Database Systems Lec7

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Chapter 7Normalization of Database

Tables


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Database Tables and

Normalization Normalization is a process for assigning

attributes to entities. It reduces dataredundancies and helps eliminate the dataanomalies.

Normalization works through a series ofstages called normal forms:

First normal form (1NF)

Second normal form (2NF) Third normal form (3NF)

Fourth normal form (4NF)

The highest level of normalization is notalways desirable.


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Database Tables and

Normalization The Need for Normalization

Case of a Construction Company

Building project -- Project number, Name,Employees assigned to the project.

Employee -- Employee number, Name, Jobclassification

The company charges its clients by billing thehours spent on each project. The hourly billing

rate is dependent on the employees position. Periodically, a report is generated.

The table whose contents correspond to thereporting requirements is shown in Table 5.1.


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Scenari

oA few employees worksfor one project.

Project Num :15Project Name :Evergreen

Employee Num :101, 102, 103,105


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Sample Form

Project Num :15Project Name : Evergreen

105

103

102

101

TotalHrs

Billed

Chr

Hours

Job

Class

Emp

Name

Emp

Num


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Table Structure Matches

the Report Format


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Problems with the Figure 5.1

The project number is intended to be a primarykey, but it contains nulls.

The table displays data redundancies. The table entries invite data inconsistencies.

The data redundancies yield the followinganomalies:

Update anomalies.

Addition anomalies.

Deletion anomalies.

Database Tables andNormalization


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Conversion to First Normal Form

A relational table must not containrepeating groups.

Repeating groups can be eliminated byadding the appropriate entry in at leastthe primary key column(s).

Database Tables andNormalization


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Data Organization: First Normal Form

Before

After


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1NF Definition

The term first normal form (1NF)describes the tabular format in which:

All the key attributes are defined.

There are no repeating groups in thetable.

All attributes are dependent on theprimary key.

First Normal Form (1 NF)


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Dependency Diagram

The primary key components are bold, underlined,and shaded in a different color.

The arrows above entities indicate all desirabledependencies, i.e., dependencies that are based on

PK. The arrows below the dependency diagram indicate

less desirable dependencies -- partial dependenciesand transitive dependencies.

Dependency Diagram


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Conversion to Second Normal Form

Starting with the 1NF format, the database can beconverted into the 2NF format by

Writing each key component on a separate line,

and then writing the original key on the lastline and

Writing the dependent attributes after eachnew key.

PROJECT (PROJ_NUM, PROJ_NAME)EMPLOYEE (EMP_NUM, EMP_NAME, JOB_CLASS,

CHG_HOUR)

ASSIGN (PROJ_NUM, EMP_NUM, HOURS)

Second Normal Form (2NF)


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Dependency Diagram


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A table is in 2NF if:

It is in 1NF and

It includes no partial dependencies;that is, no attribute is dependent ononly a portion of the primary key.

(It is still possible for a table in 2NF

to exhibit transitive dependency; thatis, one or more attributes may befunctionally dependent on nonkeyattributes.)

Second Normal Form (2NF)


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Conversion to Third Normal Form

Create a separate table with attributes ina transitive functional dependence

relationship.

PROJECT (PROJ_NUM, PROJ_NAME)


EMPLOYEE (EMP_NUM, EMP_NAME,JOB_CLASS)

JOB (JOB_CLASS, CHG_HOUR)

Third Normal Form (3 NF)


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3NF Definition

A table is in 3NF if:

It is in 2NF and It contains no transitive

dependencies.

Third Normal Form (3 NF)


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The

CompletedDatabase


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A Table That Is In 3NF

But Not In BCNF


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Sample Data for a BCNF Conversion


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Decomposition into BCNF


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BCNF Definition

A table is in BCNF if every determinantin that table is a candidate key. If atable contains only one candidate key,3NF and BCNF are equivalent.

BCNF Definition


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Normalization will help us identify

correct and appropriate TABLES. Until Now we have 4 tables

Normalization



EMPLOYEE (EMP_NUM, EMP_NAME, JOB_CLASS)

JOB (JOB_CLASS, CHG_HOUR)


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Business Rules The company manages many projects.

Each project requires the services of manyemployees.

An employee may be assigned to severaldifferent projects.

Some employees are not assigned to aproject and perform duties not specificallyrelated to a project. Some employees arepart of a labor pool, to be shared by allproject teams.

Each employee has a (single) primary jobclassification. This job classificationdetermines the hourly billing rate.

Many employees can have the same jobclassification.


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Two Initial Entities:


EMPLOYEE (EMP_NUM, EMP_LNAME,

EMP_FNAME, EMP_INITIAL,JOB_DESCRIPTION, JOB_CHG_HOUR)

Normalization and DatabaseDesign


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Three Entities After Transitive Dependency

Removed


EMPLOYEE (EMP_NUM, EMP_LNAME, EMP_FNAME,EMP_INITIAL, JOB_CODE)

JOB (JOB_CODE, JOB_DESCRIPTION, JOB_CHG_HOUR)



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The Modified ERD


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Creation of Composite Entity ASSIGN


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Attribute ASSIGN_HOUR is assigned to the compositeentity ASSIGN.

Manages relationship is created betweenEMPLOYEE and PROJECT.

PROJECT (PROJ_NUM, PROJ_NAME, EMP_NUM)

EMPLOYEE (EMP_NUM, EMP_LNAME, EMP_FNAME,EMP_INITIAL,

EMP_HIREDATE, JOB_CODE)

JOB (JOB_CODE, JOB_DESCRIPTION, JOB_CHG_HOUR)ASSIGN (ASSIGN_NUM, ASSIGN_DATE, PROJ_NUM,

EMP_NUM,

ASSIGN_HOURS)



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Higher-Level Normal Forms

4NF Definition

A table is in 4NF if it is in 3NF and has nomultiple sets of multivalueddependencies.


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A Set of Tables in 4NF


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Denormalization Normalization is only one of many

database design goals.

Normalized (decomposed) tablesrequire additional processing, reducing

system speed. Normalization purity is often difficult to

sustain in the modern databaseenvironment. The conflict between

design efficiency, informationrequirements, and processing speedare often resolved throughcompromises that includedenormalization.


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SUMMAR

Y


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Summary.

Normalization is a technique used to

design tables in which data redundancies

are minimized

The first three normal forms (1NF, 2NF

and 3NF) are most commonly

encountered

Normalization is an important part-but only

a part-of the design process


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The Initial 1NF Structure


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Table Structures Based On

The Selected PKs


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References

ROB, P. AND CORONEL, C., 2004, Database

Systems. 6th Ed., Thomson Course Technology

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Database Systems Lec7