To Query for NULL values use the syntax [FIELD] IS NULL or its complement [FIELD] IS NOT NULL.
For SQL Server and Microsoft Access, if the field is a character or text field, you may also want to include the following syntax [FIELD] = '' or its complement [FIELD] <> ''. Proper syntax must be used to formulate this OR statement. For example, ( [FIELD] IS NULL OR [FIELD] = '')
Oracle does not support the following queries [FIELD] = '', or [FIELD] <> ''. For Oracle use the NULL portion of the query only.