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Chapter 2
Database Environment
Data Independence
Sometimes the way data are physically organized
depends on the requirements of the application .
Result:
 it is impossible to change the storage structure without
affecting the application
 this is typically a problem introduced by the file handling
software , and not by the problem to solve
Data independence is needed because :
 different applications need different views on the same data
 the database administrator must have the freedom to change
the storage structure, file organization and access strategy
Definition of Data Independence
Data independence is the independence of
data and applications.
data structures can be modified without
affecting applications
applications can be modified without
affecting other applications
A possibility to achieve this:
Three level database architecture
Three level database architecture
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Conceptual level: Reflects the community user view
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External level:
Reflects the user view
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Internal level:
Close to the physical storage
It is a framework describing general concepts
not all database systems follow this architecture
ANSI-SPARC and CODASYL
Three-level Architecture
ANSI-SPARC three-level Architecture
Reasons for this architecture
user should have access to the same data , but
have a customized view of the data
users should not have to deal directly with physical
database storage details
the DBA should be able to change the database
storage structure without affecting the users’ view
DBA should be able to change the conceptual or
global structure of the database without affecting all
users
External Level
The user’s view of the database.
This level describes that part of the database that is
relevant to a particular user or group of users
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number of different external views
familiar to or efficient for the user
part of the database can be hidden for certain users
different presentations of the same data (e.g. dates)
can include derived or calculated data, not stored
in the database
Conceptual Level
The community view of the database.
This level describes what data is stored in the
database and the relationships among the data.
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Logical structure of the entire database as seen by
the DBA, independent of any storage considerations
 all entities, attributes and relationships
 constraints on the data
 semantic information about the data
 security and integrity information
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Supports all external views
Internal Level
The physical representation of the database on
the computer. This level describes how the data is
stored in the database.
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to achieve optimal run-time performance and
storage space utilization
data structures and storage devices
file organization, indexes, …
general aspects
 storage space allocation for data and indexes
 record description for storage
 record placement
 data compression and data encryption techniques
Differences between Three Levels of
ANSI-SPARC Architecture
Data Independence
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Logical Data Independence.
Refers to immunity of external schemas to changes
in conceptual schema.
Conceptual schema changes e.g. addition/removal
of entities.
Should not require changes to external schema or
rewrites of application programs.
Data Independence
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Physical Data Independence
Refers to immunity of conceptual schema to
changes in the internal schema.
Internal schema changes e.g. using different file
organizations, storage structures/devices.
Should not require change to conceptual or external
schemas.
Data Independence and the ANSI-SPARC
Three-level Architecture
Database Languages
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Data Definition Language (DDL)
Allows DBA or user to describe and name entitles,
attributes and relationships required for the
application.
Database Languages
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Data Manipulation Language (DML)
Provides basic data manipulation operations on
data held in the database.
Procedural DML - allows user to tell system exactly
how to manipulate data.
Non-Procedural DML - allows user to state what data
is needed rather than how it is to be retrieved.
Database Languages
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Fourth Generation Language (4GL)
Query Languages
Forms Generators
Report Generators
Graphics Generators
Application Generators
Host Language
Compiled
DML
module
Program
Pre-compiler
DML-statements
Object-code
Program
Procedural
Language
e.g. Pascal
including
DML-statements
Call-statements
3GL
Compiler
Call
Data Model
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Collection of concepts for describing data,
relationships between data and constraints on
the data in an organization.
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Data Model comprises:
A structural part.
A manipulative part.
Possibly a set of integrity rules.
Conceptual modeling
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The process of developing a conceptual data
model that is
a complete and accurate representation of an
organization's data requirements.
independent of implementation details.
Functions of a DBMS
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Data Storage, Retrieval and Update.
Must furnish users with the ability to store, retrieve,
and update data in the database.
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A User-Accessible Catalog.
Must furnish a catalog in which descriptions of data
items are stored and which is accessible to users.
Functions of a DBMS
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Transaction Support
Must furnish a mechanism to ensure that either all
the updates corresponding to a given transaction
are made or that none of them are made.
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Concurrency Control Services
Must furnish a mechanism to ensure that database
is updated correctly when multiple users are
updating the database concurrently.
Functions of a DBMS
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Recovery Services
Must furnish a mechanism for recovering the
database in the event that the database is damaged
in any way.
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Authorization Services
Must furnish a mechanism to ensure that only
authorized users can access the database.
Functions of a DBMS
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Support for Data Communication
Must be capable of integrating with communication
software.
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Integrity Services
Must furnish a means to ensure that both the data in
the database and changes to the data follow certain
rules.
Functions of a DBMS
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Services to Promote Data Independence
Must include facilities to support the independence
of programs from the actual structure of the
database.
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Utility Services
Should provide a set of utility services.
Components of a DBMS
Components of a DBMS
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Query processor
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Database manager (DM)
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File manager
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DML preprocessor
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DDL compiler
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Catalog manager
Components of Database
Manager (DM)
Components of Database
Manager (DM)
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Authorization control
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Command processor
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Integrity checker
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Query optimizer
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Transaction manager
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Scheduler
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Recovery manager
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Buffer manager
The Catalog
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Catalog, meta-data, data dictionary, repository
 names, types and sizes of data items
 names of relationships
 integrity constraints on the data
 authorizations
 usage statistics
 schema mappings
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Benefits
 centrally stored meta-data
 simpler communication
 identification of redundancy and inconsistency
 changes to the database can be recorded and followed-up
 security can be enforced
 integrity can be ensured
 audit information can be provided