Database Tables and Normalization

8/14/2019 Database Tables and Normalization

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Database Tables and NormalizationDatabase Tables and NormalizationNormalization is a process for assigning

attributes to entities. It reduces dataredundancies.

Normalization works through a series of stagescalled normal forms:First Normal Form (1NF)Second Normal Form (2NF)Third Normal Form (3NF) etc..

The highest level of normalization is not alwaysdesirable.


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Guidelines for Designing Relations

Guideline 1: Design relations so that the the meaning ofthe data is clear. Do not combine attributes from different

entities and relationships into a single relation.

Guideline 2: Design relations so that no insertion, deletion

or modification PROBLEMS are likely to occur.

Guideline 3: As far as possible, avoid including attributes

in a relation that have null values in a majority of the tuples.

Guideline 4: Design relations that can be JOINed with

each other using equality conditions on attributes that are

either primary keys or foreign keys.


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Keys The role of the key is based on a concept known as

determination, which is used in the definition offunctional

dependence.

An attribute B is functionally dependenton an attribute

A if the value of A determines the value of B.

An attribute that is part of a primary or candidate key isknown as a key( orprime) attribute. Else, it is a non-

key(ornon-prime) attribute.

If an attribute B is functionally dependent on a composite

key A but not on any subset of A, then B is fully

functionally dependenton A; otherwise B ispartially

dependenton A.


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Two Other Dependencies

Partial Dependency

A functional dependency A determines B is apartial

dependencyif the dependency holds even after removing

some attribute from A.

Transitive Dependency

A transitive dependencyoccurs when one non-prime attribute

depends on another non-prime attribute.


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1NF Definition

The term First Normal Form (1NF) describes the

tabular format in which:

All the prime attributes are defined and have

non-null values.

All table entries are atomic, i.e., no cell of the

table contains a group of items.All attributes are dependent on the primary key.

Database Tables and NormalizationDatabase Tables and Normalization


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Customer

Customer ID First Name Surname Tel. No. 1 Tel. No. 2 Tel. No. 3

123 Robert Ingram 2025456

124 Jane Wright 555-403-1 659555-776-

Customer

Customer ID First Name Surname

1 Robert Ingram

2 Jane Wright

Customer Telephone Number

Customer ID Telephone Number

1 2025456

2 555-403-1

124 659555-776-


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2NF DefinitionA table is in Second Normal Form (2NF) if:

It is in 1NF, and

It includes no partial dependencies, i.e., no

attribute is dependent on only a part of a

composite candidate key.

Note that it is possible for a table in 2NF toexhibit transitive dependency, i.e., a non-prime

attribute can be functionally dependent on some

other non-prime attribute.



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Consider a table describing employees' skills:

Employee Skill Current Work Location

Jones Typing 114 Main Street

Jones Shorthand 114 Main Street

Jones Whittling 114 Main Street

Roberts Light Cleaning 73 Industrial Way

The table's only candidate key is {Employee, Skill}.The remaining

attribute, Current Work Location, is dependent on only part of the

candidate key, namely Employee. Therefore the table is not in 2NF.

Note the redundancy in the way Current Work Locations are

represented: we are told three times that Jones works at 114 Main

Street. This redundancy makes the table prone to update problems.


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A 2NF alternative to this design would represent the same informationin two tables:

Employees

Employee Current Work Location

Jones 114 Main Street

Roberts 73 Industrial Way

Employees' SkillsEmployee Skill

Jones Typing

Jones Shorthand

Jones WhittlingRoberts Light Cleaning


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3NF DefinitionA table is in Third Normal Form (3NF) if:

It is in 2NF, and

It contains no transitive dependencies.



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To normalize data to achieve third normal

form-

Identify and remove any attribute which is

actually dependent on a non-key attribute

in the same relation .

Create a new relation for this attribute

identified by the relevant key.


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DenormalizationDenormalization

Normalization is only one of many database designgoals.

Normalized (decomposed) tables require additional

processing.

Normalization purity is often difficult to sustain in

the modern database environment. The conflict

between design efficiency, information require-ments, and processing speed are often resolved

through compromises that include denormalization.


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Merging Relations (View Integration)

This is required when, on large projects, the

work of several sub-teams comes together

during logical design.

Problems that may arise:

Synonyms: Different names, same

meaning.

Homonyms: Same name, different

meanings.


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SynonymsSTUDENT1 (Student_ID, Name)

STUDENT2 (SSN, Name, Address)

If Student_ID is same as SSN, the two relations can be merged to

STUDENT (SSN, Name, Address)

HomonymsSTUDENT1 (Student_ID, Name, Address)

STUDENT2 (Student_ID, Name, Phone, Address)

If one address refers to the campus address and the other to the home

address, the relations can be merged to

STUDENT (Student_ID, Name, Phone, Campus_addr, Home_addr)

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