Which of the following statements is/are correct? P : Candidate key cannot be null. Q : If the primary key is composed of more than one attribute, then these attributes are known as super attributes. R : All the candidate keys can be primary keys.

Option P is correct because a candidate key is a key that uniquely identifies a row in a relation. So, this cannot be null as it uniquely identifies a row. Option R is correct because primary key is chosen from a set of candidate keys. In other words all the candidate keys are the candidates for the primary key. So, in this way all the candidate keys are primary keys. So, this is the correct answer.