What does null mean?

Abstract

A given field in a given record can contain data, or not. If you don’t enter a value into a field in a particular record, you might think that the field was simply empty, but life isn’t that simple. Instead the field is said to contain a null value. If, for example, a field is supposed to contain the phone number of a friend but you don’t know the phone number, you don’t enter any data. The field is then said to contain a null value.