for me the rule of thumb is to display for NULL values the concept agreed upon/defined with the customer, in your case "unknown". Give an option to set to "unknown" for fields where previously information was entered know known to be false, but no correct value known by the operator. Do not allow ambiguity to enter via same display for null and zero or null and empty(), it will bite you later...


>The question is for each field the valid values
>can be actual value (know to the user) or null (for value not known to the user) in the sql field.
>
>e.g. if we have a field called salary then the valid
>values can be Zero to some figure. But if the user
>does not have the information for that field then
>the value should be null.
>
>Please note that in above example even Zero
>is a valid value. Hence Zero and null value
>have there own meaning.
>
>Zero salary can be valid for a director of a company.
>Null value can be valid for a person for whom we don't have any information.
>
>Question here is how should be our data entry screen.
>