For all practical purposes we need only the first three normal forms. The chances of you encountering the higher normal forms are very rare. But there are no hard and fast rules about defining normal forms. Historically, the process of normalization and the process of discovering undesirable dependencies were carried out as part of the database design activity. Usually one would go up to the 3NF and in some cases up to 4NF or 5NF. But is possible to define other normal forms that take into account additional types of dependencies and constraints. The idea behind the domain-key normal form (DKNF) is to specify the ultimate normal form that takes into account all the possible types of dependencies and constraints. A relation is said to be in domain-key normal form, if all possible types of dependencies that should hold on the relation can be enforced simply by enforcing the domain constraints and key constraints on the relation. For a relation in DKNF, it becomes easy to enforce all database constraints by simply checking that each attribute value in a tuple is of the appropriate domain and that every key constraint is enforced. Even though the DKNF is intended to be the ultimate normal form, because of the difficulty in including complex constraints in a DKNF relation and the difficulty in specifying general integrity constraints, its practical utility is limited.

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