1. Every primary key is candidate key, and a super key
2. One of the candidate key is chosen as primary key,
example R(A,B,C,D,E,F)
suppose candidate key =AB, CD, F
now anyone of these three can be primary keys
3. any set of attribute which contains candidate key or primary key is super key
example ABE, FE, ABCDEF like these are super keys
Now if a attribute is unique, that it can uniquely identify each row then that attribute can be a candidate or primary key.
example
A B
1 q
2 r
3 s
3 t
here A can not be a key, because values in A can not uniquely identify each row in B
A(3) = [s,t], on other hand column B can be unique attribute