JLearner JLearner - 3 months ago 19
SQL Question

Superkey, candidate key & primary key

Can any kind soul clarify my doubts with a simple example below and identify the superkey, candidate key and primary key?

I know there are a lot of posts and websites out there explaining the differences between them. But it looks like all are generic definitions.

Example:

Student (StudentNumber, FamilyName, Degree, Major, Grade, PhoneNumber)


So from the above example, I can know
StudentNumber
is a primary key.

But as for superkey, I'm a bit confused what combination of attributes could be grouped into the superkey?

As for candidate key, I'm confused by the definition given as any candidate key can qualify as a primary key.

Does it mean that attributes such as
PhoneNumber
are a candidate key and can be a primary key? (Assuming that a
PhoneNumber
only belongs to one student)

Thanks for any clarification!

Answer

Since you don't want textbook definitions, loosely speaking, a super key is a set of columns that uniquely defines a row.

This set can have one or more elements, and there can be more than one super key for a table. You usually do this through functional dependencies.

In your example, I'm assuming:

StudentNumber    unique
FamilyName     not unique
Degree     not unique
Major      not unique
Grade      not unique
PhoneNumber    not unique

In this case, a superkey is any combination that contains the student number.

So the following are superkeys

StudentNumber
StudentNumber, FamilyName
StudentNumber, FamilyName, Degree
StudentNumber, FamilyName, Degree, Major
StudentNumber, FamilyName, Degree, Major, Grade
StudentNumber, FamilyName, Degree, Major, Grade, PhoneNumber
StudentNumber, Degree
StudentNumber, Degree, Major
StudentNumber, Degree, Major, Grade
StudentNumber, Degree, Major, Grade, PhoneNumber
StudentNumber, Major
StudentNumber, Major, Grade
StudentNumber, Major, Grade, PhoneNumber
StudentNumber, Grade
StudentNumber, Grade, PhoneNumber
StudentNumber, PhoneNumber

Now assume, if PhoneNumber is unique (who shares phones these days), then the following are also superkeys (in addition to what I've listed above).

PhoneNumber
PhoneNumber, Grade, 
PhoneNumber, Major, Grade
PhoneNumber, Degree, Major, Grade
PhoneNumber, FamilyName, Degree, Major, Grade
PhoneNumber, Major
PhoneNumber, Degree, Major
PhoneNumber, FamilyName, Degree, Major
PhoneNumber, StudentNumber, FamilyName, Degree, Major
PhoneNumber, Degree
PhoneNumber, FamilyName, Degree
PhoneNumber, StudentNumber, FamilyName, Degree
PhoneNumber, FamilyName
PhoneNumber, StudentNumber, FamilyName

A candidate key is simply the "shortest" superkey. Going back to the 1st list of superkeys (i.e. phone number isn't unique), the shortest superkey is StudentNumber.

The primary key is usually just the candidate key.