Fourth Normal Form (4NF)
Fourth Normal Form (4NF) is introduced by Ronald Fagin based on Multivalued Dependency. 4NF definition is "a table is in 4NF if and only if, for every one of its non-trivial multivalued dependencies X hold Y, X is a superkey that is, X is either a candidate key or a superset thereof."
4NF is used to decompose many non-trivial multivalued dependencies into individual non-trivial multivalued dependencies that avoid redundent data.
An instance, a Project_Department_Task entity is maintains data through Project_Name, Department_Name and Task_Name attributes. Project_Name, Department_Name and Task_Name are key attributes that entity does not have any non-key attributes.
The Project_Name hold Department_Name and Task_Name. Department_Name and Task_Name are two non-trivial multivalued dependencies on the Project_Name attribute that Project_Name is not a superkey of entity.
These non-trivial multivalued dependencies on a non-superkey are producing redundant data. According to the 4NF, the Project_Department_Task entity can decompose into Project_Department and Project_Task entities to avoid redundant data.
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