Data models in DBMS and differences between Relational, Network and Hierarchical models

 Various Data Models in DBMS:

 

 Data Model:

A data model is a collection of conceptual tools for describing data - its relationships, semantics and consistency constraints. Data models can be distinguished from one another on the basis of the relationship among data, that is defined and the way the data is conceptually defined.

A primary objective that a data model serves is to evolve a high level data model. It should assist the designer to incorporate a major portion of semantics of database in the schema.

In DBMS, the data models are broadly classified as:

1.    Object based data models

2.    Record based data models and;

3.    Physical data models

 

1.    Object Based Data Models:

These models are used in describing data at a logical and view level. They have properties like they provide flexible structure in the capabilities and allow us to specify data constraints explicitly. This model highlights that everything is object having set of attributes.

 

Examples of data models that fall in this category are:

a)    Entity relationship model

b)    Object oriented model

 

2.    Record Based Data Models:

The record based data models are used to describe data at a logical and view level. However, they revolve around the records of database and specify the overall structure of database with the help of value of records. This is a key distinguishing factor between the record based and object based model.

 

These models have got their name record based models because the database is structured in fixed format records of several types. Each record type defines a fixed number of fields or attributes and each field is usually of a fixed length. This results in simple physical level implementations of the database.

Examples of data models that fall in this category are:

a)    Relational model

b)    Network model

c)    Hierarchical model

 

3.    Physical Data Models:

 

This model is used to describe data at the lowest level. It actually describes the behaviour of data at disk level. Furthermore, it also describes the way data and data relationships that are maintained while storing them on the disk. As such this leads to the way a DBMS is going to use secondary storage devices for storing and accessing data.

 

Examples of data models that fall in this category are:

a)    Unifying model

b)    Frame memory model

 

A.  Network Model:

 

The data in network model are represented by collection of records. The relationships among data are represented by links, which can be viewed as pointers.

Simply speaking, a pointer is a physical address which identifies where the next record can be found on the disk or we can say that it points to the next address location.

 

The network model allows each record to have multiple parent and child records forming a lattice structure. Thus, network model supports many to many relationships. The relationship between different records is called sets. This model is used as an easy way of representing objects and their relationships. Following figure shows an example for network database:

B.  Hierarchical Model:

 

This model is very similar to network model, as in both the models, the data and relationships among data are represented by using records and links respectively. In this model, records are organized as collection of trees rather than arbitrary graphs.

 

The data storage is in the form of parent child relationship. An origin of the tree is called the root.  Data that follows the root is called a node and the last node in the series is called as a leaf node. Usually, there is one to many relationship found in this model. Following figure shows an example for hierarchical database:

Differences between Relational, Network and Hierarchical Models:

Parameter	Relational model	Network model	Hierarchical model
Relationship	It supports one to many and many to many relationships.	It supports many to many relationships.	It supports one to many relationships.
Nature	It is a table based model, where a table is a collection of rows and columns.	It is based on records and links.	It is based on tree like structure having one root, several nodes and leaf nodes.
Popularity	This model is more popular.	This model is less popular.	This model is less popular.
Applications	It has many applications, almost unlimited.	It is an upgraded version of hierarchical model, so used in the network.	It is mainly used in mainframe and database system.
Data relation	This model uses values to relate data. It does not use pointers or links.	This model uses links to relate data.	This model uses pointers to relate data.
Storage	The data is stored in the form of tables.	The data is stored in the form of arbitrary graph.	The data is stored in the form of a tree having parent - child relationship.