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Question: Explain Normalization and Denormalization with examples.

Answer: Normalization

Record design process that identifies and avoids data problems and redundancy
Specifies the fields and the primary key
Normalization analyzes record structure through four stages
Unnormalized records
First normal form (1NF) records
Second normal form (2NF) records
Third normal form (3NF) records
Normalization

First normal form
Unnormalized records contain a repeating group
A repeating group refers to a single record that has multiple values in a particular field
Example: multiple product numbers in a single order record
A 1NF record cannot have a repeating group
Normalization

First normal form
To convert an unnormalized record to 1NF, the repeating group must be removed
Expand the primary key to include the primary key of the repeating group
The new primary key is a combination of the original primary key and the key of the repeating group
Instead of a single record with a repeating group, the result is many records, one for each instance of the repeating group
Normalization

Second normal form (2NF)
To be in second normal form, a record must be in 1 NF, and all nonkey fields must be functionally dependent on the entire primary key - not just part of it
Functional dependency means that a value in one field determines a value in another field
If the primary key is a single field, then any record in 1 NF is automatically in 2 NF
In 2NF, all nonkey fields are functionally dependent on the entire primary key
Normalization

Second normal form (2NF)
To convert a 1NF record to 2NF
Create a new record design for each field (or combination of fields) in the primary key
Place remaining fields with the appropriate record
The result will be several records, each with a primary key field (or combination of fields) that determines the values of the other fields in that record
Normalization

Third normal form (3NF)
To be in 3NF, a record must be in 2NF and no nonkey field is functionally dependent on another nonkey field
In 3NF, all nonkey fields are functionally dependent on the primary key, the entire key, and nothing but the key
Normalization

Third normal form (3NF)
To convert a 2NF record to 3NF
Remove all nonkey fields that depend on another nonkey field and place them in a new record that has the determining field as a primary key
Normalization

A normalization example
Identify the entities
ADVISOR
STUDENT
COURSE
Identify the relationships
One advisor advises many students (1:M)
Students take one or more courses, and courses have one or more students (M:N)
Normalization

A normalization example
Identify the entities
ADVISOR
STUDENT
COURSE
Identify the relationships
One advisor advises many students (1:M)
Students take one or more courses, and courses have one or more students (M:N)
Document the unnormalized record
Note the repeating group of courses
Normalization

A normalization example
Convert the unnormalized record to 1 NF
Remove the repeating group
Create a primary key composed of the original primary key (student number) and the primary key of the repeating group (course number)
The result is one record for each instance of the combination primary key
Normalization

A normalization example
Convert the 1 NF record to 2NF
Create a separate record design for each field and combination of fields in the primary key
Place functionally dependent fields with an appropriate primary key
The result is three records instead of one, each with a unique primary key
Now all nonkey fields are dependent on the entire primary key, not just a portion of it
Normalization

A normalization example
Convert the 2NF record to 3NF
The STUDENT record contains a nonkey field (advisor name) that is dependent on another nonkey field (advisor number)
Create a new record with advisor number as the primary key
Remove the dependent nonkey field (advisor name) and include it in the new record
Normalization

A normalization example
Convert the 2NF record to 3NF
The STUDENT record contains a nonkey field (advisor name) that is dependent on another nonkey field (advisor number)
Create a new record with advisor number as the primary key
Remove the dependent nonkey field (advisor name) and include it in the new record
Now all nonkey fields are dependent on the entire primary key, and nothing but the key

Normalization

Record design process that identifies and avoids data problems and redundancy
Specifies the fields and the primary key
Normalization analyzes record structure through four stages
Unnormalized records
First normal form (1NF) records
Second normal form (2NF) records
Third normal form (3NF) records
Normalization

First normal form
Unnormalized records contain a repeating group
A repeating group refers to a single record that has multiple values in a particular field
Example: multiple product numbers in a single order record
A 1NF record cannot have a repeating group
Normalization

First normal form
To convert an unnormalized record to 1NF, the repeating group must be removed
Expand the primary key to include the primary key of the repeating group
The new primary key is a combination of the original primary key and the key of the repeating group
Instead of a single record with a repeating group, the result is many records, one for each instance of the repeating group
Normalization

Second normal form (2NF)
To be in second normal form, a record must be in 1 NF, and all nonkey fields must be functionally dependent on the entire primary key - not just part of it
Functional dependency means that a value in one field determines a value in another field
If the primary key is a single field, then any record in 1 NF is automatically in 2 NF
In 2NF, all nonkey fields are functionally dependent on the entire primary key
Normalization

Second normal form (2NF)
To convert a 1NF record to 2NF
Create a new record design for each field (or combination of fields) in the primary key
Place remaining fields with the appropriate record
The result will be several records, each with a primary key field (or combination of fields) that determines the values of the other fields in that record
Normalization

Third normal form (3NF)
To be in 3NF, a record must be in 2NF and no nonkey field is functionally dependent on another nonkey field
In 3NF, all nonkey fields are functionally dependent on the primary key, the entire key, and nothing but the key
Normalization

Third normal form (3NF)
To convert a 2NF record to 3NF
Remove all nonkey fields that depend on another nonkey field and place them in a new record that has the determining field as a primary key
Normalization

A normalization example
Identify the entities
ADVISOR
STUDENT
COURSE
Identify the relationships
One advisor advises many students (1:M)
Students take one or more courses, and courses have one or more students (M:N)
Normalization

A normalization example
Identify the entities
ADVISOR
STUDENT
COURSE
Identify the relationships
One advisor advises many students (1:M)
Students take one or more courses, and courses have one or more students (M:N)
Document the unnormalized record
Note the repeating group of courses
Normalization

A normalization example
Convert the unnormalized record to 1 NF
Remove the repeating group
Create a primary key composed of the original primary key (student number) and the primary key of the repeating group (course number)
The result is one record for each instance of the combination primary key
Normalization

A normalization example
Convert the 1 NF record to 2NF
Create a separate record design for each field and combination of fields in the primary key
Place functionally dependent fields with an appropriate primary key
The result is three records instead of one, each with a unique primary key
Now all nonkey fields are dependent on the entire primary key, not just a portion of it
Normalization

A normalization example
Convert the 2NF record to 3NF
The STUDENT record contains a nonkey field (advisor name) that is dependent on another nonkey field (advisor number)
Create a new record with advisor number as the primary key
Remove the dependent nonkey field (advisor name) and include it in the new record
Normalization

A normalization example
Convert the 2NF record to 3NF
The STUDENT record contains a nonkey field (advisor name) that is dependent on another nonkey field (advisor number)
Create a new record with advisor number as the primary key
Remove the dependent nonkey field (advisor name) and include it in the new record
Now all nonkey fields are dependent on the entire primary key, and nothing but the key
Source: CoolInterview.com

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