What is a super key in database?

Answer 1

S a subset of A, is a super key if S -> A.

That is, the combination of all attributes in S, determines all attributes in A.

Example:

R(studentNo, firstName, lastName, course, instructor)

FDs:

studentNo -> firstName, lastName, course

course -> instructor

Is studentNo a super key?

Yes, because studentNo yields the attributes firstName, lastName and course and course yield instructor. So with studentNo as super key, we are able to retrieve all the other attributes in relation R.

Is course a super key?

No, because we can only retrieve the instructor attribute and nothing else.

Answer 2

Super key is a key which uniquely identifies each row of the relation.

The key may consists of one or more attributes.

Ex:-

student {Roll no., Name, City, Marks}

F = {Roll no. -> Name, Roll no. -> City, Marks, Roll no., Name -> City}

Super key = {Roll no., Name}

Candidate key = {Roll no.}