Database Reference
In-Depth Information
Entity Definition
Entities are real-world “things” of interest to an organization. Note the following
businesses and a few of the things of interest to the particular organizations:
Airline:
AIRCRAFT, AIRCRAFT TYPE, MANUFACTURER, PASSENGER,
FLIGHT
Pharmacist:
CUSTOMER, PRODUCT, PRESCRIPTION, SUPPLIER, BILL
Car Wash:
WASH TYPE, WASH, EQUIPMENT
Farm Market:
PRODUCE, SUPPLIER, SALE
Orchard:
TREE, VARIETY, SPECIES, ORCHARD, CUSTOMER
Observe the following features of an entity:
Independent existence.
Notice that in each case, an entity in the real world has
independent existence. That means an entity does not depend on the presence of
another entity for its existence. The entity PASSENGER exists irrespective of
whether it is related to another entity called FLIGHT. There could potentially be
prospective passengers who have not been associated with any flights at all. Still,
the entity exists in the database system.
Distinguishable.
In the real world, one entity is distinguishable from another.
That means that one passenger named Tom James is distinguishable from another
passenger named Joe Rawlins. Each entity is unique, and you can differentiate one
entity from another.
Physical and Conceptual Entities.
When you review the examples of entities
listed above, you observe that some of the things may be seen or touched—things
that are tangible. When you examine the others on the list, you realize that some
of the other things cannot be really seen or experienced. These are not tangible.
Nevertheless, both categories are things that are of interest to an organization. Make
sure you include both types as indicated below in your data model.
Things with physical existence.
An employee, a specific automobile, a particular
building, an explicit machine, and a particular aircraft are examples of this type.
Things with conceptual existence.
Examples include a university course, a visit to
the doctor, an invoice sent to a customer, a bank account, and a type of aircraft.
Entity Types
So far in our discussion we have indicated one particular thing of interest to an orga-
nization as an entity. An entity is a single thing. One customer is a “thing” that an
organization is interested in. Of course, there would be thousands of customers for
an organization. So database practitioners make a distinction and refer to the notion
that refers to the collection of entities as an entity type. Entities are individual
“things,” whereas entity types refer to the group of entities.
Figure 7-3 illustrates the two notions of entity types and entities.
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