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PREPARED BYPREPARED BY:-:-SUDIN SHAKYASUDIN SHAKYA

BSCITBSCIT

FIRST SEMESTERFIRST SEMESTER

SECTION ASECTION A

COLLEGE OF PROFESSIONALCOLLEGE OF PROFESSIONAL

STUDIESSTUDIES

UNDER SIKKIM MANIPALUNDER SIKKIM MANIPAL

UNIVERSITYUNIVERSITY

MAITIDEVI ,KATHMANDUMAITIDEVI ,KATHMANDU

THREE LEVEL SCHEMAS OF DATA BASETHREE LEVEL SCHEMAS OF DATA BASE

MANAGEMENT SYSTEMMANAGEMENT SYSTEM


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INTRODUCTION

The outline proposed for the buffer betweenapplication programs, end users and a database

to fulfill features of data independence is called three

level schemas. It was proposed in 1975 by theAmerican National Standards Institute StandardPlanning And Requirements Committee (ANSI-SPARC).


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OBJECTIVES

The objective of the three-level architecture is to separate theusers view(s) of the database from the way that it isphysically represented. This is desirable since:

It allows independent customised user views

It hides the physical storage details from usersThe database administrator should be able to change the

database storage structures without affecting the usersviews

The internal structure of the database should be unaffectedby changes to the physical aspects of the storage


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THREE-LEVEL ARCHITECTURE

EXTERNAL LEVEL

The way users

perceive the data.

CONCEPTUAL

LEVEL

INTERNAL LEVEL

DATABASE

Mapping and independence

between external and

internal levels.

The way the DBMS and

OS perceive the data.


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EXTERNAL LEVEL (highest level)

The users view of database

Consists of a no of different external views of DB.

Describes part of the DB for particular group of users

Provides a powerful and flexible security mechanism by

hiding parts of the DB from certain users. The user is not

aware of the existence of any attributes that are missing

from the view.

It permits users to access data in a way that is customized totheir needs, so that the same data can be seen by different

users in different ways, at the same time.


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CONCEPTUAL LEVEL (LOGICAL SCHEMA)

The logical structure of the entire database as seen byDBA.

What data is stored in the database.

The relationships among the data.Complete view of the data requirements of the

organization, independent of any storageconsideration.


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INTERNAL LEVEL (PHYSICAL SCHEMA)

Physical representation of the DB on the computer.

How the data is stored in the database.

Physical implementation of the DB to achieve optimal

run time performance and storage space utilization.- Storage space allocation for data and indexes

- Record description for storage

- Record placement

- Data compression, encryption


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Sno FName LName Age Salary Staff_No LName BNo

External View1 External View2

Staff_No FName LName DOB Salary Branch NoConceptual Level

Struct STAFF {

int Staff_No;

int Branch_No;

charFName(15);

charLName(15);

struct date Date_of_Birth;

float Salary;

struct STAFF * next;

};

Internal Level


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DATA INDEPENDENCE

The ability to modify a scheme definition in one level without affecting ascheme definition in a higher level is called data independence.

There are two kinds:

1 Logical data independence

The ability to modify the conceptual scheme withoutcausing application programs to be rewritten.

Immunity of external schemas to changes in the

conceptual schema.

Usually done when logical structure of database is

altered.


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2. PHYSICAL DATA INDEPENDENCE

The ability to modify the internal scheme without

having to change the conceptual or externalschemas.

Modifications at this level are usually to improveperformance.


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