Database Reference
In-Depth Information
EMPLOYEE
SKILL
EmployeeNumber
Name
EmployeeName
Phone
Email
HireDate
ReviewDate
EmpCode
Description
SalaryRange
EMPLOYEE_SKILL
EmployeeNumber
SkillNumber
Figure 5-35
Mixed Relationship Pattern
for Employee Skills
ProficiencyLevel
CourseTaken
Only.” For example, Figure 5-36 shows two grayed-out sections, one for commercial fishers
and another for sport fishers. The presence of these grayed-out sections indicates the need for
subtype entities.
The data model for this form is shown in Figure 5-37. Observe that each grayed-out sec-
tion has a subtype. Notice that the subtypes differ not only in their attributes, but that one has
a relationship that the other does not have. Sometimes the only differences between subtypes
are differences in the relationships they have.
The nonidentifying relationship from VESSEL to COMMERCIAL_LICENSE is shown as
1:N, mandatory to mandatory. In fact, this form does not have sufficient data for us to con-
clude that the maximum cardinality from VESSEL to COMMERCIAL_LICENSE is N. This
fact was determined by interviewing users and learning that one boat is sometimes used by
more than one commercial fisher. The minimum cardinalities indicate a commercial fisher
must have a vessel and that only vessels that are used for licenses are to be stored in this
database.
The point of this example is to illustrate how forms often suggest the need for subtypes.
Whenever you see a grayed-out or otherwise distinguished section of a form with the words
“For use by . . . ,” think “subtype.”
Figure 5-36
Data Entry Form Suggesting
the Need for Subtypes
Resident Fishing License
2013 Season
State of Washington
License No:
03-1123432
Name:
Street:
City:
State:
Zip:
For Use by Commercial Fishers Only
For Use by Sport Fishers Only
Vessel Number:
Number Years at
This Address:
Vessel Name:
Prior Year License
Number:
Vessel Type:
Tax ID:
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