Database Reference
In-Depth Information
StudentMajor
SocSecNo
StudentID
STUDENT
StudentPhone
StudentName
StudentFax
Figure 6-4
STUDENT object: attributes.
StudentFax
StudentID
StudentName
SocSecNo
StudentPhone
StudentMajor
111245
Kristin Rogers
214-56-7835
516-777-9584
Networking
516-777-9587
121365
Rob Smith
123-44-5546
718-312-4488
Web Design
123456
Mary Williams
101-54-3838
212-313-1267
Networking
212-313-1267
234754
Shaina Gonzales
213-36-7854
212-126-3428
Social Science
388910
Andrew McAllister 311-33-4520
718-567-4321
Engineering
718-567-4322
400500
Kassy Goodman
512-22-8542
732-346-5533
Programming
732-346-5538
511675
Rob Smith
111-22-3344
908-212-5629
Liberal Arts
908-212-5630
Figure 6-5
STUDENT object: sample attribute values.
its relationship with another object called COURSE. Therefore, CourseNo does not
qualify to be an attribute of STUDENT.
Unique Values for an Instance Take a few samples of the attributes for the
object STUDENT as shown in Figure 6-5.
Look at the set of values shown in one row of the figure. These values for the
attributes StudentID, StudentName, SocSecNo, and StudentPhone relate to one
student, a specific instance of the object STUDENT. These attributes do not relate
to any random instance of the object; they relate to one particular student.
Let us say that this student is Rob Smith. Then these attributes are characteris-
tics of Rob Smith. If you observe a specific instance of the object STUDENT,
namely Rob Smith, then you will note a unique set of values for the attributes. This
unique set of values describes Rob Smith. In the data model, each instance of an
object possesses a unique set of values for its attributes.
Changeable Values We have noted that each instance of an object is described
by a unique set of values for its attributes. Review the unique set of values for the
attributes describing the instance represented by Mary Williams in Figure 6-5. Let
us say that Mary marries John Pearson and changes her name to Mary Pearson.
Also, she changes her phone number. What are the implications?
What you notice is that although each instance is described by a unique set of
values for its attributes, these values may change over time. The values for Stu-
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