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Third normal form is a normal form used in normalizing a database design to reduce the duplication of data and ensure referential integrity by ensuring that # the entity is in second normal form and # all the attributes in a table are determined only by the candidate keys of that table and not by any non-prime attributes.〔("What is Third Normal Form?" ) Cory Janssen, Technopedia, retrieved 24 April 2014〕 ==Definition of third normal form== The third normal form (3NF) is a normal form used in database normalization. 3NF was originally defined by E.F. Codd in 1971.〔Codd, E.F. "Further Normalization of the Data Base Relational Model." (Presented at Courant Computer Science Symposia Series 6, "Data Base Systems," New York City, May 24th–25th, 1971.) IBM Research Report RJ909 (August 31st, 1971). Republished in Randall J. Rustin (ed.), ''Data Base Systems: Courant Computer Science Symposia Series 6''. Prentice-Hall, 1972.〕 Codd's definition states that a table is in 3NF if and only if both of the following conditions hold: * The relation R (table) is in second normal form (2NF) * Every non-prime attribute of R is non-transitively dependent on every key of R. A non-prime attribute of R is an attribute that does not belong to any candidate key of R.〔Codd, p. 43.〕 A transitive dependency is a functional dependency in which ''X'' → ''Z'' (''X'' determines ''Z'') indirectly, by virtue of ''X'' → ''Y'' and ''Y'' → ''Z'' (where it is not the case that ''Y'' → ''X'').〔Codd, p. 45–46.〕 A 3NF definition that is equivalent to Codd's, but expressed differently, was given by Carlo Zaniolo in 1982. This definition states that a table is in 3NF if and only if, for each of its functional dependencies ''X'' → ''A'', at least one of the following conditions holds: * ''X'' contains ''A'' (that is, ''X'' → ''A'' is trivial functional dependency), or * ''X'' is a superkey, or * Every element of ''A''-''X'', the set difference between A and X, is a prime attribute (i.e., each attribute in ''A''-''X'' is contained in some candidate key)〔Zaniolo, Carlo. "A New Normal Form for the Design of Relational Database Schemata." ''ACM Transactions on Database Systems'' 7(3), September 1982.〕〔Abraham Silberschatz, Henry F. Korth, S. Sudarshan, ''(Database System Concepts ) (5th edition), p. 276-277〕 Zaniolo's definition gives a clear sense of the difference between 3NF and the more stringent Boyce–Codd normal form (BCNF). BCNF simply eliminates the third alternative ("Every element of A-X, the set difference between A and X, is a prime attribute"). 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「Third normal form」の詳細全文を読む スポンサード リンク
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