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| DATABASE SYSTEMS |
1. Introduction to DBMS
- The evolution of database technology
- Characteristics of the database approach
- Components of a DBMS
- Advantages of using the DBMS approach - Explain the basic concepts of DBMS
Introduction
The database is now such an integral part of our day-to-day life that often we are not aware we are using one. We consider a database to be a collection of related data and the Database Management System (DBMS) to be the software that manages and controls access to the database. A database application is simply a program that interacts with the database at some point in its execution.
Example of
Database using day -
to
day life
· Purchases from the supermarket
· Purchases using your credit card
· Booking a holiday at the travel agents
· Booking a holiday at the travel agents
· Using the local library
· Using the Internet
Traditional File-Based Systems
A collection of application programs that perform services
for the end-users such as
the production of reports. Each
program
defines and manages its own data.
File-based systems were an early attempt to
computerize the manual
filing system
that we are all familiar with. For example, in an organization a manual file is set up to hold all external
and
internal
correspondence relating to a project,
product,
task, client, or
employee. Typically, there are many such
files, and for safety
they are labeled and
stored in one or more cabinets.
For security, the cabinets may have locks or may
be located in secure areas of the building. In our
own home, we
probably have some sort
of filing system which contains receipts,
guarantees, invoices, bank statements, and
The manual filing system works well while the number of items
to
be stored is small. It even works quite adequately when there are
large numbers of items
and
we have only to
store and retrieve them. However, the manual filing system breaks down when we have to cross-reference or process the information in the files.
Problem of the File-Based Approach
· Separation and isolation of
data
· Duplication of
data
· Data dependence
· Incompatible file formats
Separation and isolation of data
When data
is isolated in separate files, it is more difficult to access data that should be
available.
Duplication of data
Owing to the decentralized approach taken
by each department, the file-based approach encouraged, if not necessitated,
the
uncontrolled duplication of data.
Why we don't need duplication?
· Duplication is wasteful. It costs time
and money to enter the
data more
than once.
· It takes up additional
storage space
· Perhaps more importantly, duplication
can
lead to loss of data integrity; in other
words, the data is no longer consistent.
Data dependence
As we have already mentioned, the physical
structure
and storage of the data files and records
are
defined in the application code. This means
that changes to an existing
structure are
difficult to make.
Incompatible file formats
Because the structure of
files is embedded in
the application programs, the structures are dependent
on
the application programming
language.
The Database
A shared collection of logically related data, and a description of this data, designed to
meet the information needs
of an organization.
The database
is
a
single,
possibly large
repository of data that can be used simultaneously by many users.
The database holds not only the
organiza- tion’s
operational
data but also a description of this data. For this reason, a database is also defined as a self-describing collection of
integrated records. The description of the data is known as the system catalog or data
dictionary or metadata (the ‘data about
data’). It is the self-describing nature of a database that provides program–data independence.
Data abstraction, separates
the structure of
the
data from the application programs and
stores it in the database.
If new data structures are added or existing structures are modified then the application programs are unaffected,
provided they do
not
directly
depend upon what has been modified. For example, if we add a new field
to a record or create
a new file, existing
applications are unaffected. However, if we
remove a field from a file that an application
program
uses, then
that application program is affected by this change and must be
modified accordingly.
The Database Management System (DBMS) A software system that enables users
to define, create, maintain, and control access
to
the database.
The DBMS is the software that interacts with
the users’ application
programs and the
database. Typically, a DBMS provides
the
following facilities:
It allows users to define the database, usually through a Data Definition Language
(DDL). The
DDL allows users to specify the data types
and structures and the con- straints on the
data to be stored in the database.
It allows users to insert, update, delete,
and
retrieve
data from the database, usually through a Data Manipulation Language
(DML). Having a central
repository for
all data and data descriptions allows the DML to provide a general
inquiry facility to this data,
called a query language.
The provision of a query
language alleviates
the prob-lems with file-based systems where the user has
to
work with a fixed set of queries or there is
a proliferation of programs, giving major
software management problems.
The most
Database Designers
In large database design projects, we can
distinguish between
two types of designer: logical database
designers
and
physical database designers.
The logical database designer
is concerned
with identifying the data (that is, the entities
and attributes), the
relationships between
the
data, and
the constraints on
the data
that is to be stored in the database. The logical database designer must have
a thorough and complete understanding of the organization’s data and
any constraints
on this data.
The physical database designer decides
how the logical database design is to be
physic-
ally realized.
Application Developers
Once the database
has
been implemented, the application programs that provide
the
required functionality for
the
end-users must be implemented.
This is the responsibility of the application developers.
End-Users
The end-users
are
the ‘clients’ for the
database, which has been designed and implemented, and is being maintained to
serve their information needs.
Advantages of DBMS
· Control of data redundancy
· Data consistency
· Sharing of data
· Improved data
integrity
· Improved security
Control of data redundancy
Redundancy mean
storing the
same data multiple times. As we discussed in early, traditional file-based systems waste
space by
storing
the same information in more than
one
file. The database approach attempts
to eliminate the redundancy by integrating the files so
that multiple
copies of the same
data are not stored. However,
the
database
approach does not eliminate redundancy
entirely, but controls
the
amount of
redundancy
inherent in the database.
Sometimes, it
is necessary to duplicate key data items to model relationships. At other times, it is desirable to duplicate some
data items to improve performance.
Data consistency
By eliminating or controlling redundancy, we
reduce the risk of inconsistencies occurring.
If a
data item is stored only once in the
database, any update to its value has to be performed only once and the new
value is
available immediately to all users. If a data item is stored more
than
once
and
the
system is aware of
this, the system can
ensure that all copies of the item
are
kept consistent. Unfortunately,
many
of today’s
DBMSs do not automatically ensure this type
of consistency.
Sharing of data
More users share more of the data.
Improved data integrity
Database integrity refers to the validity and
consistency of stored
data.
Integrity is
usually expressed in terms
of
constraints,
which are
consistency
rules that
the database is not permitted to violate. Constraints may apply to data items
within a
single record or they may apply to relationships between records.
Improved security
Database security
is the protection of the database from unauthorized users. Without suitable security
measures, integration
makes
the data more vulnerable than file-
based systems.
Characteristics of
the Database
Approach
· Self-describing nature of a database
system
· Insulation between programs
and
data, and data abstraction
· Support of multiple views of the data
· Sharing of data and multiuser
transaction processing
Self-Describing Nature of a
Database
System
A fundamental characteristic of the database
approach is that the database system
contains not only the database itself but also
a complete
definition or description
of
the database structure and
constraints.
This definition is stored in the DBMS catalog,
which contains information such as
the structure of each file, the type and storage
format of each data item, and various
constraints on
the
data. The
information
stored in the catalog is called meta-data, and it describes the structure
of the
primary database
Insulation between Programs and Data
In traditional file
processing, the structure of data files is embedded
in the application
programs, so any changes to the structure
of a
file may require changing all programs that
access that file. By contrast,
DBMS access programs do not require such changes in most cases. The structure of data files is stored in the DBMS
catalog
separately from the access programs.Support of Multiple Views of the Data
A database typically has many users, each of whom may require a different perspective or view of the database. A view may be a subset of the database or it may contain virtual data that is derived from the database files but is not explicitly stored.
Sharing of Data
A multi user DBMS, as its name implies,must allow multiple users to access the database at the same time. This is essential if data for multiple applications is to be integrated and maintained in a single database.
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BY: C.SIVAKARAN
JAFFNA COLLEGE OF TECHNOLOGY
Sri Lanka
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