Editing Fourth normal form (section)

==Multivalued dependencies==
If the column headings in a relational database table are divided into three disjoint groupings ''X'', ''Y'', and ''Z'', then, in the context of a particular row, we can refer to the data beneath each group of headings as ''x'', ''y'', and ''z'' respectively. A [[multivalued dependency]] ''X'' <math>\twoheadrightarrow</math> ''Y'' signifies that if we choose any ''x'' actually occurring in the table (call this choice ''x<sub>c</sub>''), and compile a list of all the ''x<sub>c</sub>yz'' combinations that occur in the table, we will find that ''x<sub>c</sub>'' is associated with the same ''y'' entries regardless of ''z''. So essentially the presence of z provides no useful information to constrain the possible values of ''y''.

A '''trivial multivalued dependency''' ''X'' <math>\twoheadrightarrow</math> ''Y'' is one where either ''Y'' is a subset of ''X'', or ''X'' and ''Y'' together form the whole set of attributes of the relation.

A [[functional dependency]] is a special case of multivalued dependency. In a functional dependency ''X'' → ''Y'', every ''x'' determines ''exactly one y'', never more than one.