ISNULL( )
Tests if a value in a specified data field is a null value. A null value means no value exists.
Syntax
ISNULL(value)
Argument
value
The data field in which to check for null values.
Returns
True if a value in the specified data field is a null value; returns false otherwise.
Example
The following example uses ISNULL( ) in conjunction with the IF( ) function to test for null values in the BirthDate data field. If there is a null value, the computed column displays No date specified; otherwise the BirthDate value is displayed.
IF(ISNULL([BirthDate]), "No date specified", [BirthDate])