如何将具有两个不同主键的两个表联接到另一个表中？

Question

I have two tables: students and courses , assuming that each student can be in more than one course and that each course can have more than one student.

[Table Students]             [Table Courses]
id(PK)                       id(PK)
name                         name
age                          duration
etc...                       etc...

and what I want to do it is to relate both tables into another table, for example, studying , in which I will store the course or courses that is doing each student. Like this:

[Table studying]
idStudent
idCourse

What I have deduced

I think that idStudent and idCourse should be foreign keys because the information it is stored in students and courses respectively with an unique primary key and to respect the consistency of the database. It cannot exist a relation without information neither of the student nor the course or just without the information of one of them.

I also know that some tables has two primary keys to allow that in the table could exist more than one repeated value of a primary key, but not of both primary keys at the same time.

My questions

• These ids ( idStudent , idCourse ). Have to be primary keys or foreign keys?
• Should the table studying has another column with an ID?
• Is my deduction in the good way?

PS: I do not need sql statements, I just need help to clarify my confusion.

Thanks in advance!

Answer 1

These ids (idStudent, idCourse). Have to be primary keys or foreign keys?

You want them to be foreign keys, because the existence of each record on your third table depends on the availability of the first, that is, there cannot be a "Student Course" or a "Course with Students" without either the course or the student. It could (if you don't make those keys) but you would break referential integrity

On the other hand, having FK's is usually a good thing because you make sure that you don't remove dependable records by mistake (which is what the constraint is for on the first place) unless you did something like cascade deleting

Should the table studying has another column with an ID?

No, it does not have to but again, sometimes it is a good practice because some software like Object Relational Mappers, Diagram Software, etc. may rely on the fact that they always needs a by-convention primary key. Some others don't even support composite keys so while it is not mandatory it can help in the future and it does not hurt. Of course this all depends on what you are using the database for and how (pure SQL, which engine you use, if you use it with a framework etc.)

Is my deduction in the good way?

All is relative. But I think your logic is good. My advice is that you always design your data schemas as flexible as you can because if a project grows its harder (and more costly) to do those changes down the road. Invest time on thinking how you may expand your application functionality and think if the schema will adapt to it.

Answer 2

If you like you can make them primary keys on studying table. But this is unnecesary, because relation (role of studying table) is many to many and this kind of table dont need primary keys. You need to know that also when you make them pk (pair of student id and course id) , thats mean that theee could be only one pair of each, thats equivalent to constrain unique - student can take a course only ones. In the future you maybe would like to add to this table start_date and this kind of pk could be a problem, you will need to modify them.

Answer 3

Your deduction is correct. In fact, you should have a composite primary key consisting of both (idStudent, idCourse) columns, because this tuple is the identifier of row in the table, you do not need additional ID column (of course, you can also take that approach to add additional ID column that would be your primary key, but you do not need it if one student can have one course assigned only once)

To respect the integrity, both columns (separately) should be foreign keys - idStudent should be referencing id column of Students table and idCourse should reference id column of Courses table.