Question

An instance of relational schema R(A, B, C) has distinct values of A including NULL values. Which one of the following is true?
1. A is candidate key
2. A is not a candidate key
3. A is a primary key
4. Both "A is a candidate key" and "A is a primary key"

Answer 1

Correct Answer - Option 1 : A is candidate key

The correct answer is option 1.

•  The primary key can not have a NULL value.
• The candidate key can have a NULL value

Hence the correct answer is 1. A is candidate key.

Candidate key:

A candidate key is the minimal set of attributes that can uniquely determine/identify each tuple in a relation.

Super key:

A super key is a set of keys that uniquely define a row or a tuple. As a consequence, a super key is the superset of a Candidate key. It means that a candidate key can only be accessed from a super key.

Primary key:

An attribute or set of attributes is called a primary key (PK) that uniquely identifies each row in the table. If you want to create a primary key, you should define a PRIMARY KEY constraint when you create or modify a table.