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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.〔"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." 〕 ==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'' ''Y'' signifies that if we choose any ''x'' actually occurring in the table (call this choice ''xc''), and compile a list of all the ''xcyz'' combinations that occur in the table, we will find that ''xc'' 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'' ''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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fourth normal form」の詳細全文を読む スポンサード リンク
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