Page "Fourth normal form" Paragraph 0

Fourth normal form ( 4NF ) is a normal form used in database normalization.

Introduced by Ronald Fagin in 1977, 4NF is the next level of normalization after Boyce – Codd normal form ( BCNF ).

Whereas the second, third, and Boyce – Codd normal forms are concerned with functional dependencies, 4NF is concerned with a more general type of dependency known as a multivalued dependency.

A Table is in 4NF if and only if, for every one of its non-trivial multivalued dependencies X Y, X is a superkey — that is, X is either a candidate key or a superset thereof .< ref name =" Fagin ">" A relation schema R * is in fourth normal form ( 4NF ) if, whenever a nontrivial multivalued dependency X Y holds for R *, then so does the functional dependency X → A for every column name A of R *.

Intuitively all dependencies are the result of keys.