Fifth Normal Form (5NF), also known as Project-Join Normal Form (PJNF), is a level of database normalization that further refines the structure of a database to eliminate certain types of redundancy and anomalies. It addresses cases where information is redundantly stored across multiple rows due to a combination of candidate keys and overlapping multivalued dependencies.
To achieve 5NF, a database should already be in Fourth Normal Form (4NF). The primary goal of 5NF is to eliminate redundancy caused by join dependencies between candidate keys and multivalued dependencies. In 5NF, no nontrivial join dependency exists.
In simpler terms, 5NF ensures that the database structure is free from certain complex forms of redundancy and that there are no hidden dependencies that could lead to data anomalies.
Achieving 5NF often requires decomposing tables and creating additional tables to store certain combinations of attributes. The process can become quite complex and may involve a deep understanding of the data and the specific use cases.
It’s important to note that achieving 5NF might not always be necessary or practical for every database. The decision to normalize to 5NF should be made based on the specific requirements, complexity of the data, and trade-offs between normalization and performance.
It’s also worth mentioning that normalization beyond 3NF or BCNF, such as 4NF and 5NF, is less commonly practiced and typically reserved for cases where extremely high data integrity and elimination of all types of anomalies are critical.
Achieving Fifth Normal Form (5NF) often involves dealing with complex join dependencies and multivalued dependencies. Here’s a simplified example to illustrate the concept of 5NF normalization:
Example: Library Database
Consider a database that tracks books, authors, and the subjects the books cover. A book can have multiple authors, and each book can cover multiple subjects. We’ll work through the normalization process to achieve 5NF.
Unnormalized Table (BCNF):
|1||Book A||101||Author X||201||Science|
|1||Book A||102||Author Y||202||History|
|2||Book B||101||Author X||201||Science|
|3||Book C||103||Author Z||203||Fiction|
In this table,
(BookID, AuthorID, SubjectID) is the composite primary key.
Step 1: Identify Join Dependencies
In this case, there is a join dependency between
BookTitle and both
Step 2: Create Separate Tables
We will create separate tables for books, authors, and subjects.
Step 3: Update Original Table
SubjectName columns from the original table.
The database is now in Fifth Normal Form (5NF). We have resolved the join dependency issue by creating separate tables for authors and subjects and then creating join tables to represent the many-to-many relationships between books and authors, as well as books and subjects.
Please note that this example has been simplified for illustrative purposes. Achieving 5NF can be more complex, especially in real-world scenarios with more intricate dependencies and data structures. 5NF is generally reserved for cases where the highest level of normalization is required due to specific data integrity and anomaly elimination needs.
In summary, 5NF is a high level of normalization that focuses on eliminating certain types of redundancy and dependencies in a database, but it involves complex transformations that might not be necessary for all database designs.