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