Question

4) A

is a key which does not contain any duplicate and NULL values.​

Answer 1

Answer:

Primary key

Explanation:

The primary key is the minimal set of attributes which uniquely identifies any row of a table. The primary key cannot have a NULL value. It cannot have duplicate values. Primary key enforces the integrity of an entity

Only one primary key is allowed to use in a table. The primary key does not accept the any duplicate and NULL values.

Answer 2

Answer:

Primary key

The primary key is the minimal set of attributes which uniquely identifies any row of a table. The primary key cannot have a NULL value. It cannot have duplicate values. Primary key enforces the integrity of an entity