DataBase Analysis-junction table

Question

This matter confuses me, I have a College Information system the junction table between students table and subjects(curriculum) table, the primary key is composite key (StudentID, SubjectID) and both of them are Foreign keys but the student may be fail in exam and repeat the subject so we will have duplicate PK and we need to record all data. I have two ways to solve this matter but i don't know the best way?

1. Add new column as primary Key instead of composite key.
2. Join to the composite key Season Column and year column and the composite key will be(StudentID, SubjectID, Season, Year). I have to mention that i don't need this composite key as foreign key.

Which way is better for performance and DB integrity?

Answer 1

Subject and exam are separate (if related) concepts, so you should not try to represent them within the same table. Also, the fact that an exam has been held for the given subject is separate from the fact that any particular student took that exam. Split all these concepts into their own tables, and the model becomes more natural, for example:

Representing a student that took the same exam several times is just a matter of adding multiple rows to the STUDENT_EXAM table.

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NOTE: STUDENT_SUBJECT just records the fact that the student has enrolled in the subject, but not when (which year/semester). Keeping semester-specific information may require additional tables and more complicated relationships within the model.

NOTE: There is a diamond-shaped dependency in this model. Since SUBJECT_ID was passed from the "top" (SUBJECT), down both "sides" (STUDENT_SUBJECT, EXAM) and then merged at the "bottom" (STUDENT_EXAM) of the diamond, a student cannot take an exam on a subject (s)he has not enrolled in.