02-Data Model

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Chapter 2

Data Model

Database Systems:

Design, Implementation, and Management, Sixth Edition, Rob and Coronel

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In this chapter, you will learn:

•Why data models are important

•About the basic data-modeling building blocks

•What business rules are and how they affect database design

•How the major data models evolved, and their advantages and disadvantages

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The Importance of Data Models

•Data model

–Relatively simple representation, usually graphical, of complex real-world data

structures

–Communications tool to facilitate interaction among the designer, the applications

programmer, and the end user

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Importance of Data Modeling

•End-users have different views and needs for data

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Data Model Basic Building Blocks

•Entity is anything about which data are to be collected and stored

•Attribute is a characteristic of an entity

•Relationship describes an association among (two or more) entities

–One-to-many (1:M) relationship

–Many-to-many (M:N or M:M) relationship –One-to-one (1:1) relationship

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Business Rules

•Brief, precise, and unambiguous description of a policy, procedure, or principle within a specific organization’s environment

•Apply to any organization that stores and uses data to generate information

•Description of operations that help to create and enforce actions within that organization’s environment

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Business Rules (

)

•Must be rendered in writing •Must be kept up to date

•Sometimes are external to the organization •Must be easy to understand and widely

disseminated

•Describe characteristics of the data as viewed by the company

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Sources of Business Rules

•Company managers •Policy makers •Department managers •Written documentation –Procedures –Standards –Operations manuals

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Importance of Business Rules

•Promote creation of an accurate data model •Standardize company’s view of data

•Constitute a communications tool between users and designers

•Allow designer to understand the nature, role, and scope of data

•Allow designer to understand business processes

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The Evolution of Data Models

•Hierarchical •Network

•Relational

•Entity relationship •Object oriented

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The Hierarchical Model—Evolution

•GUAM (Generalized Update Access Method)

–Based on the recognition that the many smaller parts would come together as components of still larger components

•Information Management System (IMS)

–World’s leading mainframe hierarchical

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The Hierarchical Model—Characteristics

database development

•Limitations lead to a different way of looking at database design

•Basic concepts show up in current data models

•Best understood by examining manufacturing process

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A Hierarchical Structure

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Hierarchical Structure—Characteristics

•Each child has only one parent

•Tree is defined by path that traces parent

segments to child segments, beginning from the left

•Hierarchical path

–Ordered sequencing of segments tracing hierarchical structure

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The Hierarchical Model

•Advantages –Conceptual simplicity –Database security –Data independence –Database integrity –Efficiency

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The Hierarchical Model (

)

•Disadvantages

–Complex implementation –Difficult to manage

–Lacks structural independence

–Complex applications programming and use –Implementation limitations

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Child with Multiple Parents

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The Network Model

•Created to

–Represent complex data relationships more effectively

–Improve database performance –Impose a database standard

•Conference on Data Systems Languages (CODASYL)

•American National Standards Institute (ANSI) •Database Task Group (DBTG)

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Crucial Database Components

•Schema

–Conceptual organization of entire database as viewed by the database administrator

•Subschema

–Defines database portion “seen”by the

application programs that actually produce the desired information from data contained within the database

•Data Management Language (DML)

–Define data characteristics and data structure in order to manipulate the data

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Data Management

Language Components

•Schema Data Definition Language (DDL)

–Enables database administrator to define schema components

•Subschema DDL

–Allows application programs to define database components that will be used

•DML

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Network Model—Basic Structure

•Resembles hierarchical model

•Collection of records in 1:M relationships •Set

–Relationship

–Composed of at least two record types

•Owner

–Equivalent to the hierarchical model’s parent

•Member

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A Network Data Model

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The Network Data Model

•Advantages

–Conceptual simplicity

–Handles more relationship types –Data access flexibility

–Promotes database integrity –Data independence

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The Network Data Model (

)

•Disadvantages

–System complexity

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The Relational Model

•Developed by Codd (IBM) in 1970

•Considered ingenious but impractical in 1970 •Conceptually simple

•Computers lacked power to implement the relational model

•Today, microcomputers can run sophisticated relational database software

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The Relational Model—Basic Structure

(RDBMS)

•Performs same basic functions provided by

hierarchical and network DBMS systems, plus other functions

•Most important advantage of the RDBMS is its ability to let the user/designer operate in a human logical environment

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The Relational Model—

Basic Structure (

)

•Table (relations)

–Matrix consisting of a series of row/column intersections

–Related to each other by sharing a common entity characteristic

•Relational schema

–Visual representation of relational database’s entities, attributes within those entities, and relationships between those entities

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Relational Table

•Stores a collection of related entities

–Resembles a file

•Relational table is purely logical structure

–How data are physically stored in the

database is of no concern to the user or the designer

–This property became the source of a real database revolution

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A Relational Schema

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Linking Relational Tables

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The Relational Model

•Advantages

–Structural independence

–Improved conceptual simplicity

–Easier database design, implementation, management, and use

–Ad hoc query capability

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The Relational Model (

)

•Disadvantages

–Substantial hardware and system software overhead

–Can facilitate poor design and implementation –May promote “islands of information”problems

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The Entity Relationship Model

•Widely accepted and adapted graphical tool for data modeling

•Introduced by Chen in 1976

•Graphical representation of entities and their relationships in a database structure

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The Entity Relationship Model—

Basic Structure

•Entity relationship diagram (ERD)

–Uses graphic representations to model database components

–Entity is mapped to a relational table

•Entity instance (or occurrence) is row in table •Entity set is collection of like entities

•Connectivity labels types of relationships

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Relationships: The Basic Chen ERD

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Relationships:

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The Entity Relationship Model

•Advantages

–Exceptional conceptual simplicity –Visual representation

–Effective communication tool

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The Entity Relationship Model (

)

•Disadvantages

–Limited constraint representation –Limited relationship representation –No data manipulation language

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The Object Oriented Model

•Semantic data model (SDM) developed by Hammer and McLeod in 1981

•Modeled both data and their relationships in a single structure known as an object

•Basis of object oriented data model (OODM) •OODM becomes the basis for the object

oriented database management system (OODBMS)

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The Object Oriented Model (

)

•Object is described by its factual content

–Like relational model’s entity

•Includes information about relationships

between facts within object and relationships with other objects

–Unlike relational model’s entity

•Subsequent OODM development allowed an object to also contain operations

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Developments that

Boosted OODM’

s Popularity

reusability

•Complex data types and system requirements became difficult to manage with a traditional RDBMS

•Became possible to support increasingly sophisticated transaction & information requirements

•Ever-increasing computing power made it possible to support the large computing

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Object Oriented Data Model—

Basic Structure

•Object: abstraction of a real-world entity

•Attributes describe the properties of an object •Objects that share similar characteristics are

grouped in classes

•Classes are organized in a class hierarchy •Inheritance is the ability of an object within

the class hierarchy to inherit the attributes and methods of classes above it

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A Comparison of the OO Model

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The Object Oriented Model

•Advantages

–Adds semantic content

–Visual presentation includes semantic content –Database integrity

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The Object Oriented Model (

)

•Disadvantages

–Slow pace of OODM standards development –Complex navigational data access

–Steep learning curve

–High system overhead slows transactions –Lack of market penetration

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Other Models

•Extended Relational Data Model (ERDM)

–Semantic data model developed in response to increasing complexity of applications

–DBMS based on the ERDM often described as an object/relational database management

system (O/RDBMS)

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Other Models (

)

•Date’s objections to ERDM label

–Given proper support for domains, relational data models are quite capable of handling complex data

•Therefore, capability that is supposedly being extended is already there

–O/RDM label is not accurate because the

relational data model’s domain is not an object model structure

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Data Models: A Summary

•Each new data model capitalized on the shortcomings of previous models

•Common characteristics:

–Conceptual simplicity without compromising the semantic completeness of the database

–Represent the real world as closely as possible –Representation of real-world transformations

(behavior) must be in compliance with

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The Development of Data Models

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Database Models and the Internet

•Characteristics of successful “Internet age” databases

–Flexible, efficient, and secure Internet access that is easily used, developed, and supported –Support for complex data types and

relationships

–Seamless interfacing with multiple data sources and structures

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Database Models and the Internet

(

)

–Relative conceptual simplicity to make

database design and implementation less cumbersome

–An abundance of available database design, implementation, and application development tools

–A powerful DBMS graphical user interface (GUI) to help make the DBA’s job easier

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Degrees of Data Abstraction

•Way of classifying data models

•Many processes begin at high level of abstraction and proceed to an ever-increasing level of detail

•Designing a usable database follows the same basic process

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Degrees of Data Abstraction (

)

•American National Standards

Institute/Standards Planning and

Requirements Committee (ANSI/SPARC)

–Classified data models according to their degree of abstraction (1970s):

•Conceptual •External •Internal

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Data Abstraction Levels

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The Conceptual Model

•Represents global view of the database •Enterprise-wide representation of data as

viewed by high-level managers

•Basis for identification and description of main data objects, avoiding details

•Most widely used conceptual model is the entity relationship (ER) model

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A Conceptual Model for Tiny College

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Advantages of Conceptual Model

•Provides a relatively easily understood macro level view of data environment

•Independent of both software and hardware

–Does not depend on the DBMS software used to implement the model

–Does not depend on the hardware used in the implementation of the model

–Changes in either the hardware or the DBMS software have no effect on the database

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The Internal Model

•Representation of the database as “seen”by the DBMS

•Adapts the conceptual model to the DBMS •Software dependent

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The External Model

•End users’view of the data environment •Requires that the modeler subdivide set of

requirements and constraints into functional modules that can be examined within the

framework of their external models •Good design should:

–Consider such relationships between views –Provide programmers with a set of restrictions

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A Division of an Internal Model into

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Advantages of External Models

•Use of database subsets makes application program development much simpler

–Facilitates designer’s task by making it easier to identify specific data required to support each business unit’s operations

–Provides feedback about the conceptual model’s adequacy

•Creation of external models helps to ensure security constraints in the database design

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The External Model

•DBMS dependent

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The External Models for Tiny College

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The Physical Model

•Operates at lowest level of abstraction,

describing the way data are saved on storage media such as disks or tapes

•Software and hardware dependent

•Requires that database designers have a detailed knowledge of the hardware and

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Levels of Data Abstraction

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Summary

•A good DBMS will perform poorly with a poorly designed database

•A data model is a (relatively) simple abstraction of a complex real-world data-gathering environment •Basic data modeling components are:

–Entities –Attributes

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Summary (

)

•Hierarchical model

–Based on a tree structure composed of a root segment, parent segments, and child segments –Depicts a set of one-to-many (l:M) relationships

between a parent and its children

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Summary (

)

•Network model attempts to deal with many of the hierarchical model’s limitations

•Relational model:

–Current database implementation standard

–Much simpler than hierarchical or network design

•Object is basic modeling structure of object oriented model

•Data modeling requirements are a function of

different data views (global vs. local) and level of data abstraction