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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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