Normalization in Database

NORMALIZATION
Instructor:
Mehwashma Amir
E-R Model and Normalization/ Session 2
1

SQL
Server
2012
© Aptech Ltd.
SQL
Server
2012
2E-R Model and Normalization/ Session 2
 Initially, all databases are characterized by large number of columns and records.
 This approach has certain drawbacks.
 The following table consist of details of the employees and the project they are
working on.
Repetition Anomaly
 The data such as Project_id, Project_name, Grade, and Salary repeat
many times.
 This repetition hampers both, performance during retrieval of data and the storage
capacity.
 This repetition of data is called the repetition anomaly.

SQL
Server
2012
© Aptech Ltd.
SQL
Server
2012
 The repetition is shown in the following table with the help of shaded cells:
Insertion Anomaly
 Suppose the department recruits a new employee named Ann.
 Consider that Ann has not been assigned any project. Insertion of her details in the
table would leave columns Project_id and Project_name empty.
 Leaving columns blank could lead to problems later.
 Anomalies created by such insertions are called insertion anomalies as shown in
the following table:

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2012
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SQL
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2012
Deletion Anomaly
 Suppose, Bob is relieved from the project MAGNUM.
 Deleting the record deletes Bob's Emp_no, Grade, and Salary details too.
 This loss of data is harmful as all of Bob's personal details are also lost.
 This kind of loss of data due to deletion is called deletion anomaly as can be seen in
the following table:
Updating Anomaly
 Suppose John was given a hike in Salary or John was demoted.
 The change in John's Salary or Grade needs to be reflected in all projects John
works for.
 This problem in updating all the occurrences is called updating anomaly.

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Server
2012
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SQL
Server
2012
The Department Employee Details table is called an unnormalized table.
These drawbacks lead to the need for normalization.
Normalization is the process of removing unwanted redundancy and dependencies.
Initially, Codd (1972) presented three normal forms (1NF, 2NF, and 3NF), all based
on dependencies among the attributes of a relation.
The fourth and fifth normal forms are based on multi value and join dependencies
and were proposed later.

SQL
Server
2012
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SQL
Server
2012
1
• Create separate tables for each group of related data.
2
• The table columns must have atomic values.
3
• All the key attributes must be identified.
 In order to achieve the first normal form, following steps need to be performed:
 Consider the Employee Project Details table as follows:
 The table has data related to projects and employees.
 The table needs to be split into two tables, that is, a Project Details table
and an Employee Details table.
 The table columns, Project_id and Project_names, have multiple values.

SQL
Server
2012
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SQL
Server
2012
 The data needs to be split over different rows.
 The resultant tables are Project Details and Employee Details as
follows:
Project Details
Employee Details
 The Project_id attribute is the primary key for the Project Details table.
 The Emp_no attribute is the primary key for the Employee Details table.
 Therefore, in first normal form, the initial Employee Project Details table
has been reduced to the Project Details and Employee Details tables.

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Server
2012
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SQL
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2012
 The tables are said to be in second normal form if:
 Partial dependency means a non-key attribute should not be partially dependent
on more than one key attribute.
 The Project Details and Employee Details tables do not exhibit any
partial dependencies.
 The Project_name is dependent only on Project_id and Emp_name,
Grade, and Salary are dependant only on Emp_no.
 The tables also need to be related through foreign keys.
 A third table, named Employee Project Details, is created with only two
columns, Project_id and Emp_no.
They meet the requirements of the first normal form.
There are no partial dependencies in the tables.
The tables are related through foreign keys.

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2012
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SQL
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2012
 So, the project and employee details tables on conversion to second normal form
generates tables Project Details, Employee Details, and Employee
Project Details as follows:
Project Details
Employee Project Details
Employee Details
 The attributes, Emp_no and Project_id, of the Employee Project
Details table combine together to form the primary key.
 Such primary keys are called composite primary keys.

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Server
2012
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SQL
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2012
 To achieve the third normal form:
The tables should meet the requirements of the second normal form
The tables should not have transitive dependencies in them
 The Project Details, Employee Details, and Employee Project
Details tables are in second normal form.
 If an attribute can be determined by another non-key attribute, it is called a
transitive dependency.
 That is, every non-key attribute should be determined by the key attribute only.
 If a non-key attribute can be determined by another non-key attribute, it needs to
put into another table.

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Server
2012
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SQL
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2012
 On observing the different tables, it is seen that the Project Details and
Employee Project Details tables do not exhibit any such transitive
dependencies.
 The non-key attributes are totally determined by the key attributes.
 Project_name is only determined by Project_number.
 On further scrutinizing the Employee Details table, a certain inconsistency is
seen.
 The attribute Salary is determined by the attribute Grade and not the key
attribute Emp_no.
 Thus, this transitive dependency needs to be removed.
 The Employee Details table can be split into Employee Details and
Grade Salary Details tables as follows:
Employee Details Grade Salary Details

SQL
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2012
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SQL
Server
2012
 Thus, at the end of the three normalization stages, the initial Employee
Project Details table has been reduced to the Project Details,
Employee Project Details, Employee Details, and Grade Salary
Details tables as follows:
Employee Details Grade Salary Details
Project Details
Employee Project Details

SQL
Server
2012
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SQL
Server
2012
By normalizing a database, redundancy is reduced.
This, in turn, reduces the storage requirements for the database and ensures data
integrity.
However, it has following drawbacks:
Complex join queries may have to be written often to combine the data in
multiple tables.
Joins may practically involve more than three tables depending on the need
for information.
 If such joins are used very often, the performance of the database will become very
poor.
 In such cases, storing a few fields redundantly can be ignored to increase the
performance of the database.
 The databases that possess such minor redundancies in order to increase
performance are called denormalized databases and the process of doing so is
called denormalization.

Normalization in Database

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