Chapter 2

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Transcript Chapter 2

Chapter 2
Database System Concepts and
Architecture
Dr. Bernard Chen Ph.D.
University of Central Arkansas
Outline
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Data Models, Schemas, and Instances
Three-Schema Architecture
Database language and Interfaces
Centralized and client/server
architecture
Categories of data models
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One fundamental characteristic of the database approach is that
it provides some level of data abstraction
High-level or Conceptual data models:
Provide concept that are close to the way many users perceive
data
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Low-level or Physical data model:
Provide concepts that describe the details of how data is stored in
the computer
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Conceptual data models
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It uses concepts such as entities, attributes and
relationships.
Entity represents a real-world object or concept,
such as employee or project
Attribute represents some property of interest that
further describes an entity, such as employee’s name
or salary
Relation represents an association among two or
more entitles
Example of a Relation
Schemas and Database State
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In any data model, it is important to
distinguish between the description of
the data and database itself
The description of the database is called
the database schema
A displayed Schema is called a schema
diagram
Database V.S. File
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In the database approach, a single repository of
data is maintained that is defined once then
accessed by various users
The major differences between DB and File are:
Self-describing of a DB
Insulation between programs and data
Support of multiple views of the data
Sharing of data and multiuser transaction processing
University Database
Example of a Database
Schema (incomplete)
Example of a Database
Schema
Schemas and Database State
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The data in the database at a particular moment in time is
called a database state
The distinction between database schema and database state is
very important
When we define a new database, we specify its database
schema only to the DBMS
At this point, the corresponding database state is the empty
state with no data
We get the initial state of the database when the database is
first loaded
From then on, every time an update operation is applied to the
database, we get another database state
Schemas and Database State
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Valid State: a state that satisfies the structure and
constrains specified in the schema.
The database schema changes very infrequently.
The database state changes every time the
database is updated
Schema is also called intension.
State is also called extension.
Outline
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Data Models, Schemas, and Instances
Three-Schema Architecture
Database language and Interfaces
Centralized and client/server
architecture
Three-Schema Architecture
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Three of four important characteristics of
the database approach, listed in Ch.1 are:
Self-describing of a DB (Schema)
Insulation between programs and data
Support of multiple views of the data
Three-Schema Architecture : it was
proposed to help achieve and visualize
these characteristics
Three-Schema Architecture
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Defines DBMS schemas at three levels:
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Internal schema at the internal level to describe
physical storage structures and access paths (e.g
indexes).
Conceptual schema at the conceptual level to
describe the structure and constraints for the
whole database for a community of users.
External schemas at the external level to
describe the various user views.
The three-schema architecture
Outline
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Data Models, Schemas, and Instances
Three-Schema Architecture
Database language and Interfaces
Centralized and client/server
architecture
DBMS Languages
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The first step to create a database through DBMS is
to specify conceptual and internal schemas for the
database
Data Definition Language (DDL): is used by
database designers to define schemas
Data Manipulation Language (DML)
View Definition Language (VDL): is to specify
user views
In current DBMS, the preceding types of languages
are usually not considered distinct languages
DBMS Component Modules
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A DBMS is a complex software system
The figure showed in next slide is divide into
two halves.
The top half refers to the various users of the
database system
The lower half shows the internals of the
DBMS responsible for storage of data and
processing of transactions
Typical DBMS Component
Modules
Outline
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Data Models, Schemas, and Instances
Three-Schema Architecture
Database language and Interfaces
Centralized and client/server
architecture
Centralized DBMS Architecture
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A centralized DBMS in which all the
DBMS functionality, application program
execution, and user interface
processing were carried out on a single
machine
A Physical Centralized
Architecture
Basic Client/Server
Architectures
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The client/server architecture was developed
to deal with computer environment in which a
large number of PCs, workstation, file
server…
A client in this framework is typically a user
machine that provides user interface
capabilities and local processing
A server is a system containing both
hardware and software that can provide
services to the client machines.
Logical two-tier client server
architecture
Two-tier Architecture
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This is called two-tire architectures
because the software components are
distributed over two systems: client and
server
The emergence of the Web changed
the roles of client and server, leading to
the three-tier architecture
Three-tier architecture
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The intermediate layer or middle layer is
sometimes called the application server or
Web server
Three-tier Architecture Can Enhance
Security:
Database server only accessible via middle tier
Clients cannot directly access database server
Three-tier client-server
architecture
Three-tier architecture
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The presentation layer displays information to
the user
The business logic layer handles intermediate
rules and constrains before data is passed up
to the user or down to the DBMS
If the bottom layer is split into two layers (a
web server and a database server), then it is
a 4-tire architecture (possible to the n-tier)