The Entity-Relationship Model (Chapter 3a) Laku Chidambaram W.P. Wood Professor of MIS University of...

The Entity-Relationship Model(Chapter 3a)

Laku Chidambaram W.P. Wood Professor of MISUniversity of Oklahoma

© Laku Chidambaram, Ph.D.

Overview

Database Design: The E-R Model

Entities

Attributes

Relationships


Database Design

EngineerEngineer(A/C Receivable)(A/C Receivable)

EngineerEngineer(A/C Receivable)(A/C Receivable)

PilotPilot(Order Entry)(Order Entry)

PilotPilot(Order Entry)(Order Entry)

PassengerPassenger (Inventory)(Inventory)

PassengerPassenger (Inventory)(Inventory)

ExternalView

General DesignGeneral Design(Comprehensive Data Model) (Comprehensive Data Model)

General DesignGeneral Design(Comprehensive Data Model) (Comprehensive Data Model)

ConceptualModel (ERD)

BlueprintBlueprint(Database Schema) (Database Schema)

BlueprintBlueprint(Database Schema) (Database Schema)

Logical Model(Relational)

AircraftAircraft(Database)(Database)

Physical Reality(DBMS)


The Entity Relationship Diagram

An E-R diagram is a graphical representation of an organization’s data

Such data includes the people, places,

objects, events, or concepts (along with their characteristics and relationships) that are relevant to an organization


Entities

Person, place, object, event, or concept about which the organization wishes to collect and maintain data

Nouns are used to describe entitiesExamples:

Employee, Student, Nurse City, State, Country Sale, Registration, Account

Entity Type versus Entity Instance


Types of Entities

Strong Entity: can exist independently

Weak Entity: cannot exist without the “owner” entity (with which it has an identifying relationship) Has a primary key that is derived from the owner entity in

the relationship

EMPLOYEE

EMPLOYEEhas

DEPENDENT


Attributes

Properties or characteristics of entities (or relationships)

Provide the actual data that describe entities (or relationships)

EMPLOYEE

E_Address

E_Name

E_ID


Unique Attributes

Candidate keys are ALL attributes that uniquely define an instance of an entity

A primary key is the candidate key that is chosen as the “most” unique of them all!

A primary key (or identifier) is ideal if it: does not change over time has unique values that are not null does not use “intelligent” features (Note: may need to substitute simple keys for long,

i.e., composite, keys)


Composite Attributes

Attributes that are not “atomic” (or simple), i.e., those attributes that can (and need to) be broken down further

EMPLOYEE

E_Address (E_Street,

E_City,

E-Zip)

E_Name

E_ID


Multi-valued Attributes

Some attributes may have multiple values ...

… but they may need to be “fixed” before proceeding

EMPLOYEE{E_Skill}

EMPLOYEE has SKILL

S_Code

S_Name


Derived Attributes

Those attributes whose values can be calculated from the values of other attributes (either in the database or in the system).

Value is not stored in the database (since it does not take up any space) E.g., Age can be calculated, where date-of-birth is stored


RelationshipsRefer to the associations (or links) between entitiesUse verbs to describe the links May have attributes (in some cases)Are directional

STUDENTLives in

DORMITORY

EMPLOYEE Works in STORE STORE


Associative Entities

A (many-to-many) relationship that is better represented as an entity because it: contains attributes from the entities it links

and has its own attributes has independent meaning to end-users

CUSTOMEROrders PRODUCT

CUSTOMER ORDER PRODUCT

The Entity-Relationship Model (Chapter 3a) Laku Chidambaram W.P. Wood Professor of MIS University of...

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