Data Models

The way in which information is subdivided and managed within a database is referred to as the data model used by the DBMS. Each DBMS is based on a particular data model. A user must choose a DBMS which is suitable for the project.

Data models can be classified into three major groups. They are:

• Object-Based logical models.
• Record-Based logical models.
• Physical models.

A) Object Based logical Models:

 These models are used to describe data at the logical and view levels. The following are the well known models in this group.

• Entity Relationship Model.
• The Object-Oriented Model.
• The Semantic data model
• The functional data model.

Entity Relationship Model:

Entity:An Entity is an object or a thing such as person, place about which an organization keeps information. Any two objects or things are distinguishable.

Eg: Each student is an entity.

Attribute:The describing properties of an entity are called Attributes.

Eg: For a student entity, name, sex, date of birth are attributes.

Relationship:An association among entities is called a relationship.

The data model that consists of a set of entities and a set of relationships among those entities is called ER Model.

The set of all entities of the same type is called an entity set and the set of all relationship of the same type are called a relationship set.

The Object-Oriented Model:

The object oriented model is a data model based on a collection of objects.

Each object has a unique identity.The group of objects containing the same type of     values and the same methods are called classes.

The Semantic data model:

These models were based on semantic networks. Inter dependencies among the entities can be expressed in this data model.

Functional Data Model:

In this model objects, properties of objects, their relationships are viewed uniformly and are defined as functions.

B)Record Based Logical models:

This model is used to describe data at the logical and view levels. The database is structured in fixed format records of different types. Each record type has a fixed number of fields. And each field is of fixed length.The following are the three important record based logical models.

• Relational Model
• Network Model
• Hierarchical Model.

Relational Model:

A data model in which both data and their relationships are represented by means of tables is called Relational Model.

The relation is the only data structure used in this model to represent both entities and their interrelationships. A relation is a two dimensional table with a unique name.

Each row of a table is called a tuple and each column of a table is called an attribute. The set of all possible values in an attribute is called the domain of the attribute.

Network Model:

The network model uses two different structures. The data are represented by a collection of records and the relationships among data are represented by links.

Hierarchical Model:

In Hierarchical Model, data are represented by records and relationships among data are represented by links. But unlike in Network model, data are organized in an ordered tree structure, which is called   Hierarchical structure.

C) Physical Data Models:

These models are used to represent data at the lowest level.Two important physical Data Models are:

• Unifying Model
• Frame Memory Model.