DATABASE MODELLING
Database Structure
A database structure is the description
and definition of all basic structures such
as simple conceptual files, datatypes,
relationships, and constraints that should
hold on the data. In any data model it is
important to distinguish between the
description of the database and the
database itself.
1
Database Schema
The description of a database is called
the database schema (or the metadata). A database schema is specified
during database design and is not
expected to change frequently.
SUPPLIER
SNO
Sname
Status
City
Pname
Color
Weight
PART
PNO
SUPPLY
SNO
PNO
2
QTY
Database State (Instance)
The data in the database at a particular
moment of time is called the database
state (or instance).
SUPPLIER
SNO
S1
S2
S3
S4
S5
Sname
Ahmed
Badran
Aly
Saleh
Sadek
Status
20
10
10
30
20
City
Cairo
Cairo
Alex.
Tanta
Cairo
PNO
P1
P2
P3
P4
P5
P6
Pname
Nut
Bolt
Screw
Cam
Cog
Screw
Color
Red
Green
Blue
Red
Black
Black
Weight
12
15
15
17
20
14
PART
SUPPLY
SNO
S1
S1
S1
S2
S2
S4
S4
S5
PNO
P1
P2
P3
P1
P3
P2
P3
P2
3
QTY
100
200
100
150
100
200
300
100
Data Models
A data model is a set of concepts that
can be used to describe a database
structure.
Data Models
High-level
(conceptual)
Data Models
Relational
Implementation
Data Models
Hierarchical
Network
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Low-level
(physical)
Data Models
Objectoriented
Database System Architecture
The Three-level Architecture
End Users
External Level
External
View1
External
Viewn
external/conceptual
mapping
Conceptual Level
Conceptual Schema
conceptual/internal
mapping
Internal Schema
Internal Level
Stored Database
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Data Independence
Data independence is the capacity to
change the schema at one level of a
database system without having to
change the schema at the next higher
level. defined:
1.
Logical data independence is the
capacity to change the conceptual
schema without having to change
external schemas or Logical data
application programs.
2. Physical data independence is the
capacity to change the internal schema
without having to change the conceptual
(or external) schemas.
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Classification of DBMSs
Classification according to Data
Model
DBMSs
Relational
Hierarchical
Network
Objectoriented

The relational data model represents the
database as a collection of tables, where
each table can be stored as separate file.

Examples of
DBMSs are:

DB2 from IBM

ORACLE from Oracle Corporation

Informix from Informix

SyBase from OpenSoft

SQL Server from Microsoft

MS-ACCESS from Microsoft
commercial
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relational
Example of a Relational Database
SUPPLIER
SNO
S1
S2
S3
S4
S5
Sname
Ahmed
Badran
Aly
Saleh
Sadek
Status
20
10
10
30
20
City
Cairo
Cairo
Alex.
Tanta
Cairo
PNO
P1
P2
P3
P4
P5
P6
Pname
Nut
Bolt
Screw
Cam
Cog
Screw
Color
Red
Green
Blue
Red
Black
Black
Weight
12
15
15
17
20
14
PART
SUPPLY
SNO
S1
S1
S1
S2
S2
S4
S4
S5
PNO
P1
P2
P3
P1
P3
P2
P3
P2
8
QTY
100
200
100
150
100
200
300
100

The network data model represents
data as a record types. An example of a
network model is known as the
CODASYL DBTG model.
Student
Course
IS_A
Course_offerings
HAS_A
Student_grades
Section
Prerequisite
Section_grades
Grade_Report

The
hierarchical
data
model
represents data as hierarchical tree
structure.
Each
hierarchical
represents a number of related
records.
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
The object-oriented data model defines
a database in terms of objects, their
properties, and their operations. Objects
with the same structure and behavior
belong to a class, and classes are
organized into hierarchies or acyclic
graphs. The operations of each class are
specified in terms of predefined
procedures called methods.
Experimental OO prototypes:





The ORION system developed at MCC,
The OpenOODB system at Texas Instruments,
The IRIS system developed at HP laboratories,
The ODE system at ATT Bell Labs, and
The ENCORE/ObServer project at Brown
University.
Commercially available OO systems:






GEM-STONE/OPAL of SerioLogic,
ONTOS of Ontologic,
Objectivity of Objectivity Inc.,
Versant of Versant Technologies,
ObjectStore of Object Design, and,
O2 of O2 Technology.
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Classification according to
number of users
1.Single user systems support only one
user at a time and are mostly used
with personal computers.
2.Multiuser systems, which include the
majority of DBMSs, support many
users concurrently.
Classification according to
number of sites
1.Centralized DBMS where the data is
stored at a single computer site. Most
DBMSs are centralized. A centralized
DBMS can support multiple users, but
the DBMS and the database themselves
reside totally at a single computer site.
2. Distributed
DBMS (DDBMS) can have
the actual database and DBMS software
distributed over many sites, connected
by a computer network. Many DDBMSs
use a client-server architecture.
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