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30 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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1

2

Historical Development of Database Technology


1
-

Early

Database

Applications
:

The

Hierarchical

and

Network

Models

were

introduced

in

mid

1960
’s

and

dominated

during

the

seventies
.

A

bulk

of

the

worldwide

database

processing

still

occurs

using

these

models
.

2
-

Relational

Model

based

Systems
:

The

model

that

was

originally

introduced

in

1970

by

Edgar

Codd
,

was

heavily

researched

and

experimented

with

in

IBM

and

the

universities
.


3
-
Object
-
Oriented

Applications
:

OODBMS

were

introduced

in

late

1980
’s

and

early

1990
’s

to

cater

to

the

need

of

complex

data

processing

in

CAD

(Computer

Aid

Design)

and

other

applications
.

Their

use

has

not

taken

off

much
.

4
-

Data

on

the

Web
:

Web

contains

data

in

HTML

with

links

among

pages
.

This

has

given

rise

to

a

new

set

of

applications

and

E
-
commerce

is

using

new

standards

like

XML

(
eXtended

Markup

Language)
.

Basic
Definitions



Database
:

A

collection

of

related

data
.


Data
:

Known

facts

that

can

be

recorded

and

have

an

implicit

meaning
.


Mini
-
world
:

Some

part

of

the

real

world

about

which

data

is

stored

in

a

database
.

For

example,

student

grades

and

transcripts

at

a

university
.


Database

Management

System

(DBMS)
:

A

software

package/

system

to

facilitate

the

creation

and

maintenance

of

a

computerized

database
.


Database

System
:

The

DBMS

software

together

with

the

data

itself
.

Sometimes,

the

applications

are

also

included
.

3

4

A Database has the Following Implicit Properties:


1)
A

database

represents

some

aspect

of

the

real

world,

sometimes

called

the

mini
-
world
.

Changes

to

the

mini
-
world

are

reflected

in

the

database
.

2)
A

database

is

a

logically

coherent

collection

of

data

with

some

inherent

meaning
.

A

random

assortment

of

data

cannot

correctly

be

referred

to

as

a

database
.

3)
A

database

is

designed,

built,

and

populated

with

data

for

a

specific

purpose
.

It

has

an

intended

group

of

users

and

some

preconceived

applications

in

which

these

users

are

interested
.


5

Types of Database System


1
-

Traditional

Database

Applications
,

in

which

most

of

the

information

that

is

stored

and

accessed

is

either

textual

or

numeric
.


2
-

Multimedia

Databases

can

now

store

pictures,

video

clips,

and

sound

messages
.


3
-

Geographic

information

systems

(GIS)

can

store

and

analyze

maps,

weather

data,

and

satellite

images
.


4
-
Data

Warehouses

and

Online

Analytical

Processing

(OLAP)

Systems

are

used

in

many

companies

to

extract

and

analyze

useful

information

from

very

large

databases

for

Decision

Making
.


5
-

Real
-
time

and

Active

Database

technology

is

used

in

controlling

industrial

and

manufacturing

processes
.


6
-

Database

Search

Techniques

are

being

applied

to

the

World

Wide

Web

to

improve

the

search

for

information

that

is

needed

by

users

browsing

the

Internet
.

6

A Simplified Database System Environment

Stored Database
Definition
(Meta
-
Data)

Software to Process

Queries/Programs

Application Programs / Queries

Users/Programmers

DBMS

Software

Software to Access

Stored Data

Stored Database


Typical
DBMS
Functionality


1)
Define

a

Database

in

terms

of

data

types,

structures


and

constraints
.

2)
Construct

or

Load

the

Database

on

a

secondary

storage

medium
.

3)
Manipulating

the

database

querying,

generating

reports,

insertions,

deletions

and

modifications

to

its

content
.

4)
Concurrent

Processing

and

Sharing

by

a

set

of

users

and

programs


yet,

keeping

all

data

valid

and

consistent
.

5)
Protection

or

Security

measures

to

prevent

unauthorized

access
.

6)
Active

Processing

to

take

internal

actions

on

data
.

7)
Presentation

and

Visualization

of

data
.


7

8

Main Characteristics of the Database Approach


1
-
Self
-
describing nature of a database system:

A

DBMS

catalog

stores

the

description

of

the

database
:

The

description

is

called

meta
-
data)
.

This

allows

the

DBMS

software

to

work

with

different

databases
.

2
-

Insulation

between

programs

and

data
:

Called

program
-
data

independence
.

Allows

changing

data

storage

structures

and

operations

without

having

to

change

the

DBMS

access

programs
.

3
-

Data

Abstraction
:

A

data

model

is

used

to

hide

storage

details

and

present

the

users

with

a

conceptual

view

of

the

database
.

4
-

Support of multiple views of the data:

Each user may see a different view of the database, which describes
only the data of interest to that user.


9

5
-

Sharing

of

data

and

multi
-
user

transaction

processing

:

Allowing

a

set

of

concurrent

users

to

retrieve

and

to

update

the

database
.

Concurrency

control

within

the

DBMS

guarantees

that

each

transaction

is

correctly

executed

or

completely

aborted
.

OLTP

(Online

Transaction

Processing)

is

a

major

part

of

database

applications
.


Database Users


Users

may

be

divided

into

those

who

actually

use

and

control

the

content

(called

“Actors

on

the

Scene”)

and

those

who

enable

the

database

to

be

developed

and

the

DBMS

software

to

be

designed

and

implemented

(called

“Workers

Behind

the

Scene”)
.


Main Characteristics of the Database Approach [Continue..]

10

Database Users [Continue……]


1

Database

Administrators
:


Responsible

for

authorizing

access

to

the

database,

for

coordinating

and

monitoring

its

use,

acquiring

software,

and

hardware

resources,

controlling

its

use

and

monitoring

efficiency

of

operations
.

2

Database

Designers
:

Responsible

to

define

the

content,

the

structure,

the

constraints,

and

functions

or

transactions

against

the

database
.

They

must

communicate

with

the

end
-
users

and

understand

their

needs
.

3

End
-
users
:

They

use

the

data

for

queries,

reports

and

some

of

them

actually

update

the

database

content
.

4
-
Casual

:


Access

database

occasionally

when

needed
.

1
1

Database Users [Continue……]


5
-
Naïve

or

Parametric
:


They

make

up

a

large

section

of

the

end
-
user

population
.

They

use

previously

well
-
defined

functions

in

the

form

of

“canned

transactions”

against

the

database
.

Examples

are

bank
-
tellers

or

reservation

clerks

who

do

this

activity

for

an

entire

shift

of

operations
.

6
-
Sophisticated
:

these

include

business

analysts,

scientists,

engineers,

others

thoroughly

familiar

with

the

system

capabilities
.

Many

use

tools

in

the

form

of

software

packages

that

work

closely

with

the

stored

database
.

7
-

Stand
-
alone
:


Mostly

maintain

personal

databases

using

ready
-
to
-
use

packaged

applications
.

An

example

is

a

tax

program

user

that

creates

his

or

her

own

internal

database
.

12

Advantages of Using the Database Approach

1
-
Controlling

redundancy

in

data

storage
:


Redundancy

means

store

the

same

data

multiple

times

leads

to

problems
:


First
:

need

to

perform

a

single

logical

update

once

for

each

file
.


Second
:

storage

space

is

wasted

when

the

same

data

is

stored
.


Third
:

files

that

represent

the

same

data

may

become

inconsistent
.

2
-

Restricting

unauthorized

access

to

data
:

Users

or

user

groups

are

given

account

numbers

protected

by

passwords,

which

they

can

use

to

gain

access

to

the

database
.

A

DBMS

should

provide

a

security

and

authorization

subsystem
.

3
-

Providing

Storage

Structures

for

efficient

Query

Processing
:

Because

the

data

store

on

the

disk
.

Indexes

hash

data

structures,

suitably

modified

for

disk

search
.

Buffering

module

that

maintains

parts

of

the

database

in

main

memory

buffers
.

Optimization

module

of

the

DBMS

is

responsible

for

choosing

an

efficient

query

execution

plan

for

each

query

based

on

the

existing

storage

structures
.


13

Advantages of Using the Database Approach [Continue…]


4
-
Providing

Persistent

Storage

for

Program

Objects
:

Object
-
oriented

database

systems

are

compatible

with

programming

languages

such

as

C++

and

Java,

and

the

DBMS

software

automatically

performs

any

necessary

conversions
.

Hence,

a

complex

object

in

C++

can

be

stored

permanently

in

an

OODBMS
.

5
-

Providing

multiple

interfaces

to

different

classes

of

users
:

Because

many

types

of

users

with

varying

levels

of

technical

knowledge

use

a

database,

a

DBMS

should

provide

a

variety

of

user

interfaces
.

Both

forms
-
style

interfaces

and

menu
-
driven

interfaces

are

commonly

known

as

graphical

user

interfaces

(GUI

)
.

6
-

Representing

complex

relationships

among

data
:

A

DBMS

must

have

the

capability

to

represent

a

variety

of

complex

relationships

among

the

data

as

well

as

to

retrieve

and

update

related

data

easily

and

efficiently
.

14

Advantages of Using the Database Approach [Continue…]

7
-

Providing

backup

and

recovery

services
:

A

DBMS

must

provide

facilities

for

recovering

from

hardware

or

software

failures
.

8
-

Enforcing

integrity

constraints

on

the

database
:

DBMS

should

provide

capabilities

for

defining

and

enforcing

these

constraints
.

The

simplest

type

of

integrity

constraint

involves

specifying

a

data

type

for

each

data

item
.

These

constraints

are

derived

from

the

meaning

or

semantics

of

the

data

and

of

the

mini
-
world

it

represents
.

9
-

Permitting

Inferences

and

Actions

using

Rules
:

Some

database

systems

provide

capabilities

for

defining

Deduction

Rules

For

Inferencing

New

information

from

the

stored

database

facts
.

Such

systems

are

called

deductive

database

systems
.

Active

database

systems,

which

provide

active

rules

that

can

automatically

initiate

actions

when

certain

events

and

conditions

occur
.

15

Extending Database Capabilities for New Applications


1
-

Scientific

Applications

that

store

large

amounts

of

data

resulting

from

scientific

experiments

in

areas

such

as

high
-
energy

physics

or

the

mapping

of

the

human

genome
.

2
-
storage

and

Retrieval

of

Images

from

scanned

news,

Pc

photographs

or

medical

images

such

as

X
-
rays

or

MRI

(magnetic

resonance

imaging)

or

video

such

as

movies,

video

clips

or

PC

camera

video
.

3
-

Data

Mining

Applications

that

analyze

large

amounts

of

data

searching

for

the

occurrences

of

specific

patterns

or

relationships
.

4
-

Spatial

Applications

that

store

spatial

locations

of

data

such

as

weather

information

or

maps

used

in

(GIS)
.

5
-

Time

Series

Applications

for

example,

daily

sales

or

monthly

national

product

figures
.

16

WHEN NOT TO USE A DBMS

The

overhead

costs

of

using

a

DBMS

are

due

to

the

following
:

1
-

High

initial

investment

in

hardware,

software,

and

training

2
-

The

generality

that

a

DBMS

provides

for

defining

and

processing

data

3
-

Overhead

for

providing

security,

concurrency

control,

recovery,

and

integrity

functions
.

When a DBMS may be unnecessary

It

may

be

more

desirable

to

use

regular

files

under

the

following

circumstances
:

1
-

The

database

and

applications

are

simple,

well

defined,

and

not

expected

to

change
.

2
-

There

are

stringent

real
-
time

requirements

for

some

programs

that

may

not

be

met

because

of

DBMS

overhead
.

3
-

Multiple
-
user

access

to

data

is

not

required
.

17

MS Access Data

Types

Data

Type

Use

for

Size

Text

Text

or

combinations

of

text

and

numbers,

such

as

addresses
.

Also

numbers

that

do

not

require

calculations,

such

as

phone

numbers,

part

numbers,

or

postal

codes
.

Up

to

255

characters
.


Memo

Lengthy text and numbers, such as notes or
descriptions.

Up to 64,000 characters.

Number

Numeric data to be used for mathematical
calculations

1, 2, 4, or 8 bytes. 16 bytes for
Replication ID (GUID) only.

Date/Time

Dates and times.

8 bytes.

Currency

Currency values. Accurate to 15 digits to the
left of the decimal point and 4 digits to the
right.

8 bytes.

AutoNumber

Unique sequential (incrementing by 1)

4 bytes. 16 bytes for Replication ID
(GUID) only.

Yes/No

Fields that will contain only one of two values,
such as Yes/No, True/False, On/Off.

1 bit.

OLE Object

Objects (such as Microsoft Word documents,
Microsoft Excel spreadsheets, pictures, sounds,
or other binary data),

Up to 1 gigabyte (limited by disk space).

Hyperlink

Field that will store hyperlinks

Up to 64,000 characters.

Lookup
Wizard

Creates a field that allows you to choose a value
from another table or from a list of values using
a combo box

The same size as the primary key field
that is also the Lookup field; typically 4
bytes.