Wednesday, 6 August 2014

Database management system | DOMAIN KEY NORMAL FORM


DOMAIN KEY NORMAL FORM
  • 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.
  • The reason to use domain/key normal form is to avoid having general constraints in the database.
  • Most databases can easily test domain and key constraints on attributes. General constraints however would normally require special database programming in the form of stored procedures that are expensive to maintain and expensive for the database to execute. Therefore general constraints are split into domain and key constraints.
  • It's much easier to build a database in domain/key normal form .
  • While the domain/key normal form eliminates the problems found in most databases, it tends to be the most costly normal form to achieve. 
  • The third normal form, Boyce–Codd normal form, fourth normal form and fifth normal form are special cases of the domain/key normal form. All have either functional, multi-valued or join dependencies that can be converted into (super)keys. 
  • The domains on those normal forms were unconstrained so all domain constraints are satisfied. However, transforming a higher normal form into domain/key normal form is not always a dependency-preserving transformation and therefore not always possible.

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