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Copyright 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

Chapter 7

Data ModelingUsing the

Entity-Relationship

(ER) Model

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Copyright 2011 Ramez Elmasri and Shamkant Navathe

Lecture Outline

Using High-Level Conceptual Data Modelsfor Database Design

Entity Types, Attributes, and Keys

Relationship Types, Roles, and StructuralConstraints

Weak Entity Types

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Copyright 2011 Ramez Elmasri and Shamkant NavatheSlide 3- 3

Overview of Database Design Process

Two main activities: Database design

Applications design

Focus in this chapter on database design To design the conceptual schema for a database

application

Applications design focuses on the programsand interfaces that access the database

Generally considered part of softwareengineering

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Overview of Database Design Process

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Steps in Database Design

Requirements Analysis user needs; what must database do?

Conceptual Design

high level descr (often done with ER model) Logical Design

translate ER into DBMS data model

Schema Refinement consistency, normalization

Physical Design -indexes, disk layout

Security Design -who accesses what,and how

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Data Modeling Using theEntity-Relationship (ER) Model

Entity-Relationship (ER) model

Popular high-level conceptual data model

ER diagrams Diagrammatic notation associated with the ER

model

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Using High-Level ConceptualData Models for Database

Design Requirements collection and analysis

Database designers interview prospective

database users to understand and documentdata requirements

Result: data requirements

Functional requirements of the application

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Using High-Level ConceptualData Models (contd.)

Conceptual schema

Description of data requirements

Includes detailed descriptions of the entity

types, relationships, and constraints

Transformed from high-level data model intoimplementation data model

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Logical design or data model mapping

Result is a database schema in implementationdata model of DBMS

Physical design phase

Internal storage structures, file organizations,indexes, access paths, and physical design

parameters for the database files specified

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10/49Copyright 2011 Ramez Elmasri and Shamkant NavatheSlide 3- 10

Example COMPANY Database

We need to create a database schemadesign based on the following (simplified)requirementsof the COMPANY Database: The company is organized into DEPARTMENTs.

Each department has a name, number and anemployee who managesthe department. Wekeep track of the start date of the departmentmanager. A department may have several

locations. Each department controlsa number of

PROJECTs. Each project has a unique name,unique number and is located at a single

location.

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Example COMPANY Database(Contd.)

We store each EMPLOYEEs social securitynumber, address, salary, sex, and birthdate.

Each employee works forone department but maywork onseveral projects.

We keep track of the number of hours per week thatan employee currently works on each project.

We also keep track of the direct supervisorof eachemployee.

Each employee may havea number ofDEPENDENTs.

For each dependent, we keep track of their name, sex,birthdate, and relationship to the employee.

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What are the entitiesand relationshipsinthe enterprise?

What information about these entities andrelationships should we store in thedatabase?

What are the integrity constraints or

business rules that hold?

A database `schema in the ER Model can

be represented pictorially (ER diagrams).

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Entity Types, Entity Sets,Attributes, and Keys

ER model describes data as:

Entities

Relationships

Attributes

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ER Model Concepts

Entities and Attributes Entities are specific objects or things in the mini-world that

are represented in the database. For example the EMPLOYEE John Smith, the Research

DEPARTMENT, the ProductX PROJECT

Attributes are properties used to describe an entity. For example an EMPLOYEE entity may have the attributes

Name, SSN, Address, Sex, BirthDate

A specific entity will have a value for each of its attributes. For example a specific employee entity may have Name='John

Smith', SSN='123456789', Address ='731, Fondren, Houston,TX', Sex='M', BirthDate='09-JAN-55

Each attribute has a value set(or data type) associated withite.g. integer, string, subrange, enumerated type,

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ER Diagrams, NamingConventions, and Design Issues

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Entities and Attributes

Entity

Entity: Real-world object, distinguishable fromother objects. An entity is described using a set

of attributes. Entity Set: A collection of similar entities. E.g.,

all employees.

All entities in an entity set have the same set of

attributes. (Until we consider hierarchies,anyway!)

Each entity set has a key (underlined).

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Entities and Attributes (contd.)

Employees

ssnname

salary

ssn name salary

219654231 Tom 50,000 $

318689741 John 60,000 $

354856421 Mary 55,000 $

Employees

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Attributes

Particular properties that describe entity

Each attribute has a domain (the set of

permitted values for each attribute ).

Types of attributes:

Compositeversus simple(atomic) attributes

Single-valuedversus multivaluedattributes Complexattributes

Storedversus derivedattributes

Descriptive attributes

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STUDENT

Name

Last

Birth-date

Month DayYear

First


COMPOSITE ATTRIBUTE

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MULTIVALED ATTRIBUTE

Department

Location NameNumber

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Employee

Address NameNumber

St_NumSt_name City Zip

COMPLEX ATTRIBUTE

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DERIVED ATTRIBUTE

STUDENT

Name Birth-dateAge

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address

name

STUDENT

ssn

register

datecname

# of creditscno

Course

DESCRIPTIVE ATTRIBUTE

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Entity Types, Entity Sets, Keys,and Value Sets

Entity type

Collection (or set) of entities that have thesame attributes

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Entity Types, Entity Sets, Keys,and Value Sets (contd.)

Key oruniqueness constraint

Attributes whose values are distinct for eachindividual entity in entity set

Key attribute Uniqueness property must hold for every entity set

of the entity type

Value sets (or domain of values) Specifies set of values that may be assigned to

that attribute for each individual entity

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A super keyof an entity set is a set of oneor more attributes whose values uniquelydetermine each entity.

A candidate keyof an entity set is aminimal super key

IDis candidate key of student

course_idis candidate key of course Although several candidate keys may exist,

one of the candidate keys is selected to be

the primary key.

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Composite key is a key that consists of 2attributes that uniquely identify an entityoccurrence

Foreign key is a column or a combinationof columns that is used to establish andenforce a link between two tables.

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Initial Design of Entity Types forthe COMPANYDatabase Schema Based on the requirements, we can identify four

initial entity types in the COMPANY database:

DEPARTMENT

PROJECT EMPLOYEE

DEPENDENT

Their initial design is shown on the following slide The initial attributes shown are derived from the

requirements description

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Initial Conceptual Design of theCOMPANY Database

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Relationship Types, Relationship

Sets, and Roles Relationship

When an attribute of one entity type refers to

another entity type Represent references as relationships not

attributes

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Relationship Types, Sets, andInstances(contd.)

Relationship Association among two ormore entities. E.g., Tom works inPharmacy department.

relationships can have their own attributes.

Relationship Set Collection of similarrelationships.

An n-ary relationship set Rrelates nentity setsE1... En ; each relationship in Rinvolvesentities e1E1, ..., enEn

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salary

name

Employees

ssn

Works_In

sincedname

budgetd_id

Departments

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Same entity set can participate in differentrelationship sets, or in different roles in

the same set.

since

Manages

dname

budgetdid

Departments

since

Works_In

salary

name

ssn

Employees

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Relationship Degree

Degreeof a relationship type

Number of participating entity types

Binary, ternary

Binary relationship

salary

name

Employees

ssn

Works_In

dname

budgetd_id

Departments

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Relationship Degree

a ternary relation Contracts relates entitysets Parts, Departments andSuppliers, andhas descriptive attribute qty. No

combination of binary relationships is anadequate substitute.

Suppliers

qty

DepartmentsContractParts

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Relationship Degree

S can-supply P, D needs P, and D deals-with S does not imply that D has agreed to buy Pfrom S.

How do we record qty?

Suppliers

qty

DepartmentsContractParts

Suppliers

Departments

deals-with

Parts

can-supply

VS.

needs

38

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Role Names and RecursiveRelationships

Role names and recursive relationships

Role name signifies role that a participatingentity plays in each relationship instance

Recursiverelationships Same entity type participates more than once

in a relationship type in different roles

Must specify role name

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Recursive Relationships Example

COURSE

Is_Pre

is prerequisite-to

Has-prerequisites

Is-Pre COURSE

COURSE

C t i t Bi

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Constraints on BinaryRelationship Types

Cardinality ratio for a binary relationship

Specifies maximum number of relationshipinstances that entity can participate in

Participation constraint Specifies whether existence of entity depends

on its being related to another entity

Types: totaland partial

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Cardinality Examples

1:1 relationshipA person has only one IDbook and an ID book belongs to only 1person

N:1 relationshipA school can have manystudents, but a student attends only 1

school

M:N relationshipA person can own manyhouses and a house can have multipleowners

C t i t Bi

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Entity set E has total participation inrelationship set R: every entity in Eparticipates in at least one relationship in R

E.g. an ID book must have an owner

E.g. a house must have an owner

C t i t Bi

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Partial participation: only some entities in Eparticipate in relationships in R

E.g. not every person owns a house

We will use a double-line to show total

participation and a single-line to showpartial participation (NB there are manystyles of ER notation, but the ideas are thesame).

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Weak Entity Types

Can be identified uniquely only byconsidering the primary key of another(owner) entity.

Do not have key attributes of their own Identified by being related to specific entities

from another entity type

Identifying relationship Relates a weak entity type to its owner

Always has a total participation constraint

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Weak Entity Types(contd.)

Owner entity set and weak entity set mustparticipate in a one-to-many relationship set(one owner, many weak entities).

Weak entity set must have total participation in

this identifying relationship set.

salary

name

agedname

DependentsEmployees

ssn

Policy

cost

Weak entities have only a partial key (dashed underline)

1 N

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Weak Entity Types(contd.)

# of credit

cname

namesnumber

SectionCourse

cnumber

has

Weak entities have only a partial key (dashed underline)

1 N

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Summary

Basic ER model concepts of entities andtheir attributes

Different types of attributes

Structural constraints on relationships

ER diagrams represent E-R schemas

Documents

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