Database Modeling and Design - Database Systems - page 91

3. All properties of a given super-type apply to the

corresponding subtype(s), but the converse does not hold.

4. A subtype of a kernel is still a kernel; a subtype of a

characteristic is still a characteristic; a subtype of an
association is still an association.

Exercise

5.5 Database Design via the XR Model
The following approach, developed by Date and outlined in [Date, 1990], employs the
basic XR method,but relaxes the requirement that surrogates, E-relations and P-relations
are mandatory. The (partial)database model represented in Figure 5-2 will be used for the
purpose of illustration.
Before proceeding, we now introduce a notation that has become necessary: The
notation R.A will be usedto mean attribute A in relation (or entity) R. For instance,
Department.Dept# denotes attribute Dept# in the relation Department . The database
design approach involves seven steps as summarized in Figure 5-7 and clarified in the
upcoming subsections.

Figure 5-7. Database Design Procedure Using the XR Model

5.5.1 Determining the Kernel Entities
The first step involves determining the kernel entities. As mentioned, earlier, kernels are
the core relations. In the example, the kernels are Department , Employee , Supplier ,
Project , and InventoryItem . Each kernel translates to a base relation. The primary key of
each could be the user controlled ones, or surrogates may be introduced.