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Fifth Normal Form (5NF)
The 5NF (Fifth Normal Form) is also known as project-join normal form. A relation is in Fifth Normal Form (5NF), if it is in 4NF, and won’t have lossless decomposition into smaller tables.
You can also consider that a relation is in 5NF, if the candidate key implies every join dependency in it.
Example
The below relation violates the Fifth Normal Form (5NF) of Normalization −
<Employee>
| EmpName | EmpSkills | EmpJob (Assigned Work) |
| David | Java | E145 |
| John | JavaScript | E146 |
| Jamie | jQuery | E146 |
| Emma | Java | E147 |
The above relation can be decomposed into the following three tables; therefore, it is not in 5NF −
<EmployeeSkills>
| EmpName | EmpSkills |
| David | Java |
| John | JavaScript |
| Jamie | jQuery |
| Emma | Java |
The following is the <EmployeeJob> relation that displays the jobs assigned to each employee −
<EmployeeJob>
| EmpName | EmpJob |
| David | E145 |
| John | E146 |
| Jamie | E146 |
| Emma | E147 |
Here is the skills that are related to the assigned jobs −
<JobSkills>
| EmpSkills | EmpJob |
| Java | E145 |
| JavaScript | E146 |
| jQuery | E146 |
| Java | E147 |
Our Join Dependency −
| {(EmpName, EmpSkills ), (EmpName, EmpJob), (EmpSkills, EmpJob)} |
The above relations have join dependency, so they are not in 5NF. That would mean that a join relation of the above three relations is equal to our original relation <Employee>.
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