Building a Data Model - High-Performance Web Databases: Design, Development, and Deployment - page 211

• Ron is interested in selected details about each entity. For example, he
tracks the name, address, telephone number, and maximum monthly
rental amount of each renter. He is concerned with how many blocks
away the beach is from a beach property, but not from a mountain
property. These details are called attributes.
• Some attribute names appear above horizontal lines in the boxes
(e.g., NAME in the RENTER box). These attributes appear to be the
most important or most necessary details about the entity. In fact,
these attributes must be present; for example, renters must have
names but not always addresses, telephone numbers, or monthly
rental amounts.
• Some attributes have names containing asterisks (e.g., RENT-
ER*NAME, PROPERTY*STREET-ADDRESS, PROPERTY*TOWN-STATE-
ADDRESS). These attributes seem to refer to other entities (or relate
them to each other).
• The arrows are drawn with one or two arrowheads at one end and
have no arrowheads at the other end. Arrowheads presumably carry
some meaning.
Without any knowledge of data modeling, a lot of information can be
obtained about Ron's business. Therefore, it is obvious that two benefits of
the data modeling technique are that it is simple to understand, because it
uses uncomplicated diagrams, and it expresses many facts precisely and
unambiguously.
Entities and Relationships
The most important constructs within a data model are entities and rela-
tionships. An entity is a person, place, thing, or concept about which facts
are to be recorded. Examples in Exhibit 1 are RENTER, RENTAL-AGREE-
MENT, PROPERTY, BEACH-PROPERTY, and MOUNTAIN-PROPERTY. A rela-
tionship is an association between two entities. Examples are RENTER
rents RENTAL-AGREEMENT, PROPERTY is rented to RENTAL-AGREEMENT,
and PROPERTY is of type BEACH-PROPERTY or MOUNTAIN-PROPERTY. In
a data model diagram, entities are represented by boxes and relationships
by arrows.
Entities have a number of properties. For instance, each entity has a
name (e.g., RENTER) and a description (e.g., person who obtains the privi-
lege of residing on a specific property according to the terms of a rental
agreement). Entity sets (e.g., all renters) can be distinguished from entity
occurrences (e.g., renter Harry Smith and renter Josephine Morgan). AU
occurrences within an entity set have the same attributes or detailed infor-
mation items (e.g., NAME, ADDRESS, PHONE-NUMBER, and MAX-
MONTHLY-RENT-AMT-all of which are attributes of RENTER).