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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