When your data is incomplete, you can use a null to represent a missing or unknown value. A null has these characteristics:
In SQL statements, the keyword NULL represents a null.
A null is used for a value that may never be known, may be determined later, or is inapplicable. (It’s helpful to think of a null as being a marker for a missing value, rather than as a value itself.)
A null differs from zero, a string that contains only blanks, or an empty string (''). A null in the column price doesn’t mean that an item has no price or that its price is zero; it means that the price is unknown or has not been set. (Oracle is a special case with respect to empty strings; see the DBMS Tip in this section.)
Nulls don’t belong to any data type and ...