UNIT-1

Introduction: An overview of the database

management system
Chapter 1: Introduction

• Introduction:An overview of the database management system

• Purpose of Database Systems
• Database system Vs file system
• Database system concept and architecture
Database Management System (DBMS)
• Collection of interrelated data
• Set of programs to access the data
• DBMS contains information about a particular enterprise
• DBMS provides an environment that is both convenient and efficient to use.
• Database Applications:
• Banking: all transactions
• Airlines: reservations, schedules
• Universities: registration, grades
• Sales: customers, products, purchases
• Manufacturing: production, inventory, orders, supply chain
• Human resources: employee records, salaries, tax deductions
• Databases touch all aspects of our lives
Purpose of Database System
• In the early days, database applications were built on top of file
systems
• Drawbacks of using file systems to store data:
• Data redundancy and inconsistency
• Multiple file formats, duplication of information in different files
• Difficulty in accessing data
• Need to write a new program to carry out each new task
• Data isolation — multiple files and formats
• Integrity problems
• Integrity constraints (e.g. account balance > 0) become part of program code
• Hard to add new constraints or change existing ones
Purpose of Database Systems (Cont.)
• Drawbacks of using file systems (cont.)
• Atomicity of updates
• Failures may leave database in an inconsistent state with partial updates carried
out
• E.g. transfer of funds from one account to another should either complete or not
happen at all
• Concurrent access by multiple users
• Concurrent accessed needed for performance
• Uncontrolled concurrent accesses can lead to inconsistencies
• E.g. two people reading a balance and updating it at the same time
• Security problems
• Database systems offer solutions to all the above problems
 Database system Vs file system

File System Database System

• A file system is a software that manages and organizes the • DBMS or Database Management System is a software
files in a storage medium. It controls how data is stored and application. It is used for accessing, creating, and managing
retrieved. databases.
• The file system provides the details of data representation • DBMS gives an abstract view of data that hides the details
and storage of data. • DBMS is efficient to use as there are a wide variety of
• Storing and retrieving of data can’t be done efficiently in a methods to store and retrieve data.
file system. • There is a backup recovery for data in DBMS.
• It does not offer data recovery processes. • DBMS provides a crash recovery mechanism
• The file system doesn’t have a crash recovery mechanism. • DBMS offers good protection mechanism.
• Protecting a file system is very difficult. • The redundancy of data is low in the DBMS system.
• In a file management system, the redundancy of data is • Data inconsistency is low in a database management system.
greater.
• Database Management System stores data as well as defined
• The file system offers lesser security. constraints and interrelation.
• File System allows you to stores the data as isolated data • DBMS system provides backup and recovery of data even if it
files and entities. is lost.
• It doesn’t offer backup and recovery of data if it is lost. • You can easily query data in a database using the SQL
• There is no efficient query processing in the file system. language.
 Disadvantages of Traditional File System
• Data Redundancy
• Data Inconsistency
• Lack of Data Integration
• Program Dependence
• Data Dependence
• Limited Data Sharing
• Poor Data Control
• Problem of Security
• Data Manipulation Capability is Inadequate
• Needs Excessive Programming
Database system concept and architecture

• Levels of Abstraction
• View of Data
• Instances and Schemas
• Data Models
• Data Independence
Levels of Abstraction
• Physical level describes how a record (e.g., customer) is stored.
• Logical level: describes data stored in database, and the relationships
among the data.
type customer = record
name : string;
street : string;
city : integer;
end;
• View level: application programs hide details of data types. Views can
also hide information (e.g., salary) for security purposes.
View of Data
An architecture for a database system
Instances and Schemas
• Similar to types and variables in programming languages
• Schema – the logical structure of the database
• e.g., the database consists of information about a set of customers and accounts and the
relationship between them)
• Analogous to type information of a variable in a program
• Physical schema: database design at the physical level
• Logical schema: database design at the logical level
• Instance – the actual content of the database at a particular point in time
• Analogous to the value of a variable
• Physical Data Independence – the ability to modify the physical schema
without changing the logical schema
• Applications depend on the logical schema
• In general, the interfaces between the various levels and components should be well
defined so that changes in some parts do not seriously influence others.
Data Models
• A collection of tools for describing
• data
• data relationships
• data semantics
• data constraints
• Entity-Relationship model
• Relational model
• Other models:
• object-oriented model
• semi-structured data models
• Older models: network model and hierarchical model
Entity-Relationship Model
Example of schema in the entity-relationship model
Entity Relationship Model (Cont.)
• E-R model of real world
• Entities (objects)
• E.g. customers, accounts, bank branch
• Relationships between entities
• E.g. Account A-101 is held by customer Johnson
• Relationship set depositor associates customers with accounts
• Widely used for database design
• Database design in E-R model usually converted to design in the relational
model (coming up next) which is used for storage and processing
Relational Model
• Example of tabular data in the relational model
Attributes

Customer-id Customer-name Customer-street Customer-city Account-number

192-83-7465 Johnson Alma Palo Alto A-101

019-28-3746 Smith North Rye A-215

192-83-7465 Johnson Alma Palo Alto A-201

321-12-3123 Jones Main Harrison A-217

019-28-3746 Smith North Rye A-201

A Sample Relational Database
 The Three-Schema Architecture
• Internal Schema
• Describes the physical storage structure
• Uses a physical data model
• Conceptual Schema
• Describes the structure of the whole database
• Uses a conceptual or an implementation data model
• External Schema
• Includes a number user views
• Uses a conceptual or an implementation data model
• Mappings
• The process of transforming requests and results between levels
The Three-schema architecture
Data Independence
The ability to change the schema at one level of the database system
without having to change the schema at the next higher level.
Logical Data Independence:
The capacity to change the conceptual schema without having to change the
external schemas and their associated application programs.
Physical Data Independence:
The capacity to change the internal schema without having to change the
conceptual schema.
• Advantages and disadvantages
• The three -Schema Architecture can make it easier to achieve true data independence
• Mappings create an overhead during compilation or execution of a query or a
program
• Not implemented fully by DBMSs
Thank you