Denormalization produces a lower normal form

Lecture7_ch07.ppt

Database Principles: Fundamentals of Design, Implementations and Management

CHAPTER 7 Normalizing Database Designs

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Objectives

In this chapter, you will learn:

What normalization is and what role it plays in the database design process

About the normal forms 1NF, 2NF, 3NF, BCNF,

and 4NF

How normal forms can be transformed from lower normal forms to higher normal forms

That normalization and ER modeling are used concurrently to produce a good database design

That some situations require denormalization to generate information efficiently

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Database Tables and Normalization

Normalization

Process for evaluating and correcting table structures to minimize data redundancies

Reduces data anomalies

Works through a series of stages called normal forms:

First normal form (1NF)

Second normal form (2NF)

Third normal form (3NF)

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Database Tables and Normalization (cont..)

Normalization (cont..)

2NF is better than 1NF; 3NF is better than 2NF

For most business database design purposes, 3NF is as high as needed in normalization

Highest level of normalization is not always most desirable

Denormalization produces a lower normal form

Price paid for increased performance is greater data redundancy

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The Need for Normalization

Example: company that manages building projects

Charges its clients by billing hours spent on each contract

Hourly billing rate is dependent on employee’s position

Periodically, report is generated that contains information such as displayed in Table 5.1

The Need for Normalization

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The Need for Normalization

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The Need for Normalization (cont..)

Structure of data set in Figure 7.1 does not handle data very well

Table structure appears to work; report generated with ease

Unfortunately report may yield different results depending on what data anomaly has occurred

Relational database environment suited to help designer avoid data integrity problems

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The Normalization Process

Each table represents a single subject

No data item will be unnecessarily stored in more than one table

All attributes in a table are dependent on the primary key

Each table void of insertion, update, deletion anomalies

Void = depourvu de

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The Normalization Process (cont..)

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The Normalization Process (cont..)

Objective of normalization is to ensure all tables in at least 3NF

Higher forms not likely to be encountered in business environment

Normalization works one relation at a time

Progressively breaks table into new set of relations based on identified dependencies

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Conversion to First Normal Form

Repeating group

Derives its name from the fact that a group of multiple entries of same type can exist for any single key attribute occurrence

Relational table must not contain repeating groups

Normalizing table structure will reduce data redundancies

Normalization is three-step procedure

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Conversion to First Normal Form (cont.)

Step 1: Eliminate the Repeating Groups

Present data in tabular format, where each cell has single value and there are no repeating groups

Eliminate nulls: each repeating group attribute contains an appropriate data value

Step 2: Identify the Primary Key

Primary key must uniquely identify attribute value

New key must be composed

Conversion to First Normal Form (cont..)

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Conversion to First Normal Form (cont..)

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Conversion to First Normal Form (cont..)

Step 3: Identify All Dependencies

Dependencies can be depicted with help of a diagram

Dependency diagram:

Depicts all dependencies found within given table structure

Helpful in getting bird’s-eye view of all relationships among table’s attributes

Makes it less likely that will overlook an important dependency

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Conversion to First Normal Form (cont..)

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Conversion to First Normal Form (cont.)

First normal form describes tabular format in which:

All key attributes are defined

There are no repeating groups in the table

All attributes are dependent on primary key

All relational tables satisfy 1NF requirements

Some tables contain partial dependencies

Dependencies based on part of the primary key

Sometimes used for performance reasons, but should be used with caution

Still subject to data redundancies

Conversion to Second Normal Form

Relational database design can be improved by converting the database into second normal form (2NF)

Two steps

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Conversion to Second Normal Form (cont..)

Step 1: Write Each Key Component

on a Separate Line

Write each key component on separate line, then write original (composite) key on last line

Each component will become key in new table

Step 2: Assign Corresponding Dependent Attributes

Determine those attributes that are dependent on other attributes

At this point, most anomalies have been eliminated

Conversion to Second Normal Form (cont..)

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Conversion to Second Normal Form (cont..)

Table is in second normal form (2NF) when:

It is in 1NF and

It includes no partial dependencies:

No attribute is dependent on only portion of primary key

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Conversion to Third Normal Form

Data anomalies created are easily eliminated by completing three steps

Step 1: Identify Each New Determinant

For every transitive dependency, write its determinant as PK for new table

Determinant:

any attribute whose value determines other values within a row

Conversion to Third Normal Form (cont..)

Step 2: Identify the Dependent Attributes

Identify attributes dependent on each determinant identified in Step 1 and identify dependency

Name table to reflect its contents and function

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Conversion to Third Normal Form (cont.)

Step 3: Remove the Dependent Attributes from Transitive Dependencies

Eliminate all dependent attributes in transitive relationship(s) from each of the tables

Draw new dependency diagram to show all tables defined in Steps 1–3

Check new tables as well as tables modified in Step 3 to make sure that:

Each table has a determinant and that

No table contains inappropriate dependencies

Conversion to Third Normal Form (cont..)

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Conversion to Third Normal Form (cont.)

A table is in third normal form (3NF) when both of the following are true:

It is in 2NF

It contains no transitive dependencies

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Improving the Design

Table structures are cleaned up to eliminate troublesome initial partial and transitive dependencies

Normalization cannot, by itself, be relied on to make good designs

It is valuable because its use helps eliminate data redundancies

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Improving the Design (cont..)

Issues to address in order to produce a good normalized set of tables:

Evaluate PK Assignments

Evaluate Naming Conventions

Refine Attribute Atomicity

Identify New Attributes

Identify New Relationships

Refine Primary Keys as Required for Data Granularity

Maintain Historical Accuracy

Evaluate Using Derived Attributes

Improving the Design (cont..)

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Improving the Design (cont..)

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Improving the Design (cont..)

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Surrogate Key Considerations

When primary key is considered to be unsuitable, designers use surrogate keys

Data entries in Table 7.3 are inappropriate because they duplicate existing records

Yet there has been no violation of either entity integrity or referential integrity

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Higher-Level Normal Forms

Tables in 3NF perform suitably in business transactional databases

Higher order normal forms useful on occasion

Two special cases of 3NF:

Boyce-Codd normal form (BCNF)

Fourth normal form (4NF)

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The Boyce-Codd Normal Form (BCNF)

Every determinant in table is a candidate key

Has same characteristics as primary key, but for some reason, not chosen to be primary key

When table contains only one candidate key, the 3NF and the BCNF are equivalent

BCNF can be violated only when table contains more than one candidate key

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The Boyce-Codd Normal Form (BCNF) (cont..)

Most designers consider the BCNF as special case of 3NF

Table is in 3NF when it is in 2NF and there are no transitive dependencies

Table can be in 3NF and fails to meet BCNF

No partial dependencies, nor does it contain transitive dependencies

A nonkey attribute is the determinant of a key attribute

The Boyce-Codd Normal Form (BCNF) (cont...)

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The Boyce-Codd Normal Form (BCNF) (cont..)

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The Boyce-Codd Normal Form (BCNF) (cont..)

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Summary

Normalization is used to minimize data redundancies

First three normal forms (1NF, 2NF, and 3NF) are most commonly encountered

Table is in 1NF when:

All key attributes are defined

All remaining attributes are dependent on primary key

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Summary (continued)

Table is in 2NF when it is in 1NF and contains no partial dependencies

Table is in 3NF when it is in 2NF and contains no transitive dependencies

Table that is not in 3NF may be split into new tables until all of the tables meet 3NF requirements

Normalization is important part—but only part—of the design process

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Summary (continued)

Table in 3NF may contain multivalued dependencies

Numerous null values or redundant data

Convert 3NF table to 4NF by:

Splitting table to remove multivalued dependencies

Tables are sometimes denormalized to yield less I/O, which increases processing speed

Additional Slides

Please have a look a the following slides

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Fourth Normal Form (4NF)

Table is in fourth normal form (4NF) when both of the following are true:

It is in 3NF

No multiple sets of multivalued dependencies

4NF is largely academic if tables conform to following two rules:

All attributes dependent on primary key, independent of each other

No row contains two or more multivalued facts about an entity

Fourth Normal Form (4NF) (continued)

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Fourth Normal Form (4NF)

Fourth Normal Form (4NF)

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Normalization and Database Design

Normalization should be part of the design process

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

Many real-world databases have been improperly designed or burdened with anomalies

You may be asked to redesign and modify existing databases

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Normalization and Database Design (cont.)

ER diagram

Identify relevant entities, their attributes, and their relationships

Identify additional entities and attributes

Normalization procedures

Focus on characteristics of specific entities

Micro view of entities within ER diagram

Difficult to separate normalization process from ER modeling process

Two techniques should be used concurrently

Figure 7.13 in your book

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Normalization and Database Design (cont.)

Figure 7.13 in your book

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Normalization and Database Design (cont.)

Figure 7.14 in your book

Figure 7.14 in your book

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Figure 7.15 in your book

Normalization and Database Design (cont.)

Figure 7.15 in your book

Normalization and Database Design (continued)

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Normalization and Database Design (continued)

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Normalization and Database Design (continued)

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