How to make a composite key to be unique?

Question

I am making a database of students in one school.Here is what I have so far:

If you don't like reading jump to the "In short" part

The problem is that I'm not happy with this design. I want the combination of grade, subgrade and id_class to be unique and to serve as a primary key for the students table. I can remove the student_id and make a composite key from the 3 but I don't want that either. Maybe I should make another table lets say combination_id where grade, subgrade and id_class are foreign keys and there is one extra column comb_id that serves as ID for the table. And all the columns will be Primary Keys. But the problem is that those 3 columns can still repeat because of that extra column (comb_id). For example I can have the same grade, subgrade and class_id but different comb_id which will make the row valid because of the composite key of the 4 columns of the table (combination_id).

In short I want students_id to remain the only primary key of the table but to be a foreign key to another table which is somehow unique combination of grades, subgrade and class_id.

If I was not clear enough ask in the comments below and thank you in advance.

PS I'm sorry for the indescriptive title but I'm bad at naming

EDIT 1: To be more clear: grade can be 1 to 12 subgrade can be a to j id_class can be 1 to 30 and it is your number in class

So a student can be from 7b class and his number in class - 5

Answer 1

Don't mix the concepts of unique keys and primary keys. You can very well add a unique key spanning the three columns grades, subgrade and class_id. That way, no two rows could have the same values for these three columns. As you write that you don't want to have these three as a composite primary key, I'm not sure whether a composite unique supplemental key would be any better. If not, you'll have to clarify when composite keys are acceptable.

To create a unique key, you can use the following SQL statement:

ALTER TABLE students ADD UNIQUE gsc (grades, subgrade, class_id);

The word gsc there is just a name I made up from the initials of the key columns; use whatever name you want, as it hardly matters unless you want to identify the key in some EXPLAIN output or similar.