Should foreign keys become table primary key?

Question

I have a table (session_comments) with the following fields structure:

student_id (foreign key to students table)
session_id (foreign key to sessions table)
session_subject_ID (foreign key to session_subjects table)
user_id (foreign key to users table)
comment_date_time
comment

Now, the combination of student_id, session_id, and session_subject_id will uniquely identify a comment about that student for that session subject.

Given that combined they are unique, even though they are foreign keys, is there an advantage to me making them the combined primary key for that table?

Thanks again.

Answer 1

1. Making them the primary key will force uniqueness (as opposed to imply it).
2. The primary key will presumably be clustered (depending on the dbms) which will improve performance for some queries.
3. It saves the space of adding a unique constraint which in some DBMS also creates a unique index.

Whether you make these three the primary key or not, you will still need some sort of uniqueness constraint to guarantee that a student cannot be associated with the same session and session_subject_id twice. If that scenario is allowed, then you would need to expand your uniqueness constraint out to include another column.

No matter what choice you make, you should absolutely have some sort of uniqueness constraint on the table.

If you are debating as to whether to create a surrogate primary key + a unique constraint on the three columns, I would say that it depends on whether this table will have child tables. If it will, then referencing the surrogate key will be easier and smaller. If it will not, then IMO, the surrogate key does not really give you much and you might as well use the three columns as the PK.