DATA MODELING - Fundamentals of Database Management Systems - page 37

MANUFACTURER
PK

Manufacturer
Name

Manufacturer
Country
Sales Rep
Name
Sales Rep
Number

Manufactured
Manufactured by

CAR RENTAL

PK Car Serial

Number

PK Car Serial

Number

Rented
Car rented

PK Customer

Number

Model
Year
Class

Rental Date
Return Date
Total Cost

Repaired
Car Repaired

Rented
Rented by

MAINTENANCE
EVENT

CUSTOMER
PK Customer

Number

PK Repair

Number

Customer
Name
Customer
Address
Customer
Credit Rating

Date
Procedure
Mileage
Repair Time

F IGURE 2.12
Lucky Rent-A-Car entity-relationship
diagram

have rented at least one. Rental Date, Return Date, and Total Cost are intersection
data to the many-to-many relationship between CAR and CUSTOMER, as shown
in the associative entity RENTAL.

SUMMARY

Being able to express entities, attributes, and relationships is an important
preliminary step towards database management. The Entity-Relationship Model
is a diagramming technique that gives us this capability. The E-R model can
display unary relationships (relationships between entities of the same type,) binary
relationships (relationships between entities of two different types), and ternary
relationships (relationships between entities of three different types). Based on the
number of distinct entities involved in a relationship, we expand this to one-to-one,
one-to-many, and many-to-many unary relationships, one-to-one, one-to-many, and