To search for records where a particular field is empty, use IS NULL. To exclude records where the field is empty, use IS NOT NULL.
It is important to note that the number 0 (in a numeric field) and a blank (in an alphanumeric field) are not the same as an empty field. A field is empty when no value has been entered and there is no default value for this field.
In the following example, only data where the postal code in the shipping address is empty will pass the filter and be acted upon by the query.