Question

What is meant by NULL value? What are the reasons that lead to their occurrence in database relations?

Answer 1

The absence of a data item which is represented by a special value is known as NULL value.

The following are the reasons that lead to its occurrence in the database relations.

(i) When a particular attribute does not apply to an entity

(ii) When the existing value of an attribute is unknown

(iii) When the value is unknown because it does not exist