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433 L15: Normalization CS3200 Database design (sp18 s2) https://course.ccs.neu.edu/cs3200sp18s2/ 3/12/2018
Transcript of DB - ER - 180312 · 434 Announcements! • Keep bringing your name plates J • Verify your grades...
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L15:Normalization
CS3200 Databasedesign(sp18 s2)https://course.ccs.neu.edu/cs3200sp18s2/3/12/2018
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Announcements!
• KeepbringingyournameplatesJ• VerifyyourgradesandfeedbackonBB.Ifsomethingisunclearorconfusing,ordisplaysincorrectly,pleaseletusknow(e.g.,viaPiazzainstructorsonly)
• P2 ispostedandupdatedcalendar• Exam2 nextweek:contentiseverythingseenuntilthisThursday:setuplikeforExam1:laptopSQL+paperdatabasedesign+papertransactions
• Outlinetoday- HW4,Projects- Decompositions- Transactions!
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Ryan'squestion:ParkingTickets:ERDiagram
STID LName FName PhoneNo StLic LicNo
Code Fine
Ticketnr Date Code@ STID@
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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
Name Price CategoryGizmo 19.99 Gadget
OneClick 24.99 CameraGizmo 19.99 Camera
Name PriceGizmo 19.99
OneClick 24.99Gizmo 19.99
Name CategoryGizmo Gadget
OneClick CameraGizmo Camera
I.e.itisaLosslessdecomposition
Sometimesadecompositionis“correct”
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LossyDecomposition
Name CategoryGizmo Gadget
OneClick CameraGizmo Camera
Price Category19.99 Gadget24.99 Camera19.99 Camera
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
