Now let me introduce the example I’ll be using as a basis for most of the discussions in the rest of the book: the familiar—not to say hackneyed—suppliers-and-parts database. (I apologize for dragging out this old warhorse yet one more time, but I believe that using essentially the same example in a variety of different books and publications can help, not hinder, learning.) Sample values are shown in Figure 1-1. To elaborate:
Suppliers: Relvar S denotes suppliers. Each supplier has one supplier number (SNO), unique to that supplier; one name (SNAME), not necessarily unique (though the SNAME values in Figure 1-1 do happen to be unique); one status value (STATUS), representing some kind of ranking or preference level among suppliers; and one location (CITY).
Parts: Relvar P denotes parts (more accurately, kinds of parts). Each kind of part has one part number (PNO), which is unique; one name (PNAME), not necessarily unique; one color (COLOR); one weight (WEIGHT); and one location where parts of that kind are stored (CITY).
Shipments: Relvar SP denotes shipments (it shows which parts are supplied, or shipped, by which suppliers). Each shipment has one supplier number (SNO), one part number (PNO), and one quantity (QTY). Also, I assume for the sake of the example that there’s at most one shipment at any one time for a given supplier and a given part, and so each shipment has a supplier-number/part-number combination that’s unique.
 For reasons that will become clear later, the values shown in Figure 1-1 differ in two small respects from those in other books of mine: The status for supplier S2 is shown as 30 instead of 10, and the city for part P3 is shown as Paris instead of Oslo.