Databases Reference
In-Depth Information
describes the target system and explains why it is a good candidate for an
ORDBMS.
ABC Corporation produces telephonic systems to the business com-
munity. All systems delivered to clients are complete and ready to run with-
out further modification from the user. Each system comprises a hardware
and a software component. The hardware component may contain an assort-
ment of parts. Some systems, depending on their complexity, include addi-
tional complex devices. A typical configuration has a Pentium-based server, a
device known as a multiplexer, and one or more networking devices. The
exact functionality of each piece is not germane to the understanding of this
example, only that certain parts are required. An additional fact to bear
in mind is that the combinations of hardware parts that will work together
properly are almost limitless.
The software component is provided as a single package, but it has
three integral parts. The first part is the operating system. The second is the
code or mechanisms that cause the telephonic system to function. The last
part is a series of drivers that facilitate the subtle differences between the
operating systems so the code mechanism can operate problem free.
The first DB requirement facing ABC Corporation is to create a DB
schema that will properly maintain all aspects of the telephony systems pro-
duced while enforcing the business rules of what parts will go with which.
Briefly, the general rules can be summarized as follows:
Each full system comprises a hardware and a software component.
·
Each hardware component has a list of required parts; the other
parts are optional, depending on the system to be delivered.
·
Each software component requires three parts, which must be cor-
rectly matched.
·
The illustration in Figure 6.1 shows the logical representation of this model-
ing problem.
This example introduces two crucial concepts to the DB design prob-
lem that are classic problem areas in the relational model: class hierar-
chies and inheritance/versioning (the concept of one or more distinct
instantiations from a single abstract class). The two criteria may sound simi-
lar, but there are aspects that make them unique.
Hierarchical structures in DB design have often been referred to as
Is-a or Is-a-type-of relationships. These relationships were so termed
because they provided a means to express different variants of the same
