Database Normalization | A Step-by-Step Guide with Examples

SATHISH ROUTHU 31 May, 2023 • 9 min read

Introduction

As an SQL Developer, you regularly work with enormous amounts of data stored in different tables that are present inside databases. It often becomes difficult to extract the information if the data is unorganized. We can solve this problem using Normalization by structuring the database in different forms or stages. This article will help you understand the concept of normalization in DBMS with step-by-step instructions and examples of tables.

We’ll discuss the functional dependencies that may exist in a table and anomalies that occur due to these functional dependencies. We will see the conversion of tables into normal forms to eliminate those anomalies.

Learning Objectives:

  • Understand the meaning of normalization and the need for it.
  • Learn about the various functional dependencies.
  • Familiarize yourself with the different stages of normalization.

This article was published as a part of the Data Science Blogathon.

What Is Normalization in DBMS?

Normalization is a technique for organizing the data into multiple related tables to minimize Data Redundancy and Data Inconsistency. It aims to eliminate anomalies in data.

Why Do We Need Normalization?

Data inconsistency results from anything that affects data integrity. This can cause the data to be correct in one place and wrong elsewhere it is stored. This can lead to unreliable and meaningless information. It occurs between tables when similar data is stored in different formats in two different tables.

For example, consider the following tables:

LibraryVisitors (StudentID, Student_Name, Student_Address, InTime, OutTime);
Students (StudentID, Student_Name, Student_Address, Department, RollNo, CourseRegistered);

In the above tables, Student_Address is stored in both tables. For each student_id, the address must be the same in those two tables. Both these relations must be considered to retrieve or update the correct address. The issues mentioned arise due to poorly designed/structured databases.

We can eliminate data inconsistency in databases by using constraints on the relations.

Data Redundancy is the condition where the same data is stored at different locations leading to the wastage of storage space.

Examples:

Student IdStudent NameCourse IDCourse Name
111JohnC08English
112AliceC08English
111JohnC02French

In the above table, we have stored student name John twice as he registered for two different courses and course name English twice as two students registered for it. This is called data redundancy. Data redundancy causes many problems in databases.

We can eliminate data redundancy in the databases by the normalization of relations.

Functional Dependency

Before diving into normalization, we need to know clearly about functional dependencies.

An attribute is dependent on another attribute if another attribute uniquely identifies it.

It is denoted by  A –> B, meaning A determines B, and B depends upon A.

Example: We can find the Student’s name using the Student_ID.

What Is an Anomaly?

An anomaly is an unexpected side effect of trying to insert, update, or delete a row. Essentially more data must be provided to accomplish an operation than expected.

Consider the following relation:

Retail_Outlet_IDOutlet_LocationItem_CodeDescriptionQty_AvailableRetail_Unit_Price
R1001King Street,
Hyderabad, 540001
I1001Britannia
Marie Gold
251600
R1002Rajaji Nagar,
Bangalore, 600341
I1106Cookies581289
R1003MVP Colony,
Visakhapatnam,
500021
I1200Best Rice222000
R1001King street,
Hyderabad
I1309Dal201500

Types of Anomalies

Here are some of the most common anomalies that happen in database management.

1. Insertion anomalies: These occur when we cannot insert a new tuple into the table due to a lack of data.

What happens if we try to insert(add) the details of a new retail outlet with no items in its stock?

  • NULL values would be inserted into the item details columns, which is not preferable.

2. Deletion anomalies: They happen when the deletion of some data deletes the other required data also (Unintended data loss)

What happens if we try to delete the item of item code I1106?

  • The details of the retail outlet R1002 will also be deleted from the database.

3. Update anomalies: These happen when an update of a single record requires an update in multiple records.

How many rows will be updated if the retail outlet location of R1002 is changed from King Street to Victoria Street?

  • 2 Rows will be updated

4. Data redundancy: This happens when new items are supplied to a retail outlet.

What details do we need to insert?

  • Apart from all necessary details, retail_outlet_location will also be inserted, which is redundant.

We have seen insert, delete, update anomalies, and data redundancy in the above-given example. Functional dependencies may lead to anomalies. To minimize anomalies, there is a need to refine functional dependencies using normalization.

What Is Normalization?

Database normalization is the process of organizing a relational database in accordance with a series of so-called normal forms in order to reduce data redundancy and improve data integrity. It was first proposed by Edgar F. Codd.

“Normal Forms” (NF) are the different stages of normalization

  • 1 NF (First Normal Form)
  • 2 NF (Second Normal Form)
  • 3 NF (Third Normal Form)
  • BCNF (Boyce -Codd Normal Form)
  • 4 NF (Fourth Normal Form)
  • 5 NF (Fifth Normal Form)
  • 6 NF (Sixth Normal Form)

4NF to 6NF applies to multivalued dependencies and complex table scenarios. In this article, we discuss up to BCNF.

Different forms or stages of normalization in DBMS

1 NF: First Normal Form

A relation R is said to be in 1 NF (First Normal) if and only if:

  1. All the attributes of R are atomic.
  2. It does not contain any multi-valued attributes.

In the above-taken example of the Retail_Outlets table, we have stored multiple values in an address field, such as street name, city name, and pin code.

What if we want to know about all retail outlets in a given city? We may need to perform some string operations on the address field, which is not preferable. So we need to store all these atomic values in separate fields.

A multi-valued attribute is an attribute that can have multiple values like Contact numbers. They should also be separated like ContactNo1, ContanctNo2,.. to achieve 1st Normal form.

1 NF | First normal form
1st Normal Form

Advantage: 1 NF allows users to use the database queries effectively as it removes ambiguity by removing the non-atomic and multi-valued attributes, which creates major issues in the future while updating and extracting the data from the database.

Limitation: Data redundancy still exists even after 1st Normal form, so we need further normalization.

2 NF: Second Normal Form

A relation R is said to be in 2 NF (Second Normal) form if and only if:

  1. R is already in 1 NF
  2. There is no partial dependency in R between non-key attributes and key attributes.

Suppose we have a composite primary or candidate key in our table. Partial dependency occurs when a part of the primary key (Key attribute) determines the non-key attribute.

In the Retail Outlets table, the Item_Code and Retail_Outlet_ID are key attributes. The item description is partially dependent on Item_Code only. Outlet_Location depends on Retail_Outlet_ID. These are partial dependencies.

To achieve normalization, we need to eliminate these dependencies by decomposing the relations.

2 NF | Second normal form
2nd Normal Form

From the above decomposition, we eliminated the partial dependency.

Advantage: 2 NF attempts to reduce the amount of redundant data in a table by extracting it, placing it in a new table(s), and creating relationships between those tables.

Limitation: There are still some anomalies, as there might be some indirect dependencies between Non-Key attributes, leading to redundant data.

3 NF: Third Normal Form

A relation R is said to be in 3 NF (Third Normal Form) if and only if:

  1. R is already in 2 NF
  2. There is no transitive dependency that exists between key attributes and non-key attributes through other non-key attributes.

A transitive dependency exists when another non-key attribute determines a non-key attribute. In other words, If A determines B and B determines C, then automatically, A determines C.

Transitive dependency
Transitive Dependency

Some other examples:

  • The Year of birth determines the Age of the person
  • The price of an Item determines the class of the Item
  • The ZIP code of a city determines the City’s Name
3 NF | Third normal form
3rd Normal Form

Advantage: 3 NF ensures data integrity. It also reduces the amount of data duplication.

Boyce-Codd Normal Form

It is an upgraded version of the 3rd Normal form. It is also called as 3.5 Normal Form.

A relation R is said to be in 3 NF (Third Normal Form) if and only if:

  1. R is already in 3 NF
  2. For any dependency A –> B, then A should be the Super key.

In simple words, if A –> B, then A cannot be a non-prime Attribute if B is a prime attribute which means that A non-prime attribute cannot determine a prime attribute.

You must be wondering how’s this possible. but Yes, there can be some cases in which the Non-Prime attribute will determine the prime attributes even if the relationship was in the 3rd Normal form. BCNF does not allow this kind of dependency.

Sample Table

Let us understand this better with an example. Look at the below Relation of Student Enrollments table.

Student_IDCourse_NameProfessor
101JAVAProf. Java
102C++Prof. CPP
101PythonProf. Python
103JAVAProf. Java_2
104PythonProf. Python_2

In the above relation:

  • One student can enroll in multiple courses.
  • Multiple professors can teach one course.
  • One professor can be assigned only one course.

So the (Student_ID & Course_Name) will form the primary key. These 2 will compositely determine all other attributes in the relation. In our case, it is only the professor.

  • The Relation is clearly in 1st Normal Form as there are No Multivalued attributes, and all attributes have atomic values.
  • The Relation is in 2nd Normal Form as there are No Partial dependencies.
  • Student_Id cannot determine Course_Name as one student can enroll in multiple courses.
  • Course_Name cannot determine the professor, as multiple professors may teach the same course.
  • The relation is in 3rd normal form as there are no transitive dependencies.

If we observe here, the “Professor” attribute, a non-prime attribute, can determine the Course_Name as each professor teaches only one course. But Course_Name is a prime attribute, and Professor is not a Super Key. That means a non-prime attribute determines the prime attribute.

This is not allowed in BCNF. So, how do we decompose this relation?

BCNF | Boyce-Codd Normal Form | DBMS normalization
Boyce-Codd Normal Form

Until here, we have seen normal forms up to BCNF. Here are some guidelines to follow while normalizing the database.

Guidelines for Using Normalization

  • Depending on the business requirements, we can normalize the tables up to the 2nd normal form or the 3rd normal form.
  • Prefer tables in 3 NF in applications with extensive data modifications.
  • Prefer tables in 2 NF in applications with extensive data retrieval.
  • Reason: retrieving data from multiple tables is a costly operation.
  • Converting the tables from higher normal form to lower normal form is called “Denormalization”.

The below picture summarizes how to reach the third normal form from an unnormalized form:

unnormalized to the third stage of normalization

Any relational database without normalization may lead to problems like large tables, difficulty maintaining the database as it involves searching many records, poor disk space utilization, and inconsistencies. If we fail to eliminate this kind of problem, it would lead to data integrity and redundancy problems. Normalization of a relational database helps to solve these problems. Normalization applies to a series of transformations in terms of normal forms. Any relation in a database must be normalized to get efficient access to the database. Each Normal form eliminates each type of dependency and improves the data integrity.

Advantages of Normalization

Normalization helps a lot with organizing data. Here are some of its advantages:

  • It reduces data redundancy: Normalisation assists in removing redundant data from tables, using less storage space, and increasing database effectiveness.
  • It improves data consistency: Normalisation guarantees that the data stays organized and consistent, lowering the possibility of data errors and inconsistencies.
  • It makes database design simple: Normalization offers rules for arranging tables and data linkages. This facilitates database design and maintenance.
  • It handles queries faster: Faster query performance is a result of normalized tables’ generally easier search and data retrieval capabilities.
  • It simplifies database maintenance: By dividing a database’s complexity into smaller, more manageable tables, normalization makes it simpler to add, change, and delete data.

Conclusion

This article was aimed at making you understand the normalization process and how to apply it when you design a database system. There is another multi-valued dependency that 4NF and 5NF can eliminate. Try to explore those also.

I hope this article helped you to understand the concept of normalization better. If you have any questions, please let me know in the comments.  I wish you great learning ahead.

Key Takeaways:

  • Normalization is a technique for organizing the data into multiple related tables to minimize Data Redundancy and Data Inconsistency.
  • Insertion, deletion, update, and data redundancy are the various possible anomalies that may occur when building a database.
  • There are seven different stages of normalization known as Normal Forms.

Frequently Asked Questions

Q1. What is normalization with an example?

A. Normalization is a technique for organizing the data into multiple related tables to minimize data redundancy and inconsistency. It aims to eliminate anomalies in data. There are a few examples discussed in this article that explain this better.

Q2. What is 1st 2nd and 3rd normalization?

A. 1st, 2nd, and 3rd normal forms are the fundamental stages of database normalization.
1st normalization: In the first stage, each attribute in the connection is atomic.
2nd normalization: By this stage, the non-prime qualities become functionally reliant on the entire candidate key.
3rd normalization: In this stage, the non-prime attributes become directly (non-transitively) reliant on candidate keys.

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SATHISH ROUTHU 31 May 2023

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Shingi
Shingi 04 May, 2023

Thanks brother this was helpful

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