o, Date, Status）Invoice Line（Invoice No, Line No, Product Code, Qty Shipped）Product（Product Code, Product Description）在这个设计中我们有六个关系变量：Customer, Product, Order, Order Line, Invoice,和Invoice Line.粗体字有下划线的属性是"候选键 (码)"。非粗体字有下划线的属性是"外键 (码)"。通常任意选择一个候选键