Database Reference
In-Depth Information
VehicleID
LEASE
AGREEMENT
VEHICLE
covered by
Make
LeaseNo
MnthlyPymnt
ExprtnDate
CAR
TRUCK
VAN
Model
Model
Capacity
Options
Type
NoSeats
Figure 6-20
Subsets: inheritance of relationships.
utes not shared with the other two types. You have now come across the situation
of superset and subsets in the information requirements. VEHICLE is the superset
and CAR, TRUCK, and VAN are the subsets.
What about the association of the instances of VEHICLE with instances of
AGREEMENT? In the same way, do instances of CAR have associations with
instances of AGREEMENT? They do, because cars are covered by lease agree-
ments. You note that if the superset VEHICLE has a relationship with another
object, AGREEMENT, then its subset CAR also has the same relationship with the
same object AGREEMENT.
Figure 6-20 illustrates this principle of inheritance of relationships by the subsets
from the superset. Note that each of the subsets inherits the relationship with the
object AGREEMENT.
Special Cases
In real-world situations, you will run into a few special cases of specialization.
Usually, these special cases deal with the presence of particular instances in just
one subset or in more than one subset. Let us examine these special cases. We will
describe each special case with an example. Each example shows the links between
instances in the superset and instances in the subsets. The notations to represent the
special cases in a data model diagram vary. You need not be unnecessarily confused
with variation in the notations. Please pay attention to the concepts; concepts are
more important than notations.
Exclusive Subsets Figure 6-21 describes this special case. In this case, an
instance of the superset can be an instance of only one of the subsets. The subsets
are exclusive.
Nonexclusive Subsets Figure 6-22 shows nonexclusive subsets. In this case, an
instance of the superset can be an instance of more than one subset. The subsets
are nonexclusive.
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