PPT Slide
- Keep in mind that the preceding E-R diagram reflects a set of business rules that may easily be modified
- For example, if customers are supplied via a commercial customer list, many of the customers on that list will not (yet!) have bought anything, so INVOICE would be optional to CUSTOMER
- We are assuming here that a product can be supplied by many vendors and that each vendor can supply many products. The PRODUCT may be optional to VENDOR if the vendor list includes potential vendors from which you have not (yet) ordered anything.
- Some products may never sell, so LINE is optional to PRODUCT... because an unsold product will never appear in an invoice line.
- LINE may be shown as weak to INVOICE, because it borrows the invoice number as part of its primary key and it is existence-dependent on INVOICE
- The design depends on the exact nature of the business rules.
E-R Modeling & Transformation Exercise