A. The Area being Modelled is :-
An e-Commerce Web Site for a specialist Bookseller.

B. The Things of Interest, ('THINGS'), include :-
B.1 Customers
B.2 Customer Orders
B.3 Customer Rentals
B.4 Games
B.5 Other Rental Items
B.6 Rental Items
B.7 Videos

C. These THINGS_OF_INTEREST are Related as follows :-
C.1 A CUSTOMER can be associated with zero or many VIDEO_RENTALS.
    The 'zero' option provides flexibility where a CUSTOMER calls
    to place an ORDER for a RENTAL_ITEM, which is subsequently cancelled.
C.2 A CUSTOMER can place zero or many ORDERS for RENTAL_ITEMS.
C.3 A CUSTOMER_ORDER must always be associated with one and only one CUSTOMER.
C.4 A CUSTOMER_RENTAL must always be associated with one and only one CUSTOMER.
C.5 All RENTAL_ITEMS have many attributes in common, and (maybe) some which are not common.
    This is shown in the Conceptual Model by an Inheritance relationship between Rental Items
    and Videos,Games and 'Other Rental Items'.

D. Other Characteristics of these THINGS_OF_INTEREST include
D.1 A VIDEO_RENTAL has a PRICE for the rent of a Video for a specified period.

E. Sample Data includes :-
To be determined

F. Typical Enquiries include :-
F.1 How many VIDEO_RENTALS are overdue ?
F.2 Do we have any VIDEOS featuring Cher in a Movie ?

G. NOTE :
G.1 The proposed design can easily be implemented with either one Rental Item Table
or separate Tables for Videos, Games, etc..

However, a VIDEOS Table will be preferred if there are many data items are specific
to VIDEOS, and it's often better to avoid an approach which is designed to provide
generality at the expense of ease of understanding.


Barry Williams
Principal Consultant
Database Answers Ltd.
London, England

31st. January 2001