2. Database System Concepts and
Architecture
Chapter 2
2.1 Data Models, Schemas, and
Instances

Data Model: A collection of concepts used to describe
the structure of a database

Database Structure: Data types, relationships, and
constraints on data

Database Schema: The description of the database
(intension)

Schema Diagram: Diagrammatic representation of the
DB schema

Schema Construct: An object in the schema

Database State/Instance: The data in the database at a
particular moment in time (extension)
2.1 Data Models, Schemas, and
Instances
Database schema
Database Instance
2.1 Data Models, Schemas, and
Instances

Schema diagram displays some aspects of the schema


Define a new DB?



Specify the DB schema
DB state is the empty state
Populate DB with initial data?



Data types and relationships are not evident
No changes in the schema
DB state is the initial state
Subsequent update operations?


No changes in the schema
DB state changes into a new state
2.1 Data Models, Schemas, and
Instances

DBMS ensures that every DB state is a valid state


Special care must be taken when designing a DB
schema


A state that satisfies the structure and constraints
specified in the schema
A schema is not supposed to change frequently
A DBMS stores the meta-data

Schema constructs and constraints
2.2 DBMS Architecture and Data
Independence

DB architecture is specified using a
three-schema architecture
 Used to achieve the first 3 characteristics of DB
approach
1.
2.
3.

Separation between programs and data
Support of multiple user views
Use of catalog to store DB schema
Provides only a description of data
2.2.1 The Three-Schema
Architecture
1.
Physical (Internal) Level


2.
Logical (Conceptual) Level



3.
Has an internal schema
Describes the physical storage structure of the DB
Has a conceptual schema
Describes the structure of the whole DB (entities, data types,
relationships, user operations, and constraints)
Hides details of the physical level
External (View) Level


Has several external schemas or user views
Each describes the part of the DB that a particular user is
interested in
2.2.1 The Three-Schema
Architecture
External
View
…
mapping
External
View
mapping
Conceptual Schema
mapping
Internal Schema
2.2.2 Data Independence

Data Independence


Ability to change the schema at one level of a DB
system without having to change the schema at the
next higher level
Result?
1.
2.
Logical data independence
Physical data independence
2.2.2 Data Independence
1.
Logical data independence

Ability to change the conceptual schema without
having to change the external schemas or
application programs

Changes?



Expand the DB (add a record or data item)
Reduce the DB (remove a record or data item)
Changes to constraints
2.2.2 Data Independence
2.
Physical data independence

Ability to change the internal schema without
having to change the conceptual or external
schemas

Changes?

Reorganization of physical files
2.3.1 DBMS Languages

Data Definition Language (DDL)

Used by the DBA and database designers to specify the
conceptual schema of a database

In many DBMSs, the DDL is also used to define internal and
external schemas (views)

In some DBMSs, separate storage definition language
(SDL)andview definition language (VDL) are used to define
internal and external schemas

SDL is typically realized via DBMS commands provided to the DBA
and database designers
2.3.1 DBMS Languages

Data Manipulation Language (DML)

Used to specify database retrievals and updates

DML commands (data sublanguage) can be embedded in a
general-purpose programming language (host language), such
as COBOL, C, C++, or Java.


A library of functions can also be provided to access the DBMS from
a programming language
Alternatively, stand-alone DML commands can be applied
directly (called a query language).
2.3.1 DBMS Languages

Data Manipulation Language (DML)

DML Types:
 High Level or Non-procedural Language




For example, the SQL relational language
Are “set”-oriented and specify what data to retrieve rather than
how to retrieve it.
Also called declarative languages.
Low Level or Procedural Language


Retrieve data one record-at-a-time;
Constructs such as looping are needed to retrieve multiple records,
along with positioning pointers
2.3.2 DBMS Interfaces

Menu-Based Interfaces


Forms-Based Interfaces


Speech used as input query and/or as query result
Interfaces for Parametric Users


Requests are written in English
Speech Input and Output


Point and Click, Drag and Drop, etc.
Natural Language Interfaces


Designed for naïve users
Graphical User Interfaces (GUI)


Popular for browsing the Web
Ex: bank tellers using function keys
Interfaces for the DBA

Commands used for Creating user accounts, granting authorizations,
setting system parameters, changing schemas or access paths
2.4 The Database System
Environment
2.5 DBMS Architectures
1.
Centralized DBMSs Architecture


Combines everything into single system including- DBMS software,
hardware, application programs, and user interface processing software
User can still connect through a remote terminal – however, all processing
is done at centralized site
2.5 DBMS Architectures
2.
Basic Client/Server Architectures


Specialized Servers with Specialized functions (Print server, File server,
DBMS server, Web server, Email server)
Clients can access the specialized servers as needed
2.5 DBMS Architectures
3.
Two-Tier Client/Server Architectures



A client program may connect to several DBMSs, sometimes called the
data sources
Data sources can be files or other non-DBMS software that manages data
Other variations of clients are possible
2.5 DBMS Architectures
4.
Three-Tier and n-tier Architectures



Common for Web applications
Intermediate Layer called Application Server or Web Server:
Such an architecture can enhance security:


Database server only accessible via middle tier
Clients cannot directly access database server
2.6 Classification of Database
Management Systems

Several criteria are used

Data Model

Traditional
1.
2.
3.

Emerging
4.
5.

Relational data model
Network data model
Hierarchical data model
Object data model
Object-relational data model
Number of Users
1.
2.
Single-user systems
Multi-user systems
2.6 Classification of Database
Management Systems

Number of Sites
1.
2.
Centralized DBMS
Distributed DBMS



Homogeneous DDBMS
Heterogeneous DDBMS
Federated DBMS (multidatabase system)
Any Questions?