UNITII–A.ENTITYRELATIONSHIPMODEL

Agenda•  En0ty&En0tySets

•  A6ributes•  Rela0onship&Rela0onshipSets

•  Constraints–MappingCardinali0es,Par0cipa0onConstraints,Keys

•  E-RDiagrams&DesignofDatabaseschema

•  DesignIssues

•  WeakEn0tySets•  ExtendedE-RFeatures–Specializa0on,Generaliza0on,Aggrega0on

•  Reduc0onofanE-RschematoTables

En0ty&En0tySets•  Adatabasecanbemodeledas:

— acollec0onofen00es,— rela0onshipamongen00es.

•  An en2ty is an object in the real world that is dis0nguishable fromanotherobjects.

•  Everya6ributeisdefinedbyitssetofvaluescalleddomain.Forexample,in a school database, a student is considered as an en0ty. Student hasvariousa6ributeslikename,age,class,etc.

•  An en(ty set is a set of en00es of the same type that share the sameproper0es.

— Example:setofallpersons,companies,trees,holidays

En0tySetscustomerandloan

customer-idcustomer-customer-customer-loan-amountnamestreetcitynumber

A6ributes•  An en0ty is represented by a set of a6ributes, that is descrip0ve

proper0espossessedbyallmembersofanen0tyset.

Example: customer = (customer-id, customer-name, customer-street, customer-city) loan = (loan-number, amount)

A6ributesTypesofA6ributes:

• Simple a6ribute − Simple a6ributes are atomic values, which cannot bedividedfurther.Example-astudent'sphonenumberisanatomicvalueof10digits.

• Composite a6ribute − Composite a6ributes are made of more than onesimplea6ribute.Example-astudent'scompletenamemayhavefirst_nameandlast_name.

• Deriveda6ribute−Deriveda6ributesarethea6ributesthatdonotexistinthe physical database, but their values are derived from other a6ributespresent in the database. Example - average_salary in a department shouldnotbesaveddirectlyinthedatabase,insteaditcanbederived.Foranotherexample,agecanbederivedfromdata_of_birth.

• Single-value a6ribute − Single-value a6ributes contain single value.Example−Social_Security_Number.

• Mul2-valuea6ribute −Mul0-valuea6ributesmaycontainmore thanonevalues. Example − a person can have more than one phone number,email_address,etc.

Example

Rela0onship&Rela0onshipSets•  Rela2onship:The associa0on among en00es is called a rela0onship. For example, an

employeeworks_at a department, a student enrolls in a course. Here,Works_atandEnrollsarecalledrela0onships.

•  Rela2onshipSet:Asetofrela0onshipsofsimilartypeiscalledarela0onshipset.Likeen00es,

a rela0onship too can have a6ributes. These a6ributes arecalleddescrip2vea6ributes.

Rela0onshipSets(Cont.)•  Ana#ributecanalsobepropertyofarela0onshipset.•  Forinstance,thedepositorrela0onshipsetbetweenen0tysetscustomer

andaccountmayhavethea6ributeaccess-date

Rela0onshipSetswithA6ributes

Rela0onshipSets•  Rela0onshipsetsthatinvolvetwoen0tysetsarebinary(ordegreetwo).

Generally,mostrela0onshipsetsinadatabasesystemarebinary.

•  Rela0onshipsetsmayinvolvemorethantwoen0tysets.

•  Rela0onshipsbetweenmorethantwoen0tysetsarerare.

�  E.g.Supposeemployeesofabankmayhavejobs(responsibili0es)atmul0plebranches,withdifferent jobsatdifferentbranches. Thenthere is a ternary rela0onship set between en0ty sets employee,jobandbranch

Constraints:MappingCardinali0es•  Referstonumberofen2tysetsthatpar2cipateinarela2onshipset.•  Forabinaryrela0onshipsetthemappingcardinalitymustbeoneofthe

followingtypes:

— Onetoone— Onetomany

— Manytoone

— Manytomany

MappingCardinali0es

One to one One to many

Note: Some elements in A and B may not be mapped to any elements in the other set

MappingCardinali0es

Many to one Many to manyNote: Some elements in A and B may not be mapped to any elements in the other set

MappingCardinali0esExample•  Rela0onshipfromaccounttocustomerismanytoone,•  Likewise,customertoaccountisonetomany

E-RDiagrams•  Rectanglesrepresenten0tysets.•  Diamondsrepresentrela0onshipsets.•  Lineslinka6ributestoen0tysetsanden0tysetstorela0onshipsets.•  Ellipsesrepresenta6ributes•  Doubleellipsesrepresentmul0valueda6ributes.

•  Dashedellipsesdenotederiveda6ributes.•  Underlineindicatesprimarykeya6ributes

E-RDiagramWithComposite,Mul0valued,andDerivedA6ributes

CompositeA6ributes

Mul0valuedA6ributes

DerivedA6ributes

PrimaryKey

ComponentA6ributes

Roles•  En0tysetsofarela0onshipneednotbedis0nct

•  The labels “manager” and “worker” are called roles; they specify howemployeeen00esinteractviatheworks-forrela0onshipset.

•  Roles are indicated in E-R diagrams by labeling the lines that connectdiamondstorectangles.

•  Role labels are op0onal, and are used to clarify seman0cs of therela0onship

CardinalityConstraints•  Weexpresscardinalityconstraintsbydrawingeitheradirectedline(→),

signifying “one,” or an undirected line (—), signifying “many,” betweentherela0onshipsetandtheen0tyset.

•  One-to-onerela2onship:— A customer is associatedwith atmost one loan via the rela0onship

borrower— Aloanisassociatedwithatmostonecustomerviaborrower

One-To-ManyRela0onship•  In the one-to-many rela0onship a loan is associated with at most one

customerviaborrower,acustomerisassociatedwithseveral(including0)loansviaborrower

Many-To-OneRela0onships•  Inamany-to-onerela0onshipaloanisassociatedwithseveralcustomers

viaborrower,acustomerisassociatedwithatmostoneloanviaborrower

Many-To-ManyRela0onship•  Acustomerisassociatedwithseveralloansviaborrower

•  Aloanisassociatedwithseveralcustomersviaborrower

Par0cipa0onofanEn0tySetinaRela0onshipSet

•  Totalpar2cipa2on(indicatedbydoubleline):everyen0tyintheen0tysetpar0cipatesinatleastonerela0onshipintherela0onshipset

—  E.g.par0cipa0onofloaninborroweristotal—  everyloanmusthaveacustomerassociatedtoitviaborrower

•  Par2alpar2cipa2on:someen00esmaynotpar0cipateinanyrela0onshipintherela0onshipset

—  E.g.par0cipa0onofcustomerinborrowerispar0al

Keys•  SuperKey−Asuperkeyofanen0tysetisasetofoneormorea6ributes

whosevaluesuniquelydetermineeachen0ty.

•  CandidateKey−Aminimalsuperkeyiscalledacandidatekey.Anen0tysetmayhavemorethanonecandidatekey. Inordertobeeligible foracandidatekeyitmustpasscertaincriteria.

—  Itmustcontainuniquevalues

—  Itmustnotcontainnullvalues

—  Itcontainstheminimumnumberoffieldstoensureuniqueness

—  Itmustuniquelyiden0fyeachrecordinthetable

•  PrimaryKey−Aprimarykeyisoneofthecandidatekeyschosenbythedatabasedesignertouniquelyiden0fytheen0tyset.

•  Foreign Key – A foreign key is a column or group of columns in arela0onaldatabasetablethatprovidesalinkbetweendataintwotables.It acts as a cross-reference between tables because it references theprimarykeyofanothertable,therebyestablishingalinkbetweenthem.

Example1

Candidate Key

Example2•  CandidateKey:

•  Asanexamplewemighthaveastudent_idthatuniquelyiden0fiesthestudentsinastudenttable.Thiswouldbeacandidatekey.Butinthesametablewehavethestudent’sfirstnameand lastname that also,when combined,uniquely iden0fythestudentinastudenttable.Thesewouldbothbecandidatekeys.

Example•  PrimaryKey:

Example•  ForeignKey:

TABLE:student

TABLE: course

DesignIssues•  Useofen0tysetsvs.a6ributes

Choicemainlydependsonthestructureoftheenterprisebeingmodeled,andontheseman0csassociatedwiththea6ributeinques0on.

•  Useofen0tysetsvs.rela0onshipsetsPossibleguidelineistodesignatearela0onshipsettodescribeanac0onthatoccursbetweenen00es

•  Binaryversusn-aryrela0onshipsetsAlthoughitispossibletoreplaceanynonbinary(n-ary,forn>2)rela0onshipsetbyanumberofdis0nctbinaryrela0onshipsets,an-aryrela0onshipsetshowsmoreclearlythatseveralen00espar0cipateinasinglerela0onship.

•  Placementofrela0onshipa6ributes

WeakEn0tySets•  Anen0ty set thatdoesnothaveaprimarykey is referred toasaweak

en0tyset.

•  The existence of a weak en0ty set depends on the existence of aiden0fyingen0tyset

—  itmust relate to the iden0fying en0ty set via a total, one-to-manyrela0onshipsetfromtheiden0fyingtotheweaken0tyset

—  Iden0fyingrela0onshipdepictedusingadoublediamond

•  The discriminator (or par0al key) of a weak en0ty set is the set ofa6ributesthatdis0nguishesamongalltheen00esofaweaken0tyset.

•  Theprimarykeyofaweaken0tysetisformedbytheprimarykeyofthestrong en0ty set on which the weak en0ty set is existence dependent,plustheweaken0tyset’sdiscriminator.

OR•  PrimaryKeyof theweaken0ty set=Discriminator+Primarykeyof

strongen0tyset

WeakEn0tySets(Cont.)•  Wedepictaweaken0tysetbydoublerectangles.

•  Weunderlinethediscriminatorofaweaken0tysetwithadashedline.•  payment-number–discriminatorofthepaymenten0tyset•  Primarykeyforpayment–(loan-number,payment-number)

WeakEn0tySets(Cont.)•  Note:theprimarykeyofthestrongen0tysetisnotexplicitlystoredwith

theweaken0tyset,sinceitisimplicitintheiden0fyingrela0onship.

•  If loan-numberwere explicitly stored,payment could bemade a strongen0ty, but then the rela0onship between payment and loan would beduplicated by an implicit rela0onship defined by the a6ribute loan-numbercommontopaymentandloan

ExtendedE-RFeatures:Specializa0on•  Itisatop-downapproachinwhichonehigherlevelen0tycanbebroken

down into two lower level en0ty. In specializa0on, some higher levelen00esmaynothavelower-levelen0tysetsatall.

•  Depicted by a triangle component labeled ISA (E.g. customer “is a”person).

•  A6ributeinheritance–alower-levelen0tysetinheritsallthea6ributesandrela0onshippar0cipa0onofthehigher-levelen0tysettowhichitislinked.

Specializa0onExample

Generaliza0on•  Generaliza2onisabo6om-upapproachinwhichtwolowerlevelen00es

combinetoformahigher levelen0ty. Ingeneraliza0on,thehigher levelen0ty can also combine with other lower level en0ty to make furtherhigherlevelen0ty.

•  For example, pigeon, house sparrow, crow and dove can all begeneralizedasBirds.

•  Specializa0onandgeneraliza0onaresimpleinversionsofeachother;theyarerepresentedinanE-Rdiagraminthesameway.

•  GENERALIZATION:

•  SPECIALIZATION:

Aggrega0on•  Aggrega0on is an abstrac0on in which rela0onship sets are treated as

higherlevelen0tysets.

•  Here,arela0onshipsetisembeddedinsideanen0tyset,andtheseen0tysetscanpar0cipateinrela0onships.

E-RDesignDecisions•  Theuseofana6ributeoren0tysettorepresentanobject.

•  Whether a real-world concept is best expressed by an en0ty set or arela0onshipset.

•  Theuseofaternaryrela0onshipversusapairofbinaryrela0onships.

•  Theuseofastrongorweaken0tyset.

•  Theuseofspecializa0on/generaliza0on–contributestomodularityinthedesign.

•  Theuseofaggrega0on–cantreattheaggregateen0tysetasasingleunitwithoutconcernforthedetailsofitsinternalstructure.

Example–ERDiagram•  The branch en0ty set, with a6ributes branch-name, branch-city, and

assets.•  The customer en0ty set, with a6ributes customer-id, customer-name,

customerstreet,andcustomer-city.•  The employee en0ty set, with a6ributes employee-id, employee-name,

telephonenumber, salary, andmanager. Addi0onal descrip0ve featuresare themul0valued a6ributedependent-name, thebase a6ribute start-date,andthederiveda6ributeemployment-length.

•  Two account en0ty sets—savings-account and checking-account—withthe common a6ributes of account-number and balance; in addi0on,savings-accounthasthea6ribute interest-rateandchecking-accounthasthea6ributeoverdra:-amount.

•  The loan en0ty set, with the a6ributes loan-number, amount, andorigina;ngbranch.

•  The weak en0ty set loan-payment, with a6ributes payment-number,paymentdate,andpayment-amount.

Example–ERDiagramcontd..•  borrower,amany-to-manyrela0onshipsetbetweencustomerandloan.•  loan-branch, a many-to-one rela0onship set that indicates in which

branchaloanoriginated.loan-payment,aone-to-manyrela0onshipfromloantopayment,whichdocumentsthatapaymentismadeonaloan.

•  depositor, with rela0onship a6ribute access-date, a many-to-manyrela0onship set between customer and account, indica0ng that acustomerownsanaccount.

•  cust-banker,withrela0onshipa6ribute type,amany-to-onerela0onshipsetexpressingthatacustomercanbeadvisedbyabankemployee,andthat a bank employee can advise one ormore customers.works-for, arela0onshipsetbetweenemployeeen00eswithroleindicatorsmanagerandworker;themappingcardinali0esexpressthatanemployeeworksforonly one manager and that a manager supervises one or moreemployees.

SummaryofSymbolsUsedinE-RNota0on

SummaryofSymbols(Cont.)

Alterna0veE-RNota0ons

Reduc0onofanE-RSchematoTables•  Primary keys allow en0ty sets and rela0onship sets to be expressed

uniformlyastableswhichrepresentthecontentsofthedatabase.•  Adatabasewhich conforms toanE-Rdiagramcanbe representedbya

collec0onoftables.

•  Foreachen0ty set and rela0onship set there is aunique tablewhich isassignedthenameofthecorrespondingen0tysetorrela0onshipset.

•  Each table has a number of columns (generally corresponding toa6ributes),whichhaveuniquenames.

•  Conver0ng an E-R diagram to a table format is the basis for deriving arela0onaldatabasedesignfromanE-Rdiagram.

Represen0ngEn0tySetsasTables•  Astrongen0tyset(asethavingprimarykey)reducestoatablewiththe

samea6ributes.

Representa0onofWeakEn0tySet•  WeakEn0tySetCannotexistsalone

•  Tobuildatable/schemaforweaken0tyset— Construct a table with one column for each a6ribute in the weak

en0tyset

— Remembertoincludediscriminator

— Augmentoneextracolumnontherightsideofthetable,putintheretheprimarykeyoftheStrongEn0tySet(theen0tysetthattheweaken0tysetisdependingon)

— PrimaryKeyoftheweaken0tyset=Discriminator+foreignkey

Represen0ngWeakEn0tySets

WeakEn0ty

Represen0ngCompositeA6ributes•  Compositea6ributesarefla6enedoutbycrea0ngaseparatea6ributefor

eachcomponenta6ribute

customer-id

name.first-name

name.middle-ini(al

name.last-name

Represen0ngMul0valuedA6ributes•  Amul0valueda6ributeMofanen0tyEisrepresentedbyaseparatetable

EM—  Table EM has a6ributes corresponding to the primary key of E and an

a6ributecorrespondingtomul0valueda6ributeM

TABLE:Customer-phone-number

customer-id

phone-number

Represen0ngRela0onshipSet•  One-to-onerela2onshipwithoutTotalPar2cipa2on:

— Build a table with two columns, one column for each par0cipa0ngen0ty set’s primary key. Add successive columns, one for eachdescrip0vea6ributesoftherela0onshipset(ifany).

TABLE:borrower

— Primarykeycanbeeithercustomer-idorloan-number

customer-id

loan-number

Represen0ngRela0onshipSet•  One-to-onerela2onshipwithoneen2tysethavingTotalPar2cipa2on:

— Augmentoneextracolumnontherightsideofthetableoftheen0tysetwithtotalpar0cipa0on,putintheretheprimarykeyoftheen0tysetwithoutcompletepar0cipa0onaspertotherela0onship.

customer-id

customer-name

customer-street

customer-city

loan-number

Represen0ngRela0onshipSet•  ForOne-to-Manyrela2onshipwithoutTotalPar2cipa2on:

—  Samethingasone-to-one—  theprimarykeyofthe“many”en0tysetbecomestherela0on’sprimarykey.

•  For One-to-Many/Many-to-One rela2onship with one en2ty set having TotalPar2cipa2onon“many”side:—  Augmentoneextracolumnontherightsideofthetableoftheen0tyseton

the “many” side, & on the “one” side put in there the primary key of theen0tysetaspertotherela0onship.

customer-id

customer-name

customer-street

customer-city

loan-number

Represen0ngRela0onshipSet•  ForMany-to-Manyrela2onship:

— Samethingasone-to-onerela0onshipwithouttotalpar0cipa0on.

— Primary key of this new schema is the union of the foreign keys ofbothen0tysets.

Summary:Represen0ngRela0onshipSet•  Rela2onshipset.Theunionoftheprimarykeysoftherelateden0tysets

becomesasuperkeyoftherela0on.

— For binary many-to-one rela0onship sets, the primary key of the“many”en0tysetbecomestherela0on’sprimarykey.

— For one-to-one rela0onship sets, the rela0on’s primary key can bethatofeitheren0tyset.

— For many-to-many rela0onship sets, the union of the primary keysbecomestherela0on’sprimarykey

RedundancyofTables•  Many-to-one and one-to-many rela0onship sets that are total on the

many-sidecanberepresentedbyaddinganextraa6ribute to themanyside,containingtheprimarykeyoftheoneside

•  E.g.: Insteadof crea0ng a table for rela0onship account-branch, add ana6ributebranchtotheen0tysetaccount

RedundancyofTables(Cont.)•  Forone-to-onerela0onshipsets,eithersidecanbechosentoactasthe

“many”side—  Thatis,extraa6ributecanbeaddedtoeitherofthetablescorrespondingto

thetwoen0tysets

But,ifpar0cipa0onispar;alonthemanyside,replacingatablebyanextraa6ributeintherela0oncorrespondingtothe“many”sidecouldresultinnullvalues.

•  Thetablecorrespondingtoarela0onshipsetlinkingaweaken0tysettoitsiden0fyingstrongen0tysetisredundant.—  E.g.Thepaymenttablealreadycontainstheinforma0onthatwouldappearin

the loan-payment table (i.e., the columns loan-number and payment-number). TABLE:payment

TABLE:loan-payment REDUNDANCY

Represen0ngGeneraliza0onTwogeneralapproachesdependingondisjointnessandcompleteness

•  Disjoint: A disjointness constraint requires that an en0ty belong to nomorethanonelower-levelen0tyset.

•  Complete:Acompletenessconstraintrequiresthateveryen0tyinhigher-levelen0tysetisalsoamemberofoneofthelower-levelen0tyset.

1.   Fornon-disjointand/ornon-completeclasshierarchy:–  createatableforeachsuperclassen0tysetaccordingtonormalen0tysettransla0onmethod.

–  Createatableforeachsubclassen0tysetwithacolumnforeachofthea6ributesofthaten0tysetplusoneforeacha6ributesoftheprimarykeyofthesuperclassen0tyset

–  This primary key from super class en0ty set is also used as theprimarykeyforthisnewtable

Example1:Fornon-disjoint•  Showthetablesfornon-disjointmapping(employeecanbestudent).Also

showthecompositea6ributesofaddressinpersonrela0on.

•  person(ID,name,street,city)

•  employee(ID,salary)•  student(ID,tot_credits)

Drawback:gennginforma0onaboutemployeerequiresaccessingtwotables

Example2

SSN SID Status Major GPA 1234 9999 Full CS 2.8 5678 8888 Part EE 3.6

SSN Name Gender 1234 Homer Male 5678 Marge Female

Student

SID Status

Major GPA

Person

Gender

SSN Name

ISA

Drawback:gennginforma0onaboutemployeerequiresaccessingtwotables

non-disjoint

Represen0ngClassHierarchy2.   FordisjointANDcompletemappingclasshierarchy:

— DONOTcreateatableforthesuperclassen0tyset— Createatableforeachsubclassen0tysetincludealla6ributesofthat

subclassen0tysetanda6ributesofthesuperclassen0tyset

Example1:Fordisjoint

•  employee(ID,name,street,city,salary)

•  student(ID,name,street,city,tot_credits)

Example2

SSN Name Cid Street City

1234 John 9999 OldBroad

London

5678 Mary 8888 Queen London

Disjoint and Complete mapping

Notablecreatedforsuperclassen0tyset.

Customer Cid

Street City

Bank

person

SSN Name

ISA Employee

Salary Street

City

SSN Name Salary Street City

1234 John 1000$ OldBroad

London

5678 Mary 2000$ Queen London

9123 Homer 3000$ Polo NewYork

DrawbackifitwouldhavebeenNon-Disjoint:StreetandCitymaybestoredredundantlyforpersonswhoarebothcustomersandemployees

Example1•  ConstructanE-Rdiagramforacar-insurancecompanywhosecustomers

ownoneormorecarseach.Eachcarhasassociatedwith it zero toanynumberofrecordedaccidents.

Example2•  Construct appropriate tables for the E-R diagrams of Car Insurance

company.

Sol.Carinsurancetables:person(driver-id,name,address)

car(license,year,model)

accident(report-number,date,loca0on)

par0cipated(driver-id,license,report-number,damage-amount)

Example3•  ConstructanE-Rdiagramforahospitalwithasetofpa0entsandasetof

medicaldoctors.Associatewitheachpa0entalogofthevarioustestsandexamina0onsconducted.

p-id

d-id

Example4•  ConstructappropriatetablesfortheE-RdiagramofHospital.

Sol.Hospitaltables:pa0ents(p-id,name,insurance,date-admi6ed,date-checked-out)

doctors(d-id,name,specializa0on)

test(test-id,test_name,date,0me,result)

doctor-pa0ent(pa0ent-id,doctor-id)

test-log(test-id,test_name,date,0me,result,p-id)performed-by(test-id,d-id)

dr-pa0ent(p-id,d-id)