A model is a representation of reality, ‘real world’ objects and events, and their associations. It is an abstraction that concentrates on the essential, inherent aspects of an organization and ignore the accidental properties. A data model represents the organization itself. Let should provide the basic concepts and notations that will allow database designers and end users unambiguously and accurately to communicate their understanding of the organizational data.
Data Model can be defined as an integrated collection of concepts for describing and manipulating data, relationships between data, and constraints on the data in an organization.
A data model comprises of three components:
• A structural part, consisting of a set of rules according to which databases can be constructed.
• A manipulative part. Defining the types of operation that are allowed on the data (this includes the operations that are used or updating or retrieving data from the database and for changing the structure of the database).
• Possibly a set of integrity rules, which ensures that the data is accurate.
The purpose of a data model is to represent data and to make the data understandable.
There have been many data models proposed in the literature. They fall into three broad categories:
• Object Based Data Models
• Physical Data Models
• Record Based Data Models
The object based and record based data models are used to describe data at the conceptual and external levels, the physical data model is used to describe data at the internal level.
A DBMS is a software package that carries out many different tasks including the provision of facilities to enable the user to access and modify information in the database. The database is an intermediate link between the physical database, computer and the operating system and the users. To provide the various facilities to different types of users, a DBMS normally provides one or more specialized programming languages called database languages.
Database languages come in different forms. They are: -
1. Data Description Language (DDL)
2. Data Manipulation Language (DML)
Data Description Language (DDL)
As the name suggests, this language is used to define the various types of data in the database and their relationship with each other.
The basic functions performed by DDL are: -
• Create tables, files, databases and data dictionaries.
• Specify the storage structure of each table on disk.
• Integrity constraints on various tables.
• Security and authorization information of each table.
• Specify the structure of each table.
• Overall design of the Database.
Data Manipulation Language (DML)
A language that enables users to access or manipulate data (retrieve, insert, update, delete) as organized by a certain Data Model is called the Data Manipulation Language (DML). It can be of two types: -
1. Procedural DML - It describes what data is needed and how to get it. For example: - Relational Algebra.
2. Non Procedural DML - It describes what data is needed without specifying how to get it. For example: - Relational calculus.