Information Technology Reference
In-Depth Information
HKID
Patient_name
n
1
Insurance
Patient
Insuredby
1
belong
n
1
n
Medical
Record
Record
folder
Medical_Rec_No
Create_date
Sub_type
Contain
Folder_no
location
G
AE
Record
Ward
Record
Outpatient
Record
Medical_Rec_No
Ward_no
Admission_date
Discharge_date
Medical_Rec_No
Outpatient_No
Specialty
Medical_Rec_No
AE_No
Fig. 2.6
An extended entity relationship (EER) model for hospital patient record
2.4
Object-Oriented Model
To date, numerous object-oriented data models have been proposed. In an object-
oriented data model (Hughes
1991
), the world is viewed as a set of objects that com-
municate with each other by exchanging messages, and can be described as follows.
Object-oriented model is a logical schema in the form of objects with name,
properties, and behavior. An object represents “a thing” that is important to users
in the portion of reality that the users want to model. Each object belongs to a fam-
ily, or class, of similar objects. A class is a template (containing code and data) that
describes the common characteristics of a set of objects. Object-oriented database
systems simplify programming database updates and provide faster access to stored
data by blurring the distinction between programming language and database.
Objects that have similar properties are grouped together into a class. Each class
has an ID that is called the object identifier (i.e., OID). The OIDs are unique. The
IDs need not necessarily be the same as the primary key values used to identify the
tuples in the relational model.
A class has properties that describe the objects of the class. These properties are
called instance variables (i.e., attributes). A link is a physical or conceptual connec-
tion between object instances (i.e., classes). A link is an instance of an association.
Classes have methods associated with them. Methods are procedures that are
executed when they are invoked. Methods are invoked when appropriate messages
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