The first SELECT statement given on p.97 is copied below.

SELECT InvoiceTotal, PaymentTotal, CreditTotal, InvoiceTotal - PaymentTotal - CreditTotal AS BalanceDue FROM Invoices;

The BalanceDue in the output of this statement is called a derived or calculated column because it is not defined in the Invoices table but in this statement. In other words, there is no BalanceDue data can be found in the Invoices table.

Assume that, there are some invoices in the Invoices table, their PaymentTotal or CreditTotal or both have no data value for some reasons. Discuss, for these invoices, what will be the reasonable value of BalanceDue at the output of this statement and explain why.