What is 5NF?

The fifth normal form, or 5NF, is also called the project-join normal form. If a relation is in 4NF and does not have lossless decomposition into smaller tables, it is in Fifth Normal Form (5NF). If redundancy is avoided, 5NF is satisfied when all tables are divided into as many tables as possible.

Objective:

• 5NF aims to guarantee that no redundant data is stored and that all data dependencies in a database are explicitly represented in the schema.
• This can help to eliminate update anomalies, insertion anomalies, and deletion anomalies that can occur when data is stored in a non-normalized or partially normalized database.

Note: one can also consider that a relation is in 5NF if the candidate key implies every join dependency.

Example:

Let’s take a look at an example for a better understanding of the Fifth Normal Form.

SUBJECT	LECTURER	SEMESTER
Computer	Anshika	Semester 1
Computer	John	Semester 1
Math	John	Semester 1
Math	Akash	Semester 2
Chemistry	Praveen	Semester 1

In the above table, John takes both Computer and Math classes for Semester 1, but he doesn’t take Math classes for Semester 2. To identify valid data in this situation, a combination of all these fields is needed.

Therefore,  to make the above table into 5NF, we can decompose it into three relations:

Relation 1:

SEMESTER	SUBJECT
Semester 1	Computer
Semester 1	Math
Semester 1	Chemistry
Semester 2	Math

Relation 2:

SUBJECT	LECTURER
Computer	Anshika
Computer	John
Math	John
Math	Akash
Chemistry	Praveen

Relation 3:

SEMESTER	LECTURER
Semester 1	Anshika
Semester 1	John
Semester 1	John
Semester 2	Akash
Semester 1	Praveen

Now we can say that all 3 relations are in 5NF.

Note: also read about Fourth Normal Form (4NF)

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