Chapter 8 Normalization. © 2001 The McGraw-Hill Companies, Inc. All rights reserved....

Chapter 8Chapter 8Normalization

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Outline Outline

Modification anomaliesFunctional dependenciesMajor normal formsRelationship independencePractical concerns

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Modification AnomaliesModification Anomalies

Unexpected side effectInsert, modify, and delete more data than

desiredCaused by excessive redundanciesStrive for one fact in one place

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Big University Database TableBig University Database Table

StdSSN StdClass OfferNo OffYear Grade CourseNo CrsDesc

S1 JUN O1 2000 3.5 C1 DB S1 JUN O2 2000 3.3 C2 VB S2 JUN O3 2000 3.1 C3 OO S2 JUN O2 2000 3.4 C2 VB

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Functional DependenciesFunctional Dependencies

Constraint on the possible rows in a tableValue neutral like FKs and PKsAssertedUnderstand business rules

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FD DefinitionFD Definition

X YX (functionally) determines YX: left-hand-side (LHS) or determinantFor each X value, there is at most one Y

valueSimilar to candidate keys

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FD Diagrams and ListsFD Diagrams and Lists

StdSSN StdCity StdClass OfferNo OffTerm OffYear EnrGradeCourseNo CrsDesc

StdSSN StdCity, StdClass

OfferNo OffTerm, OffYear, CourseNo, CrsDesc

CourseNo CrsDesc

StdSSN, OfferNo EnrGrade

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FDs in DataFDs in Data

• Prove non-existence (but not existence) by looking at data

• Two rows that have the same X value but a different Y value

StdSSN StdClass OfferNo OffYear Grade CourseNo CrsDesc

S1 JUN O1 2000 3.5 C1 DB S1 JUN O2 2000 3.3 C2 VB S2 JUN O3 2000 3.1 C3 OO S2 JUN O2 2000 3.4 C2 VB

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NormalizationNormalization

Process of removing unwanted redundancies

Apply normal forms– Identify FDs– Determine whether FDs meet normal form– Split the table to meet the normal form if there

is a violation

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Relationships of Normal FormsRelationships of Normal Forms1NF

2NF

3NF/BCNF

4NF

5NF

DKNF

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1NF1NF

Starting point for SQL2 databasesNo repeating groups: flat rows

StdSSN StdClass OfferNo OffYear Grade CourseNo CrsDesc

S1 JUN O1 2000 3.5 C1 DB O2 2000 3.3 C2 VB S2 JUN O3 2000 3.1 C3 OO O2 2000 3.4 C2 VB

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Combined Definition of Combined Definition of 2NF/3NF2NF/3NFKey column: candidate key or part of

candidate keyAnalogy to the traditional justice oathEvery nonkey depends on a key, the whole

key, and nothing but the keyUsually taught as separate definitions

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2NF2NF

Every nonkey column depends on a whole key, not part of a key

Violations– Part of key nonkey– Violations only for combined keys

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2NF Example2NF Example

Many violations for the big university database table– StdSSN StdCity, StdClass– OfferNo OffTerm, OffYear, CourseNo,

CrsDescSplitting the table

– UnivTable1 (StdSSN, StdCity, StdClass) – UnivTable2 (OfferNo, OffTerm, OffYear,

CourseNo, CrsDesc)

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3NF3NF

Every nonkey column depends only on a key not on nonkey columns

Violations: Nonkey NonkeyAlternative formulation

– No transitive FDs– A B, B C then A C– OfferNo CourseNo, CourseNo CrsDesc

then OfferNo CrsDesc

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3NF Example3NF Example

One violation in UnivTable2– CourseNo CrsDesc

Splitting the table– UnivTable2-1 (OfferNo, OffTerm, OffYear,

CourseNo, CrsDesc)– UnivTable2-2 (CourseNo, CrsDesc)

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BCNFBCNF

Every determinant must be a candidate keySimpler definitionApply with simple synthesis procedureSpecial case not covered by 3NF

– Part of key Part of key– Special case is not common

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BCNF ExampleBCNF Example

Many violations for the big university database table– StdSSN StdCity, StdClass– OfferNo OffTerm, OffYear, CourseNo,

CrsDesc– CourseNo CrsDesc

Splitting into four tables

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Simple Synthesis ProcedureSimple Synthesis Procedure

1. Eliminate extraneous columns from the LHSs.

2. Remove derived FDs. 3. Arrange the FDs into groups with each

group having the same determinant. 4. For each FD group, make a table with the

determinant as the primary key.5. Merge tables in which one table contains

all columns of the other table.

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Simple Synthesis ExampleSimple Synthesis Example

Step 1: no extraneous columnsStep 2: eliminate OfferNo CrsDescStep 3: already arranged by LHSStep 4: four tables (Student, Enrollment,

Course, Offering)Step 5: no redundant tables

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Relationship Independence and Relationship Independence and 4NF4NFM-way relationship that can be derived

from binary relationships Split into binary relationshipsSpecialized problem4NF does not involve FDs

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Relationship Independence Relationship Independence ProblemProblem

StdSSNStdName

StudentOfferNoOffLocation

Offering

TextNoTextTitle

Textbook

EnrollStd-Enroll

Offer-Enroll

Text-Enroll

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Relationship Independence Relationship Independence SolutionSolution

StdSSNStdName

Student

OfferNoOffLocation

Offering

TextNoTextTitle

Textbook

Enroll Orders

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MVDs and 4NFMVDs and 4NF

MVD: difficult to identify– A B | C (multi-determines)– A associated with a collection of B and C

values– B and C are independent– Nontrivial MVD: not also an FD

4NF: no nontrivial MVDs

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Higher Level Normal FormsHigher Level Normal Forms

5NF for M-way relationshipsDKNF: absolute normal formDKNF is an ideal, not a practical normal

form

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Role of NormalizationRole of Normalization

Refinement– Use after ERD– Apply to table design or ERD

Initial design– Record attributes and FDs– No initial ERD– May reverse engineer an ERD

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Normalization ObjectiveNormalization Objective

Update biasedNot a concern for databases without

updates (data warehouses)Denormalization

– Purposeful violation of a normal form– Some FDs may not cause anomalies– May improve performance

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SummarySummary

Beware of unwanted redundanciesFDs are important constraintsStrive for BCNFUse a CASE tool for large problemsImportant tool of database developmentFocus on the normalization objective