DATABASE SYSTEM CONCEPTS AND ARCHITECTURE

Chapter 2

Contents

Data Models, Schemas, and InstancesCategories of Data ModelsSchemas, Instances, and Database State

Three Schema Architecture and Data Independence

Database languagesDBMS InterfacesThe Database System Environment

Data Model, Schemas and Instances

The data model is a collection of concepts that can be used to describe the structure of a database, provides necessary means to achieve this abstraction.

By structure of a database we mean data types, relationships, and constraints that hold a database.

Categories of Data Model

High-level or conceptual data model Provide concepts that are close to the way many

users perceive data. Low level or physical data model

Provides concepts that describe the details of how data is stored in the computer

Representational or implementational data model Provide concepts that may be understood by user ,

but that are not too far removed from the way data is organized within the computer.

Schemas,Instances and database State

It is important to distinguish between the description of the database and the database itself.

The description of a database is called the database schema which is specified during database design and is not expected to change frequently

The actual data in a database may change quite frequently and is called a database state or snapshot.

3Schema Architecture

3 Schema Architecture

Internal Level has internal schema, which describes the physical storage structure of the database.

Conceptual Level has conceptual schema, and hides the physical storage structures and concentrates on describing entities, data types, relationships , user operations and constraints.

External or View Level describes a part of the database that is particular to a group of users interest in it and hides the rest of the database from that user group.

Data Independence

Logical Data IndependenceCapacity to change the conceptual schema

without changing the application programs or external schema.

Physical data independenceCapacity to change the internal schema

without changing the conceptual schema

DBMS Languages

DDL Used by DBA and Database designers to define

both conceptual and internal schemas. Storage Definition Language (sdl): used to

define the internal schema View Definition Language(VDL): used to

specify the user views and their mapping to the conceptual schema.

DML( data manipulation language): for retrieval , insertion,deletion and modification of the data.

Interfaces

Menu based InterfacesForms-based InterfacesGraphical User InterfacesNatural Language InterfacesInterfaces for Parametric UsersInterfaces for DBA

The Database System Environment

Classification of Database Management System

Several criteria are normally used to classify DBMS’sData Model Number of UsersNumber of SitesCost