An instance of relational schema R (A, B, C) has distinct values of A including NULL values. Which one of the following is true ?

The correct answer is A is a candidate key.

Key Points

• A candidate key is a column, or set of columns, in a table that can uniquely identify any database record without referring to any other data. A candidate key's values are unique and can be used as a primary key.
• In the given relational schema R (A, B, C), column A has distinct values including NULL values. This means that each value in column A is unique among all entries in that column, which is a requirement for being a candidate key.
• A candidate key must have unique values, and while it can have NULL values, it means it is not suitable to be a primary key (as primary keys cannot have NULL values). Hence, column A qualifies as a candidate key.
• A primary key is a candidate key that is chosen by the database designer to uniquely identify records in a table. Since A contains NULL values, it cannot be a primary key.

Additional Information

• Relational database systems typically require that each table has a primary key. This key is used to enforce entity integrity.Other candidate keys can be used as alternative keys or for creating unique constraints.
• NULL values in candidate keys can be useful in certain scenarios where not all records need to have a value for the key column.
• In SQL, the UNIQUE constraint can be used to designate a column (or combination of columns) as a candidate key.
• When designing a database schema, it's essential to identify candidate keys to ensure data integrity and efficient data retrieval.