DATA MODELING - Fundamentals of Database Management Systems - page 34

the unique identifier. The attribute Quantity is intersection data in the many-to-many
relationship and so becomes an attribute in the associative entity SALES that links
salespersons with the products they have sold in a many-to-many relationship.

EXAMPLE: GOOD READING BOOK STORES

Figure 2.10 shows the E-R diagram for Good Reading Bookstores. Good Reading
is a chain of bookstores that wants to keep track of the topics that it sells, their
publishers, their authors, and the customers who buy them. The topic entity has
four attributes. Book Number is the unique identifier. A book has exactly one
publisher. Publisher Name is the unique identifier of the PUBLISHER entity. A
publisher may have (and generally has) published many books that Good Reading
carries; however, Good Reading also wants to be able to keep track of some
publishers that currently have no books in Good Reading's inventory (note the
zero-modality symbol from PUBLISHER towards BOOK). A book must have at
least one author but can have many (where in this case ''many'' means a few,
generally 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

PUBLISHER
PK

BOOK WROTE

Publisher
Name

PK Book

Number

PK Book

Number

Published
Published by

Written by

PK Author

Number

City
Country
President
Year Founded

Book Name
Publication
Year
Pages

Wrote

Sold
In sale

Wrote
Written by

SALE

CUSTOMER
PK

AUTHOR

PK Book

Number

Customer
Number

PK Author

Number

Bought
Bought by

PK Customer

Number

Customer
Name
Street
City
State
Country

Author Name
Year Born
Year Died

Date
Price
Quantity

F IGURE 2.10
Good Reading Bookstores entity-relationship diagram