Let me close by discussing the important topic of Business Rules which must underly any Schema you look at.
These are often implemented as 'Constraints' in the Database.
These Business Rules are important because they define the conditions that the Database must meet.
For example, every Order must be associated with a valid Product.
This prevents invalid Orders being entered into the Database.
These Rules can be understood by both the Users and the Database designer.
When you extend a Database Schema, it is always valuable to keep these Rules up-to-date.
This is not always done meticulously but is necessary where Users will 'sign-off' on their requirements.
A. SCOPE OF THE DATABASE SCHEMA :
A.1 The Scope is buying products from Amazon.com and Starbucks.
B. THE THINGS OF INTEREST include :-
C. These THINGS are Related as follows :-
C.1 A Customer can have zero,one or many ORDERS.
C.2 A CUSTOMER_ADDRESS can be associated with only one ADDRESS.
C.3 A CUSTOMER_ADDRESS can be associated with only one CUSTOMER.
C.4 A DELIVERY is associated with just one ORDER.
C.5 An ORDER must be associated with one and only one CUSTOMER.
In different applications in the real world, it is possble that an ORDER can
be associated with more than one CUSTOMER,but in our example, it's only one.
C.6 An ORDER can be associated with one or many PRODUCTS.
C.7 A PAYMENT must be associated with one and only one ORDER.
D. Other Characteristics of these THINGS include :-
D.1 A BOOK can have one and only one ISBN.
D.2 A FRAPPUCCINO can have a Flavor.
D.3 A PANINI can have Ingredients.
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