A null represents an undefined or unknown value and is specified by the keyword null. A null is not the same as a zero, a blank, or an empty string. A null can be assigned to any nullable column when no other value is specifically assigned. More information about defining nullable columns is provided in the Create Table section in the “OpenSQL Statements” chapter.

The IS NULL predicate allows nulls to be handled in queries. For details, see Exists Predicate in the “OpenSQL Statements” chapter.