Database and Expert System Technology - Information Systems Reengineering, Integration and Normalization - page 56

• Inheritance—Atriangularsymbolthatlinksclassesiscalledtheinheritancesym-
bol. The apex of the triangle is linked to the superclass with the subclasses being
linked to the base of the triangle. Figure 2.9 shows that class AE_Record is a sub-
class that inherits the attributes and methods of its superclass Medical_Record.
• Navigation—Givenanassociationbetweentwoclasses,itispossibletonavigate
from objects of one kind to another kind. Unless otherwise specified, navigation
across an association is bidirectional.

In UML, a class diagram is a collection of declarative model elements, such as
classes, their contents and relationships. It is a graphical view of the static structural
model. It shows the relationship between classes and the attributes and operations
of a class.
A class is a description of a set of objects that share the same data structure (at-
tributes), behaviors (operations), methods, and relationships. Objects are instances
of classes. An attribute is a class property that specifies the data structure that in-
stances of the class may contain. An operation is a class interface that is invoked on
an object to manipulate attributes. A method is the implementation of an operation.
It specifies a program or procedure associated with an operation. Relationship is a
connection among model elements.
Association and generalization are useful relationships specified in UML. As-
sociation is the semantic relationship between two or more classes that specifies
connections among their instances. It consists of at least two ends, each specify-
ing a connected class and a set of properties such as navigability and multiplicity
that must be fulfilled for the relationship to be valid. Association class is a model
element that has both association and class properties. It allows the additions of at-
tributes and operations to an association.
In summary, there are basically five major data semantics in UML class diagram
as shown in Fig. 2.9 , in the following semantics:

a. One-to-many association between classes Patient and class Record folder
b. Subclass AE record, subclass Ward record, and subclass Outpatient record is a
subset of superclass Medical record.

2.4.2 Object-Oriented DDL

There are many commercial object-object databases in the industry. In this topic,
we choose UniSQL (UniSQL 1992 ) as a representative for illustration purposes.
To implement the abstract data type, we must first define each class. A class is a
collection of many objects with similar properties. Each object is an instance of a