7. ENTITY RELATIONSHIP MODELLING
When a database design is implemented, one of the important things to know for each attribute of an entity type is whether or not that attribute must have a value. For example, when a book is borrowed from a library the date the book is borrowed is known, but the returned date is not known. Sometimes you will not know the value of an attribute until a certain event occurs.
Consider an educational environment and when a student registers for a course. The date the student registers would be known, but the grade is yet to be determined.
When an entity is created but some attribute does not have a value, we say it is null. Null represents the absence of a value; null is different from zero or from blank.