Databases Reference
In-Depth Information
PUBLISHER
BOOK
AUTHOR
Publisher Name
City
Country
Telephone
Year Founded
Book Number
Book Name
Publication
Year
Pages
Author
Number
Author Name
Year Born
Year Died
*
*
*
Published
Wrote
Date
Price
Quantity
Bought
CUSTOMER
*
Customer
Number
Customer
Name
Street
City
State
Country
Good Reading Bookstores E-R diagram.
FOR EXAMPLE
Following Up on Your Design
You might discover during the conversion process that your E-R design isn't
as complete as it should be. When this happens, you may find it necessary
to go back to your original data to look for missing attributes.
Consider the following situation. You are designing a database for a cus-
tomer support call center. When customers call they are automatically
routed to the next available customer support agent. The agent with whom
the customer speaks is, in this case, completely random. Your first choice
as an associative entity might be the combination of the customer and
employee identities. You would use this as the primary key.
When a customer calls back, he or she might get a different customer
service agent or, by luck of the draw, the same customer service agent. This
would mean the same combination of customer and employee identities,
and would mean a duplicate value in the primary key. This is an illegal con-
dition. You need an additional value, such as the date and time of the call,
to produce a unique primary key value.
