Database Reference
In-Depth Information
The Object-Relationship Model
As you are aware, or will soon learn (from your software engineering course), there are,
broadly speaking, two alternate paradigms for software construction: the functional
approach (which is the traditional approach) and the object-oriented (OO) approach. In
an object-oriented environment, the comparative methodology for the E-R diagram is the
object-relationship diagram (ORD or O-R diagram). The concept of an ORD is similar to
that of an ERD, and the diagrams are also similar, but there are a few exceptions:
In the OO paradigm, the
object type replaces the entity (type) of
the relational model. Like the entity, an object type is a concept
or thing about which data is stored. Additionally, the object type
defines a set of operations, which will be applicable to all objects
(instances) of that type.
The symbol used to denote an object type is similar to an
entity symbol, except that it has two extended areas — one for
the attributes of the object type, and the other for its defined
operations.
The preferred diagramming convention is the UML
(Unified Modeling Language) notation.
Depending on the OO development tool, there might be
additional notations regarding the cardinality (more precisely,
multiplicity) of the relationships represented.
A full treatment of the OO approach is beyond the scope of this course. You are
no doubt familiar with using UML diagrams in your OO programming courses. For a
quick review of the fundamentals, please see references [Lee, 2002] and [Martin, 1993].
However, in the interest of comprehensive coverage, an overview of the approach is
provided in chapters 5 and 23.
Database Tree
A database tree (hierarchy) is a traditional alternative, which used to be employed prior to
the introduction or the E-R model; it was successfully employed in a system called RAMIS
(the original acronym stands for “Random Access Management Information System”).
A database tree (hierarchy) is a collection of entities and 1:M relationships arranged such
that the following conditions hold:
The root has no parent
Each subsequent node has a single parent
Figure 3-6 illustrates a database hierarchy. Observe that it looks like a general tree
(review your data structures). Except for the root (node A), each node has a parent node
that it references. Note also that all the relationships are 1:M relationships (traditionally
referred to as parent-child relationships ).
 
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