DATA MODELING - Fundamentals of Database Management Systems - page 31

products. Going in the reverse direction, a product can be composed of no other
products or be composed of several other products.

TERNARY RELATIONSHIPS

A ternary relationship involves three different entity types. Assume for the
moment that any salesperson can sell to any customer. Then, Figure 2.8 shows
the most general, many-to-many-to-many ternary relationship among salespersons,
customers, and products. It means that we know which salesperson sold which
product to which customer. Each sale has intersection data consisting of the date of
the sale and the number of units of the product sold.

EXAMPLE: THE GENERAL HARDWARE COMPANY

Figure 2.9 is the E-R diagram for the General Hardware Company, parts of which
we have been using throughout this chapter. General Hardware is a wholesaler
and distributor of various manufacturers' tools and other hardware products. Its
customers are hardware and home improvement stores, which in turn sell the
products at retail to individual consumers. Again, as a middleman it buys its goods
from the manufacturers and then sells them to the retail stores. How exactly does

CUSTOMER
PK

One
Customer

Customer
Number
Customer
Name
HQ City

Many
Customers

Purchased
Sold to

One
Salesperson

One
Product

SALESPERSON
PK

SALE PRODUCT

Salesperson
Number
Salesperson
Name
Commission
Percentage
Year of Hire

PK Salesperson

Number

PK Product

Number
Product
Name
Unit Price

Sold
Sold by

Sold
Sold
Product

PK Product

Number

PK Customer

Number
Date
Quantity

Many
Salespersons

Many
Products

FIGURE2.8
Ternary relationship