTLY Preparatory_Advanced_Required_Course_ID_UX
ON Preparatory(Advanced_CourseID,Advanced_CourseID);
ALTER TABLE Preparatory
ADD CONSTRAINT Unique_Preparatory_Advanced_Required_Course_ID
UNIQUE USING INDEX Preparatory_Advanced_Required_Course_ID_UX;