Database Reference
In-Depth Information
10,000 physical copies of various topics, the object set BOOK COPY contains these
10,000 instances.
Conceptual Objects
AIRCRAFT TYPE, BOOK, and APPLIANCE TYPE do
not represent any tangible things. You cannot touch or see an AIRCRAFT TYPE.
What you can see is a physical aircraft that is of the Boeing 747 type. So these are
not examples of physical objects. The objects represented by these are conceptual;
these are conceptual objects.
What kinds of instances does a conceptual object set contain? These instances
are not physical things. Each instance within the object set BOOK refers to a par-
ticular title in the library. The library may have the topic,
A Brief History of Time
by Stephen Hawking. This title will be an instance of the conceptual object BOOK.
If the library holds four copies of this topic, then you will have four corresponding
instances in the physical object BOOK COPY.
Data Model Diagrams
Figure 6-25 presents the data model diagrams for the three
examples discussed.
Note the relationships between the conceptual object and the corresponding
physical object in each example. Also, observe the cardinality indicators and under-
stand the association of the instances.
Recursive Relationships
EMPLOYEE is a fairly common object in many real-world information require-
ments. Think about the instances for this object set. Every employee in the organi-
zation will be represented by an instance. In the set of instances, you will find
workers who supervise other workers. An instance representing a supervisor asso-
ciates with those instances of employees under his or her supervision. What you find
here is instances of an object associating with instances of the same object. Associ-
ations recur within the same object. These are recursive associations, and this type
CONCEPTUAL OBJECT
PHYSICAL OBJECT
AIRCRAFT
TYPE
PHYSICAL
AIRCRAFT
contains
1
*
BOOK
COPY
BOOK
contains
1
*
APPLIANCE
TYPE
PHYSICAL
APPLIANCE
contains
1
*
Figure 6-25
Data model diagrams: conceptual and physical objects.
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