What Is Domain-key Normal Form (dk/nf) & Fifth Normal Form (5nf)?


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Muhammad Azhar answered
In 1981, Fagin proposed a simple normal form kno\^n as Domain-Key Normal Form (DK/NF). This normal form involves the concepts of domain- key, and constraints. This normal form of relation cannot have insertion, deletion, and modification anomalies. So the Domain-Key Normal Form is defined as:A relation is in DK/NF, if every constraint on the relation is a logical consequence of the definition of keys and domains.

The definition of DK/NF uses the terms domain key, and constraint. The domain of an attribute is the set of allowable values for that attribute. A key is a unique identifier of the tuple. Similarly, constraint defines any rule or restriction applied to the attributes of the relations. Although functional dependencies, multi-valued dependencies, and join dependencies are constraints but "general constraints" are also other types of constraints. We have rules about relationships between attributes of a relation that are not expressed as dependencies. Unfortunately, the concept of Domain-Key Normal Form does not provide a methodology (or algorithm) for converting a relation to DK/NF..

A relation is in Fifth Normal Form (5NF) if it is in Fourth Normal Form and does not have a join dependency. It is also known as Project-Join Normal Form (PJNF).
A relation that has a join dependency cannot be divided into two or more relations such that the resulting relations can be re-combined to form the original relation.
In all the Normal Forms, the technique to divide the relation into two or more relations is used, in order to eliminate the anomalies. In each of the examples, there were no join dependencies. So it was possible to re-create the original relation.

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