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Chapter 7Chapter 7Normalization
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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 EnrGrade CourseNo CrsDesc
S1 JUN O1 2003 3.5 C1 DB S1 JUN O2 2003 3.3 C2 VB S2 JUN O3 2003 3.1 C3 OO S2 JUN O2 2003 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 EnrGrade CourseNo CrsDesc
S1 JUN O1 2003 3.5 C1 DB S1 JUN O2 2003 3.3 C2 VB S2 JUN O3 2003 3.1 C3 OO S2 JUN O2 2003 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 SQL:1999 databasesNo repeating groups: flat rows
StdSSN StdClass OfferNo OffYear EnrGrade CourseNo CrsDesc
S1 JUN O1 2003 3.5 C1 DB O2 2003 3.3 C2 VB S2 JUN O3 2003 3.1 C3 OO O2 2003 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 non key 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 ExampleMany 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 non key columns
Violations: Nonkey NonkeyAlterative formulation
– No transitive FDs– A B, B C then A C– OfferNo CourseNo, CourseNo CrsDesc
then OfferNo CrsDesc
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3NF Example3NF ExampleOne violation in UnivTable2
– CourseNo CrsDescSplitting the table
– UnivTable2-1 (OfferNo, OffTerm, OffYear, CourseNo)
– UnivTable2-2 (CourseNo, CrsDesc)
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BCNFBCNF
Every determinant must be a candidate key.
Simpler definitionApply with simple synthesis procedureSpecial cases not covered by 3NF
– Part of key Part of key– Nonkey Part of key– Special cases are not common
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BCNF ExampleBCNF ExampleMany violations for the big university
database table– StdSSN StdCity, StdClass– OfferNo OffTerm, OffYear, CourseNo– 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
Begin with FDs shown in Slide 7Step 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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Multiple Candidate KeysMultiple Candidate Keys
Multiple candidate keys do not violate either 3NF or BCNF
Step 5 of the Simple Synthesis Procedure creates tables with multiple candidate keys.
You should not split a table just because it contains multiple candidate keys.
Splitting a table unnecessarily can slow query performance.
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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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Extension to the Relationship Extension to the Relationship Independence SolutionIndependence Solution
StdSSNStdName
StudentOfferNoOffLocation
Offering
TextNoTextTitle
Textbook
Enroll Orders
PurchaseStd-Purch
Offer-Purch
Text-Purch
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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– Non trivial MVD: not also an FD
4NF: no non trivial MVDs
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MVD RepresentationMVD Representation
A B C A1 B1 C1 A1 B2 C2 A1 B2 C1 A1 B1 C2
A B | C
OfferNo StdSSN TextNo O1 S1 T1 O1 S2 T2 O1 S2 T1 O1 S1 T2
OfferNo StdSSN | TextNo
Given the two rows above the line, the two rows below the line arein the table if the MVD is true.
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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 after
normalization
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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
