Databases Reference
In-Depth Information
the Pay_Method and Payment objects. the Pay_Method object is the super-
class for the Check, Cash, and Credit_Card subclasses.
COMPARISON OF THE MODELING METHODS
The three modeling techniques are somewhat similar-they all use the
same basic elements: objects/entities, attributes, and relationships. When
modeling the accounting system used as an example in this article, the
three techniques produce essentially the same diagram; the only major
difference is found in the notation. It is not true that this is the case for all
applications, because there probably are certain scenarios where the
models would look different.
The following sections compare the basic elements of these three mod-
eling methodologies.
Entities and Objects
The most basic element of all three models is the object or entity. All
three define this element in generally the same terms.
To help analysts determine valid objects, the Information Model meth-
odology lists five different categories of objects: tangible items, roles, inci-
dents, interactions, and specifications. This methodology also provides
tests to determine the validity of an object. These tests include a unifor-
mity test, more-than-a-name test, an or test, and a more-than-a-list test. All
of these tests review the object definition or attributes to check the cor-
rectness and uniformity of an object.
OMT also provides guidelines for removing incorrect object classes.
Invalid classes include those that are redundant, irrelevant, vague,
attributes, operations, roles, and implementation constructs. In the Entity
Relationship model a rule governing entities states that a valid entity must
have at least one attribute and one instance.
An entity in the ER model and an object in the Information Model are
very similar. An object in Object Modeling Technique has a major differ-
ence. An OMT object has operations defined for it. Defining operations for
an object in a system can give an analyst greater insight into the use of the
object in the system. The other models are strictly concerned with the data
associated with the object or entity.
One special type of entity in the ER model is the weak entity type. This
type is different from a normal entity in that it does not have an independent
existence. The weak entity cannot exist without its parent entity. For exam-
ple, an Employee_Dependent entity relies on an Employee entity for part of
its unique identity. Although a unique attribute could be created for a weak
entity, it should not be because of the strong bond between the entities. In
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