2.ER Diagram

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S. Nandagopalan, BITS. Nandagopalan, BIT 11

ChapterChapter--22


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Requirement Analysis:Requirement Analysis: CollectingCollecting

information from the users what theyinformation from the users what they

want to store in the database.want to store in the database. Conceptual Database Design:Conceptual Database Design: High levelHigh level

description of data to be stored in thedescription of data to be stored in the

database. ER model is the ideal one.database. ER model is the ideal one.

Logical Database Design:Logical Database Design: Selecting anSelecting an

appropriate DBMS for implementation.appropriate DBMS for implementation.


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Schema Refinement:Schema Refinement: This is done throughThis is done through

normalization theories.normalization theories.

Physical Database Design:Physical Database Design: Low levelLow levelimplementation like indexing and clusteringimplementation like indexing and clustering

of tables.of tables.

Application and Security Design:Application and Security Design: DevelopingDeveloping

a complete application using fronta complete application using front--end toolsend tools

like VB, Java, etc. Unified Modelinglike VB, Java, etc. Unified Modeling

Language (UML) may be used.Language (UML) may be used.


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Entities:Entities:An entity is anything that exists inAn entity is anything that exists in

a real world with an independent existence.a real world with an independent existence.EgEg.. MarutiMaruti Car, Accounts Department , DBMSCar, Accounts Department , DBMS

Book, etc.Book, etc.

Entity Set:Entity Set: Collection of similar entities.Collection of similar entities.

EgEg. Employees. EmployeesCollection of AccountsCollection of Accounts

department employees and HR departmentdepartment employees and HR departmentemployees.employees.


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Attributes:Attributes: The properties of an entity can beThe properties of an entity can bedescribed by what is known as attributes.described by what is known as attributes.

EgEg. SSN, Name, Age may be the attributes of. SSN, Name, Age may be the attributes of

the entity set Employees. These attributesthe entity set Employees. These attributesare common for all the entities in this entityare common for all the entities in this entityset.set.

Attribute Types: Simple or AtomicAttribute Types: Simple or Atomic -- holdhold

single value and not composed of any othersingle value and not composed of any otherattributes.attributes.

Example:Example: RegNoRegNo, Age,, Age, Date_of_BirthDate_of_Birth,,Dept_NoDept_No, Sex, etc., Sex, etc.


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Composite:Composite: attributes that can be subattributes that can be sub--dived intodived intosome more attributes are called as compositesome more attributes are called as composite

attributes.attributes. EgEg. Address. Address

Single valued:Single valued: hold only one value for a particularhold only one value for a particular

entity.entity. EgEg. Age, Sex. Age, Sex

MultiValuedMultiValued:: attributes having more than oneattributes having more than one

value.value. EgEg.. DLocationDLocation, Qualification, Qualification

Stored:Stored: nonnon--derivablederivableEgEg. Sex, DOB. Sex, DOB

Derived:Derived: can be derived from othercan be derived from other attribute(sattribute(s).).

EgEg. Age, Total. Age, Total


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Key:Key:A key is a minimal set of attributes whoseA key is a minimal set of attributes whosevalues uniquely identify an entity in an entity set.values uniquely identify an entity in an entity set.EgEg. SSN. SSN

Candidate Key:Candidate Key: There could be more than oneThere could be more than onecandidate key for each entity set.candidate key for each entity set.

Primary Key:Primary Key: One of the candidate keys may beOne of the candidate keys may beselected as the primary key.selected as the primary key.

Domain of Attributes:Domain of Attributes: Each attribute is associatedEach attribute is associatedwith a set of values called aswith a set of values called as domdom(Ai(Ai). If the). If theattribute Age takes, say, values between 18 and 58attribute Age takes, say, values between 18 and 58years, the domain of this attribute is 18years, the domain of this attribute is 18--58.58.


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SingleSingle partialpartial

DoubleDouble --totaltotal

ParticipationParticipation

ConstraintsConstraints

88

Diamond boxDiamond boxRelationship SetRelationship Set77

UnderlinedUnderlinedPrimary keyPrimary key66

OvalOval -- thickthickDerivedDerivedAttbAttb55

OvalOval doubledoubleMulti valuedMulti valuedAttbAttb44

OvalOval singlesingleAttributesAttributes33

Rectangular BoxRectangular Box

Double LineDouble Line

Weak Entity SetWeak Entity Set22

Rectangular BoxRectangular Box

Single lineSingle lineStrong Entity SetStrong Entity Set11

MeaningMeaningER NotationER NotationEntity SetEntity SetSlSl. No. No

Name

Employees

Dependents

DLocation

SSN

Age

Manages


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RelationshipRelationship -- is an association among twois an association among two

or more entities.or more entities.

Relationship setRelationship set is a collection of similaris a collection of similarrelationships.relationships.

WorksFor

SSN

Name

Addr

Employees

DNo DLocation

Departments

DName


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Employees Works_For Departments


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The degree of a relationship type isThe degree of a relationship type is

defined as the number of participatingdefined as the number of participating

entities. If the number of participatingentities. If the number of participating

entities is only two, then it is called asentities is only two, then it is called asbinary relationshipbinary relationship..

Suppose if the number of participatingSuppose if the number of participating

entities are three, then it is called asentities are three, then it is called as

ternary relationshipternary relationship..


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Ternary Relationship SetTernary Relationship Set

Sid

SName

SAddr PNo

PName

location

PtNo PtName


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Cardinality ratio:Cardinality ratio: TheThe cardinality ratiocardinality ratio for afor a

binary relationship specifies the number ofbinary relationship specifies the number of

relationship instances that an entity canrelationship instances that an entity can

participate in.participate in.

Example: oneExample: one--toto--one (1:1) means that aone (1:1) means that a

Department can have only one Manager.Department can have only one Manager.

TypesTypes of cardinalityof cardinality mappings:mappings:

(a) one(a) one--toto--one (1:1)one (1:1)

(b) one(b) one--toto--many (1:N)many (1:N)

(c) many(c) many--toto--many (M:N)many (M:N)


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Total Participation:Total Participation: assume that the company policy isassume that the company policy isthat every department should have a manager. This impliesthat every department should have a manager. This implies

that there will be a full or total participation betweenthat there will be a full or total participation between

Departments and Manages. Therefore, this type ofDepartments and Manages. Therefore, this type of

participation is called asparticipation is called as total participationtotal participation (represented as(represented asthick or double line).thick or double line).

Total Participation

since


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When an entity set E does not participateWhen an entity set E does not participate

fully with a relationship and in turn to thefully with a relationship and in turn to the

participating entity, such a constraint isparticipating entity, such a constraint is

called as partial participation.called as partial participation.

Partial Participationsince


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AAweak entityweak entity set is an entity set such that itsset is an entity set such that its

key is composed of attributes some or all are ofkey is composed of attributes some or all are of

which belong to another entity set.which belong to another entity set.

Weak entity sets do not have their own keyWeak entity sets do not have their own keyattributes (only partial key allowed).attributes (only partial key allowed).

A weak entity will always have total participationA weak entity will always have total participation

with the identifying owner entity.with the identifying owner entity.

The owner entity set and the weak entity setThe owner entity set and the weak entity set

must participate in 1:M relationship.must participate in 1:M relationship.


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DepNameDepName partial keypartial key

SSN

Name

Addr DepName

BirthDate

Sex


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Strong entity set:

Has sufficient attributes to form a primary key

Weak entity set:

Lacks sufficient attributes to form a primary key

Hence, lacks sufficient attributes to form any key

But every entity set needs a key; What to do?

Must import attributes from strong entity set(s) A weak entity set member is subordinate to the

dominant entity from strong entity set providingattributes to complete its key


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Loan-payment

Payments

loanno

amount

Payment-amount

Payment-no Payment-date

Employees


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AnAn abstractionabstraction of some system is a model ofof some system is a model ofthat system in which certain details arethat system in which certain details aredeliberately omitted.deliberately omitted.

In some applications a system may have tooIn some applications a system may have toomany details that need to be managed andmany details that need to be managed andprovide abstraction. During such instances, itprovide abstraction. During such instances, itcan be managed by decomposing the model intocan be managed by decomposing the model intohierarchy of abstractions.hierarchy of abstractions.

A relation inA relation in Codd'sCodd's relational schema supportsrelational schema supportstwo types of abstractionstwo types of abstractions generalizationgeneralization andandaggregationaggregation..


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AAgeneralizationgeneralization is an abstraction whichis an abstraction which

enables a class of objects to be thought ofenables a class of objects to be thought of

generically as a single named object.generically as a single named object.

We can think of generalization as aWe can think of generalization as a

relationship between an entity (or class)relationship between an entity (or class)

and one or more refined versions of it.and one or more refined versions of it.

The generalization is represented throughThe generalization is represented throughwhat is known aswhat is known as ISA relationshipISA relationship


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S. Nandagopalan, BITS. Nandagopalan, BIT

2626

AccNo Balance

Account

ISA

Int_amt

Dep_amt Rec_amt Od


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2727

Employee(SSN, Name, Addr, Sex)

Fulltime_Employee(Days_worked, Salary )

Contract_Employee( Contract_period, Contract_amount )


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Aggregation is an abstraction throughAggregation is an abstraction throughwhich relationships are treated as higherwhich relationships are treated as higher--level entities.level entities.

The ER diagram lacks in establishing aThe ER diagram lacks in establishing arelation between relationships sets. Usingrelation between relationships sets. Usingour knowledge of ER diagram, we canour knowledge of ER diagram, we canconstruct the relationship among the threeconstruct the relationship among the threeentities Employees, Projects and Machinesentities Employees, Projects and Machineswithwith Works_onWorks_on and Uses relationships.and Uses relationships.


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OS

Type

SSN

Name

AddrHrs

PNo PName

Mid OS

From

To


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Why not use a ternary relationship?Why not use a ternary relationship?

CombineCombine Works_OnWorks_On and Uses to a singleand Uses to a single

relationship set? No! Because, each onerelationship set? No! Because, each onehas its onehas its one attribute(sattribute(s).).

Works_OnWorks_On has Hrs and Uses has From andhas Hrs and Uses has From and

To (dates) attributes.To (dates) attributes.

Hence, the solution is aggregation.Hence, the solution is aggregation.


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Hrs

AggregationAggregation

FromTo


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BranchOffice

SSN

Name

Addr

BranchID BranchName

Staff

Aggregation represents a "hasAggregation represents a "has--a" or "parta" or "part--of"of"

relationship between entity sets where onerelationship between entity sets where one

represents the "whole" and the other therepresents the "whole" and the other the

"part"."part".Example:Example: eacheach BranchOfficeBranchOffice ("whole") has some Staff ("part("whole") has some Staff ("part--of").of").


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StepStep--1:1: I dent ify t he St rong and WeakI dent ify t he St rong and WeakEnt it y SetsEnt it y Sets

StepStep--2:2: I dent i fy theI dent i fy the relevantrelevant Attr ibutesAttr ibutes

StepStep--3:3: I dent ify t he Relationship SetsI dent ify t he Relationship Sets

StepStep--4:4: I dent ify t he Cardinality Rat io andI dent ify t he Cardinalit y Rat io andPart icipat ion Const raint sPart icipat ion Const raint s

StepStep--5:5: I dent i fy the I SI dent i fy the I S--A and HasA and Has--AARelationship SetsRelationship Sets


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Case St udy

Insurance Policy Company Database

Library System

Case St udy

Insurance Policy Company Database

Library System


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WorkFor

BranchID Addr

BranchNamePhone

Hold

Date

Po li cy Name Pr emAmoun t

Tenure

Bonus

MatAmountPolicyNo

Name

Phone

AddrAge

Policy

Branches

Customers

Staff

Interact

Managers SectAssts DevOfficers FullTime Agents

StaffID

StaffName

Age

Phone

M

1

M1

M 1


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S. Nandagoapalan, BIT

Start date

Employees

NameSSN

Bdate

Sex

Addr

SalaryDName

WorksFor Departments

Manages

DNumber

Locations

Supervises

Name

Has

Dependents

Sex

Birth_date

WorksOn

Hours

No. of Employees

CONTROLS

PName

PNumberLocation

Projects

N 1

11

NM

1

NRelationship

N

1


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ISBNTitle

Author

PublPrice

AquisNo DatePur

Date

Custid

Name

BirthDate

ResNo ResDate

Priority

Dept Desig

Salary

Branch Fee

1 M

M

M1

1

Library System


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Design an ER diagram for the following problem:Design an ER diagram for the following problem:

It is required to store the information regarding Students, TeacIt is required to store the information regarding Students, Teachers,hers,and Subjects in an Engineering Department of a College orand Subjects in an Engineering Department of a College orUniversity.University.

Every student belongs to a particular semester. There are eightEvery student belongs to a particular semester. There are eight

semester in each discipline.semester in each discipline. A minimum of 15 students are required to opt for an electiveA minimum of 15 students are required to opt for an elective

subject.subject.

A teacher may handle more than one subject in any givenA teacher may handle more than one subject in any givensemester. However, for any given semester he/she can handle asemester. However, for any given semester he/she can handle amaximum of two theory and four practical subjects. It is requiremaximum of two theory and four practical subjects. It is required tod tostore the salary and leave details of the teachers.store the salary and leave details of the teachers.

We need to maintain the attendance and class marks of everyWe need to maintain the attendance and class marks of everystudent in each semester.student in each semester.

You may assume any other details not supplied with this questionYou may assume any other details not supplied with this question..


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A Bank wants to computerize all of its transactions. ItA Bank wants to computerize all of its transactions. Itoffers the following account types: Savings bank (SB),offers the following account types: Savings bank (SB),Recurring deposit (RD), Fixed deposit (FD)Recurring deposit (RD), Fixed deposit (FD)

The Bank also wishes to keep track of loans given to theThe Bank also wishes to keep track of loans given to the

customers. Identify the entities and its attributes with allcustomers. Identify the entities and its attributes with allpossible relationships. Write the ER diagram and statepossible relationships. Write the ER diagram and stateclearly the assumptions that you make. The followingclearly the assumptions that you make. The followingassumptions may be considered:assumptions may be considered:

A customer can have only one type of account.A customer can have only one type of account.

Joint accounts are not allowed. Loans can be taken onlyJoint accounts are not allowed. Loans can be taken onlywhen the customer has at least one of the accountwhen the customer has at least one of the accounttypes.types.


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Amazon.comAmazon.com has decided to reorganize its database. Informationhas decided to reorganize its database. Informationabout users, books and sales are stored. Amazon stores as muchabout users, books and sales are stored. Amazon stores as muchinformation as possible on user activity, in order to analyze aninformation as possible on user activity, in order to analyze anddimprove its site. Below are few requirements:improve its site. Below are few requirements:

A user has a unique id, name, password and a single email addresA user has a unique id, name, password and a single email address.s.

Amazon contacts users periodically by email, so it is importantAmazon contacts users periodically by email, so it is important totoknow whether a userknow whether a users email has been bouncing back messages ands email has been bouncing back messages andwhether the user is willing to be spammed.whether the user is willing to be spammed.

The last date of a userThe last date of a users visit is stored, so that Amazon can displays visit is stored, so that Amazon can displayto the user a list of items that are new to the site since his lto the user a list of items that are new to the site since his last visit.ast visit.

Books have an ISBN number, title, author's name, publisher's namBooks have an ISBN number, title, author's name, publisher's nameeand cost.and cost.

For each sale, Amazon stores the date of sale, the items bought,For each sale, Amazon stores the date of sale, the items bought, thethecustomer (that has to be a user), his/her address (street, numbecustomer (that has to be a user), his/her address (street, number,r,city, state, country,city, state, country, zipcodezipcode), telephone number, and credit), telephone number, and credit--cardcardnumber.number.


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Users who have bought at least one book, can place commentsUsers who have bought at least one book, can place commentsabout every book (although it is not a book that he has bought aabout every book (although it is not a book that he has bought att

Amazon), by giving a rate to the book from 1 to 10.Amazon), by giving a rate to the book from 1 to 10.

Amazon stores, for each comment, the content of the comment andAmazon stores, for each comment, the content of the comment andthe percentage of users who were helped by this comment.the percentage of users who were helped by this comment.

A book can be on a "A book can be on a "wishwish--listlist" of a user. This is a book that the user" of a user. This is a book that the userwould like to buy at Amazon. Books from awould like to buy at Amazon. Books from a wishwish--listlist can be boughtcan be boughtfor the user by himself, or by a friend. The friend has also tofor the user by himself, or by a friend. The friend has also to be abe auser of Amazon. Amazon wants to keep track of whether books on auser of Amazon. Amazon wants to keep track of whether books on awishwish--listlist were bought, and by whom they were bought.were bought, and by whom they were bought.

Draw an entity relationship diagram to model the informationDraw an entity relationship diagram to model the informationdescribed above. Remember to put constraints, key attributes, etdescribed above. Remember to put constraints, key attributes, etc.c.

If you use the ISA relationship, state any covering and overlapIf you use the ISA relationship, state any covering and overlapconstraints that hold. Make any necessary and logical assumptionconstraints that hold. Make any necessary and logical assumptions.s.State any such assumptions clearly.State any such assumptions clearly.


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2.ER Diagram