What are the various situations in which an attribute can use a null value?

There are various situations in which an attribute can use a NULL value. One such situation is when a particular entity does not have an appropriate value for an attribute. In such a situation, the attribute takes a special value called the NULL value. A null value of an attribute may indicate "not applicable". For example, consider an entity type PERSON with three attributes First_name, Middle_name and Last_name. Since not every person has a middle name, the person without any middle name will have a NULL value for the attribute Middle_name. Another situation when the NULL value is used is when the value of a particular attribute is unknown. An unknown category can further be classified as missing and not known. For example, if the attribute Book_title has the value NULL, it indicates that the title of the book is missing, as every book must have a title. However, if the attribute Phone has a NULL value, it indicates that it is not known whether the value for Phone exists or not.