OBJECT - BASED LOGICAL MODELS

   Object - based logical models are used in describing data at logical level and view level.Logical level and view level are used to retrieve the data.Logical level describes what data are stored in the database and what relationships exist among those data,Logical level is used by DBA,who must decide what information is to be kept in the database.View level describes only part of the entire database to be viewed by the user of the database hiding the details of the information stored.

OBJECT - BASED LOGICAL MODELS ARE DESCRIBED IN THE DIFFERENT FOLLOWING MODELS:

• The Entity - Relationship Model
• The Object - Based Logical Model
• The Semantic Data Model
• The Functional Data Model.

THE ENTITY - RELATIONSHIP MODEL

   An entity is a "thing" or "object" in the real world that is distinguishable from other objects.The Entity - Relationship Model (E - R Model) is based on a collection of basic objects,called entities,and the relationship among these objects.

* Rectangles represent entities

* Diamonds represent relationship among entities

* Ellipse represents attributes ( characteristics of entities)

* Lines represents link of attributes to entities to relationships

   Here student and college are two different entities "things" or "Objects" and there is only one relationship between these two entities - i.e.,details.It means student details are in ID.When student details are opend it specify the student - name,student - class,student - city Known as attributes:(Characteristics to be possessed by entities).Likewise,college shoul specify the attributes:college - ID and college - name.
   Therefore,we can conclude that the overall logical structure of a database can be expressed graphically by an E-R diagram.

THE OBJECT - ORIENTED MODEL

   Like the E-R model,the Object-oriented Model is based on a collection of objects.An object contains values stored in instance variables,within the object also contains bodies of code that operates on the object.These bodies of code are called as methods.

   Objects that contain the same types of values and the same methods are grouped together into classes.A class may be viewed as a definition for objects.This combination of data and methods comprising a definition is similar to a programming-language abstract data type.

   The only way in which one object can access the data of another object is by invoking a method of that other object.This action is called sending a message to the object.Thus,the call interface of the methods of an object defines that objects are externally visible.The internal part of the object-the instance variables and method code -are not visible externally.The result is two levels of abstraction.

THE SEMANTIC DATA MODEL

   A semantic Data Model is a more high-level data model that makes it easier for a user to give starting description of data in an enterprise.These models contain a wide variety of relations that helps to describe a real application scenario.A DBMS cannot support all these relations directly;so it is built only with few relations Known as relational model in DBMS.A widely used semantic data model is the Entity-Relationship (ER) data model which allows us to graphically denote entities and relationship between them.

THE FUNCTIONAL DATA MODEL

   The functional Data Model makes it easier to define functions and call them wherever necessary to process data.