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

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NIELIT Scientific Assistant CS 5 Dec 2021 Official Paper

Answer 1

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.