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. You can assign a null to any nullable column when no other value is specifically assigned. For more information about defining nullable columns, see CREATE--Create a Table on page 27.

The ifnull function and the is null predicate enable you to handle nulls in queries. For details, see Ifnull on page 22 and Is Null Comparison on page 24.