Domain/key normal form

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Domain/key normal form (DKNF) is a normal form used in database normalization which requires that the database contains no constraints other than domain constraints and key constraints.

A domain constraint specifies the permissible values for a given attribute, while a key constraint specifies the attributes that uniquely identify a row in a given table.

The domain/key normal form is achieved when every constraint on the relation is a logical consequence of the definition of keys and domains, and enforcing key and domain restraints and conditions causes all constraints to be met. Thus, it avoids all non-temporal anomalies.

It's much easier to build a database in domain/key normal form than it is to convert lesser databases which may contain numerous anomalies. However, successfully building a domain/key normal form database remains a difficult task, even for experienced database programmers. Thus, while the domain/key normal form eliminates the problems found in most databases, it tends to be the most costly normal form to achieve. However, failing to achieve the domain/key normal form may carry long-term, hidden costs due to anomalies which appear in databases adhering only to lower normal forms over time.

A violation of DKNF occurs in the following table:

Wealthy Person
Wealthy Person	Wealthy Person Type	Net Worth in Dollars
Steve	Eccentric Millionaire	124,543,621
Roderick	Evil Billionaire	6,553,228,893
Katrina	Eccentric Billionaire	8,829,462,998
Gary	Evil Millionaire	495,565,211

(Assume that the domain for Wealthy Person consists of the names of all wealthy people in a pre-defined sample of wealthy people; the domain for Wealthy Person Type consists of the values 'Eccentric Millionaire', 'Eccentric Billionaire', 'Evil Millionaire', and 'Evil Billionaire'; and the domain for Net Worth in Dollars consists of all integers greater than or equal to 1,000,000.)

There is a constraint linking Wealthy Person Type to Net Worth in Dollars, even though we cannot deduce one from the other. The constraint dictates that an Eccentric Millionaire or Evil Millionaire will have a net worth of 1,000,000 to 999,999,999 inclusive, while an Eccentric Billionaire or Evil Billionaire will have a net worth of 1,000,000,000 or higher. This constraint is neither a domain constraint nor a key constraint; therefore we cannot rely on domain constraints and key constraints to guarantee that an inconsistent Wealthy Person Type / Net Worth in Dollars combination does not make its way into the database.

The DKNF violation could be eliminated by altering the Wealthy Person Type domain to make it consist of just two values, 'Evil' and 'Eccentric' (the wealthy person's status as a millionaire or billionaire is implicit in their Net Worth in Dollars, so no useful information is lost).

DKNF is frequently difficult to achieve in practice.