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L15:Normalization

CS3200 Databasedesign(sp18 s2)https://course.ccs.neu.edu/cs3200sp18s2/3/12/2018

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Ryan'squestion:ParkingTickets:ERDiagram

STID LName FName PhoneNo StLic LicNo

Code Fine

Ticketnr Date Code@ STID@

Student

TicketCode

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Violation

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Ryan'squestion:ParkingTickets:ERDiagram

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Decompositions

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Recap:Decomposetoremoveredundancies

• Wesawthatredundanciesinthedata(“badFDs”)canleadtodataanomalies

• Wedevelopedmechanismstodetectandremoveredundanciesbydecomposingtablesinto3NF orBCNF- BCNF decompositionisstandardpractice- verypowerful&widelyused!

• However,sometimesdecompositionscanleadtomoresubtleunwantedeffects…

Whendoesthishappen?

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DecompositionsinGeneral

R1 =theprojection ofRonA1,...,An,B1,...,Bm

R(A1,...,An,B1,...,Bm,C1,...,Cp)

R1(A1,...,An,B1,...,Bm) R2(A1,...,An,C1,...,Cp)

R2 =theprojection ofRonA1,...,An,C1,...,Cp

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TheoryofDecomposition

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I.e.itisaLosslessdecomposition

Sometimesadecompositionis“correct”

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LossyDecomposition

Name CategoryGizmo Gadget

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What’swronghere?

Howeversometimesitisn’tName Price Category

Gizmo 19.99 GadgetOneClick 24.99 Camera

Gizmo 19.99 Camera

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LosslessDecompositions

AdecompositionRto(R1,R2)islossless ifR=R1JoinR2

R(A1,...,An,B1,...,Bm,C1,...,Cp)

R1(A1,...,An,B1,...,Bm) R2(A1,...,An,C1,...,Cp)

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LosslessDecompositions

BCNFdecompositionisalwayslossless.Why?

Note:don’tneed{A1,...,An}à {C1,...,Cp}

If {A1,...,An}à {B1,...,Bm}Thenthedecompositionislossless

R(A1,...,An,B1,...,Bm,C1,...,Cp)

R1(A1,...,An,B1,...,Bm) R2(A1,...,An,C1,...,Cp)

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AproblemwithBCNF

Note:Thisishistoricallyinaccurate,butitmakesiteasiertoexplain

Problem:ToenforceaFD,mustreconstructoriginalrelation—oneachinsert!

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AProblemwithBCNF{Unit} à {Company}{Company,Product} à {Unit}

WedoaBCNFdecompositionona“bad”FD:{Unit}+ = {Unit, Company}

WelosetheFD{Company,Product} à {Unit}!!

Unit Company Product… … …

Unit Company… …

Unit Product… …

{Unit} à {Company}

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SoWhyisthataProblem?

Noproblemsofar.Alllocal FD’saresatisfied.

Unit CompanyGalaga99 NEUBingo NEU

Unit ProductGalaga99 DatabasesBingo Databases

Unit Company ProductGalaga99 NEU DatabasesBingo NEU Databases

Let’sputallthedatabackintoasingletableagain:

{Unit} à {Company}

ViolatestheFD{Company,Product} à {Unit}!!

{Unit} à {Company}{Company,Product} à {Unit}

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TheProblem

• WestartedwithatableRandFDsF

• WedecomposedRintoBCNFtablesR1,R2,…withtheirownFDsF1,F2,…

• Weinsertsometuplesintoeachoftherelations—whichsatisfytheirlocalFDsbutwhenreconstructitviolatessomeFDacrosstables!

PracticalProblem:ToenforceFD,mustreconstructR—oneachinsert!

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PossibleSolutions

• Variouswaystohandlesothatdecompositionsarealllossless/noFDslost- Forexample3NF: stopshortoffullBCNF decompositions.

• Usuallyatradeoffbetweenredundancy/dataanomaliesandFDpreservation…

BCNFstillmostcommon- withadditionalstepstokeeptrackoflostFDs…

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4NF and higher

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3NFMotivation

ArelationRisin3rdnormalformif:Wheneverthereisanontrivialdep.A1,A2,...,An® BforR,then{A1,A2,...,An }isasuper-keyforR,orBispartofakey.

Tradeoffs:BCNF:noanomalies,butmaylosesomeFDs3NF:keepsallFDs,butmayhavesomeanomalies

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Motivationof4NFandhigher

Course Lecturer Book

cse444 Alexandra Completebook

cse444 Wolfgang Completebook

cse444 Alexandra Cowbook

Assumeforeachcourse,wecanindependentlychoosealecturerandabook.Whatistheproblem?

Multi-valueddependency(MVD)Course®® Lecturer:Ineverylegalinstance,eachCourse valueisassociatedwithasetofLecturer valuesandthissetisindependentofthevaluesintheotherattributes(hereBook).

Classes

cse444 Wolfgang Cowbook