When entering a new record or filling out a form, you don't always have all the information at hand. For instance, if you're entering patient information in a hospital's emergency room, you might know the patient's name, but not his or her address, social security number, next of kin, and so on. At this point, these values are null-they are unknown or unavailable. In this case, null values are appropriate and necessary. Later, you can complete the record once you've had a chance to collect all the necessary information.
Not all missing information is null, meaning unknown or unavailable. Sometimes, there simply isn't an appropriate value. For example, not everyone has a fax or a cell phone number. Once you know there's no value, you might want to supply a visual clue, such as "N/A." This way, you can distinguish between truly null values and those that simply don't apply. Doing so isn't just a convenient way to alert users but can also impact criteria. For instance, a query for missing phone numbers shouldn't return records where the phone number is already known not to exist.
How you enter N/A or some equivalent, is up to you. Just don't leave it up to data entry personnel or you'll end up with inconsistent entries: na, NA, N/A, n/a, not applicable, and so on. You lose control when you let users decide.
Susan Sales Harkins is an IT consultant, specializing in desktop solutions. Previously, she was editor in chief for The Cobb Group, the world's largest publisher of technical journals.