Databases Reference
In-Depth Information
Figure 3-19
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.
two or three, at most). For a person to be of interest to Good Reading as an
author, she must have written at least one and possibly many books that Good
Reading carries. Note that there is a many-to-many relationship between the
BOOK and AUTHOR entities but no intersection data.
Looking downward from the topic entity box, a book that Good Reading
carries may not as yet have been bought by any of its customers (maybe it just
came out) or may have been bought by many of its customers. For a customer
to be of interest to Good Reading, he must have bought at least one book and
possibly (hopefully!) many. Date, Price, and Quantity are intersection data in the
many-to-many relationship between the BOOK and CUSTOMER entities. Does
this make sense? Might a customer have bought several copies of the same book
on the same date? After all, that's what the presence of the Quantity attribute
implies. Might she have then bought more copies of the same book on a later
date? Yes to both questions! A grandmother bought a copy of a book for each
of three of her grandchildren one day, and they liked it so much that she returned
and bought five more copies of the same book for her other five grandchildren
several days later.
