Rdbms xp 03

Normalizing and Denormalizing Data

Objectives
In this lesson, you will learn to:
Describe data redundancy
Describe the first, second, and third normal forms
Describe the Boyce-Codd Normal Form (BCNF)
Appreciate the need for denormalization

©NIIT RDBMS/Lesson 3/Slide 1 of 16


Understanding Data Redundancy
Redundancy means repetition of data
Redundancy increases the time involved in updating, adding,
and deleting data
It also increases the utilization of disk space and hence, disk
I/O increases



Understanding Data Redundancy (Contd.)
Redundancy can lead to the following problems:
Update anomalies—Inserting, modifying, and deleting
data may cause inconsistencies
Inconsistencies—Errors are more likely to occur when
facts are repeated
Unnecessary utilization of extra disk space



Definition of Normalization
Normalization is a scientific method of breaking down
complex table structures into simple table structures by
using certain rules
It allows you to reduce redundancy in a table and eliminate
the problems of inconsistency and disk space usage
Normalization results in the formation of tables that satisfy
certain specified rules and represent certain normal forms



Normal Forms
The most important and widely used normal forms are:
First Normal Form (1 NF)
Second Normal Form (2 NF)
Third Normal Form (3 NF)
Boyce Codd Normal Form (BCNF)



First Normal Form
A table is said to be in the 1 NF when each cell of the table
contains precisely one value
Functional Dependency
The normalization theory is based on the fundamental
notion of functional dependency
Given a relation R, attribute A is functionally dependent
on attribute B if each value of A in R is associated with
precisely one value of B



Second Normal Form (2NF)
A table is said to be in 2 NF when it is in 1 NF and every
attribute in the row is functionally dependent upon the whole
key, and not just part of the key
To ensure that a table is in 2 NF, you should:
Find and remove attributes that are functionally
dependent on only a part of the key and not on the whole
key and place them in a different table
Group the remaining attributes



Third Normal Form (3NF)
A relation is said to be in 3 NF when it is in 2 NF and every
non-key attribute is functionally dependent only on the
primary key
To ensure that a table is in 3 NF, you should:
Find and remove non-key attributes that are functionally
dependent on attributes that are not the primary key and
place them in a different table
Group the remaining attributes



Boyce-Codd Normal Form
The original definition of 3NF was inadequate in some
situations
It was not satisfactory for the tables:
that had multiple candidate keys
where the multiple candidate keys were composite
where the multiple candidate keys overlapped
Therefore, a new normal form—the Boyce-Codd Normal
Form (BCNF) was introduced
A relation is in the Boyce-Codd normal form (BCNF) if
and only if every determinant is a candidate key



Boyce-Codd Normal Form (Contd.)
In order to ensure that a table is in BCNF, you should:
Find and remove the overlapping candidate keys
Place the part of the candidate key and the attribute it is
functionally dependent on, in a different table
Group the remaining items into a table



Just a Minute...
You have received a proposed table structure for the table
Position. After testing the table structure with some data, you
find that there is a problem in inserting, deleting, and
modifying data. You will notice that the table structure could
lead to inconsistency in data and will also occupy a lot of
disk space.
Modify the table structure to optimize data storage.



Understanding Denormalization
The end product of normalization is a set of related tables
that comprise the database
However, in the interests of speed of response to critical
queries, which demand information from more than one
table, it is sometimes wiser to introduce a degree of
redundancy in tables
The intentional introduction of redundancy in a table to
improve performance is called denormalization



Just a Minute…
In the reporting system, the total amount paid to a contract
recruiter is often required. The required result can be
calculated using the two tables - ContractRecruiter and
Payment. Since the size of the Payment table is large, the join
between the two tables takes time to generate the required
output. It is, therefore, necessary to improve the performance
of this query.



Just a Minute…
Each time the salary slip for an employee is generated, the
referral bonus (if present) has to be calculated and printed in
the salary slip. The following three tables are used for this
query- MonthlySalary, Employee, and EmployeeReferrals.
However, since the table structures are large, it is necessary
to improve the performance of this query by modifying the
table structures.



Summary
In this lesson, you learned that:
Normalization is used to simplify table structures.
Normalization results in the formation of tables that satisfy
certain specified constraints, and represent certain normal
forms. The normal forms are used to ensure that various
types of anomalies and inconsistencies are not introduced in
the database. A table structure is always in a certain normal
form. Several normal forms have been identified.



Summary (Contd.)
The most important and widely used of these are:
First Normal Form (1NF)
Second Normal Form (2 NF)
Third Normal Form (3 NF)
Boyce Codd Normal Form (BCNF)
The intentional introduction of redundancy in a table in order
to improve performance is called denormalization.
The decision to denormalize results in a trade-off between
performance and data integrity.
Denormalization increases disk space utilization.


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