Chapter 4 Normalization of Database Tables. 2 Database Tables and Normalization Table is basic...

Chapter 4Chapter 4Normalization of Normalization of Database TablesDatabase Tables

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Database Tables and Database Tables and NormalizationNormalization

Table is basic building block in database Table is basic building block in database designdesign

Table’s structure is of great interestTable’s structure is of great interest Two cases:Two cases:

possible poor table structures in good database possible poor table structures in good database designdesign

Modify existing database with existing poor table Modify existing database with existing poor table structurestructure

Normalization can help recognize a poor table Normalization can help recognize a poor table and convert to good tables with good structureand convert to good tables with good structure

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Database Tables and Database Tables and NormalizationNormalization

Normalization is process for Normalization is process for assigning attributes to entitiesassigning attributes to entities Reduces data redundanciesReduces data redundancies Expending entitiesExpending entities Helps eliminate data anomaliesHelps eliminate data anomalies Produces controlled redundancies to Produces controlled redundancies to

link tableslink tables Cost more processing effortsCost more processing efforts Series steps called normal formsSeries steps called normal forms

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Database Tables and Database Tables and NormalizationNormalization

Normalization stagesNormalization stages 1NF - First normal form1NF - First normal form 2NF - Second normal form2NF - Second normal form 3NF - Third normal form3NF - Third normal form 4NF - Fourth normal form4NF - Fourth normal form

Better in dependency

Worse in performance (I/O)

Business

BioinformaticsStatistical data

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Database Tables and Database Tables and NormalizationNormalization

Example: construction companyExample: construction company Building projectsBuilding projects

Project numberProject number Project nameProject name Employees assignedEmployees assigned ……

EmployeeEmployee Employee numberEmployee number Employee nameEmployee name Job classificationJob classification

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Table 4.1 should be here.

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Figure 4.1 ObservationsFigure 4.1 Observations

PRO_NUM intended to be primary PRO_NUM intended to be primary key, but it contains null values.key, but it contains null values.

Table entries invite data Table entries invite data inconsistenciesinconsistencies

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Figure 4.1 ObservationsFigure 4.1 Observations Table displays data redundancies Table displays data redundancies

which yield the following anomalieswhich yield the following anomalies UpdateUpdate

Modifying JOB_CLASSModifying JOB_CLASS InsertionInsertion

New employee must be assigned project New employee must be assigned project (phantom project)(phantom project)

DeletionDeletion If employee deleted, other vital data lostIf employee deleted, other vital data lost

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Figure 4.2 is insert here.

Repeating group (any project can have a group of data entries) which should not to be appeared in relational table

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Data Organization: 1NFData Organization: 1NF

Figure 4.3

PK PK

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Conversion to 1NFConversion to 1NF

Repeating groups must be Repeating groups must be eliminatedeliminated Proper primary key developedProper primary key developed

Uniquely identifies attribute values (rows)Uniquely identifies attribute values (rows) Combination of PROJ_NUM and Combination of PROJ_NUM and

EMP_NUMEMP_NUM

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Conversion to 1NFConversion to 1NF Repeating groups must be eliminatedRepeating groups must be eliminated

Dependencies can be identifiedDependencies can be identified

A particular relationship between two A particular relationship between two attributes. For a given relation, attribute B is attributes. For a given relation, attribute B is functionally dependent on attribute A if, for functionally dependent on attribute A if, for every valid value of A, that value of A uniquely every valid value of A, that value of A uniquely determines the value of B. determines the value of B.

A functional dependency exists when the value A functional dependency exists when the value of one thing is fully determined by another. For of one thing is fully determined by another. For example, given the relation EMP(empNo, example, given the relation EMP(empNo, empName, sal), attribute empName is empName, sal), attribute empName is functionally dependant on attribute empNo. If functionally dependant on attribute empNo. If we know empNo, we also know the empName. we know empNo, we also know the empName.

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Desirable dependencies based on Desirable dependencies based on primary keyprimary keyLess desirable dependenciesLess desirable dependencies

Partial Partial based on part of composite based on part of composite primary keyprimary key

Transitive Transitive one nonprime attribute one nonprime attribute depends ondepends on another nonprime attribute another nonprime attribute

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Dependency Diagram Dependency Diagram (1NF)(1NF)

Figure 4.4

Above: Desired Dependencies

Below: Less Desired DependenciesComposite primary key

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PROJ_NUM,EMP_NUM PROJ_NAME, EMP_NAME, JOB_CLASS,CHG_HOUR, HOURS

PROJ_NUM PROJ_NAME

DESIRED DEPENDENCIES

EMP_NUM EMP_NAME, JOB_CLASS, CHG_HOUR

PARTIAL DEPENDENCIES

JOB_CLASS -> CHG_HOUR TRANSITIVE DEPENDENCIES

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1NF Summarized1NF Summarized

All key attributes definedAll key attributes defined No repeating groups in tableNo repeating groups in table All attributes dependent on All attributes dependent on

primary keyprimary key

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Conversion to 2NFConversion to 2NF

Start with 1NF format:Start with 1NF format: Write each key component on Write each key component on

separate lineseparate line Write original key on last lineWrite original key on last line Each component is new tableEach component is new table Write dependent attributes after Write dependent attributes after

each keyeach keyPROJECT (PROJ_NUM, PROJ_NAME)EMPLOYEE (EMP_NUM, EMP_NAME, JOB_CLASS, CHG_HOUR)ASSIGN (PROJ_NUM, EMP_NUM, HOURS)

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2NF Conversion Results2NF Conversion ResultsFigure 4.5

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2NF Summarized2NF Summarized

In 1NFIn 1NF Includes no partial dependenciesIncludes no partial dependencies

No attribute dependent on a portion No attribute dependent on a portion of primary keyof primary key

Still possible to exhibit transitive Still possible to exhibit transitive dependencydependency Attributes may be functionally Attributes may be functionally

dependent on nonkey attributesdependent on nonkey attributes

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Conversion to 3NFConversion to 3NF

Create separate table(s) to eliminate Create separate table(s) to eliminate transitive functional dependencies transitive functional dependencies

PROJECT (PROJ_NUM, PROJ_NAME)ASSIGN (PROJ_NUM, EMP_NUM, HOURS)EMPLOYEE (EMP_NUM, EMP_NAME, JOB_CLASS)JOB (JOB_CLASS, CHG_HOUR)

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3NF Summarized3NF Summarized

In 2NFIn 2NF Contains no transitive Contains no transitive

dependenciesdependencies

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Additional DB Additional DB EnhancementsEnhancements

Figure 4.6

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Boyce-Codd Normal Boyce-Codd Normal Form (BCNF)Form (BCNF)

Every determinant in the table is a Every determinant in the table is a candidate keycandidate key Determinant is attribute whose value Determinant is attribute whose value

determines other values in rowdetermines other values in row 3NF table with one candidate key is 3NF table with one candidate key is

already in BCNFalready in BCNF

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3NF Table Not in BCNF3NF Table Not in BCNF

Figure 4.7

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Decomposition of Table Decomposition of Table

Structure to Meet Structure to Meet BCNFBCNF

Figure 4.8

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Example: BCNF Example: BCNF conversionconversion

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Decomposition into Decomposition into BCNFBCNF

Figure 4.9

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Normalization and Normalization and Database DesignDatabase Design

Normalization should be part of the Normalization should be part of the design processdesign process

Make sure the proposed entities meet Make sure the proposed entities meet the required normal form before the the required normal form before the table structures are createdtable structures are created

Used to redesign or modify the Used to redesign or modify the existing table structures.existing table structures.

E-R Diagram provides macro viewE-R Diagram provides macro view

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Normalization and Normalization and Database DesignDatabase Design

Normalization provides micro view Normalization provides micro view of entitiesof entities Focuses on characteristics of specific Focuses on characteristics of specific

entitiesentities May yield additional entitiesMay yield additional entities

Difficult to separate normalization Difficult to separate normalization from E-R diagrammingfrom E-R diagramming

Business rules must be determinedBusiness rules must be determined

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Normalization and Normalization and Database DesignDatabase Design

Contracting company’s example:Contracting company’s example:

PROJECT (PROJ_NUM, PROJ_NAME)EMPLOYEE(EMP_NUM, EMP_LNAME,EMP_FNAME,EMP_INITAL, JOB_DESCRIPTION, JOB_CHG_HOUR);

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Initial ERD for Initial ERD for Contracting CompanyContracting Company

Figure 4.10

Already 3NFThere is a transitive dependency

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PROJECT (PROJ_NUM, PROJ_NAME)EMPLOYEE(EMP_NUM, EMP_LNAME,EMP_FNAME,EMP_INITAL, JOB_CODE)

JOB (JOB_CODE, JOB_DESCRIPTION, JOB_CHG_HOUR);

Removal

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Modified ERD for Modified ERD for Contracting CompanyContracting Company

Figure 4.11

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Final ERD for Final ERD for Contracting CompanyContracting Company

Figure 4.12

(M:N) converting to (1:M)

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PROJECT (PROJ_NUM, PROJ_NAME, EMP_NUM)

EMPLOYEE(EMP_NUM, EMP_LNAME,EMP_FNAME,EMP_INITAL, EMP_HIREDATE, JOB_CODE)

JOB (JOB_CODE,, JOB_DESCRIPTION, JOB_CHG_HOUR);

ASSIGN((ASSIGN_NUM, ASSIGN_DATE, ASSIGN_HOURS, ASSIGN_CHG_HOURS, ASSIGN_CHARGE, EMP_NUM, PROJ_JUM)

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DenormalizationDenormalization Normalization is one of many Normalization is one of many

database design goals database design goals Normalized table requirementsNormalized table requirements

Additional processingAdditional processing Loss of system speedLoss of system speed

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DenormalizationDenormalization

Normalization purity is difficult to Normalization purity is difficult to sustain due to conflict in:sustain due to conflict in: Design efficiencyDesign efficiency Information requirementsInformation requirements Processing Processing