... numbers).• For transactional documents (e.g., invoices, bills, and computer-generated notices): These usu-ally have some sort of number assigned when they are printed.• For products for sale: ... ■INTRODUCTION TO DATABASE CONCEPTS188662Ch01.qxp 7/28/08 3:37 PM Page 18• For companies that clients deal with: These are commonly assigned a customer/client number for tracking.• For buildings: ... addressee’s name and address and the date the item was sent.Be careful when choosing a natural key. Ideally, you are looking for something that is stable,that you can control, and that is definitely...