SQL How to not insert duplicated values

Question

I'm trying to create a procedure that inserts data into a table of registers but i don't want to repeat the second parameter, this is the table

CREATE TABLE Inscription
(
    idClass INT references tb_class,
    idStudent INT references tb_student,
)

The idea is that a student (idStudent) can register in various classes but not in the same class (idClass), I tried to add a unique constraint in the idStudent column but that only allows a student to register in one single class.

Answer 1

I always suggest that all tables have a numeric primary key. In addition, your foreign key references are not correct. And what you want to do is add a unique constraint.

The exact syntax depends on the database. The following is for SQL Server:

CREATE TABLE Inscriptions (
    idInscription int identity(1, 1) primary key
    idClass int references tb_classes(idClass),
    idStudent int references tb_students(idStudnt)
    unique (idClass, idStudent)
);

Notice that I name the tables as the plural of the entity, but the id using the singular.

The Inscriptions table probably wants other columns as well, such as the date/time of the inscription, the method, and other related information.