|
In relational databases, relvar is a term introduced by C. J. Date and Hugh Darwen as an abbreviation for relation variable in their 1995 paper ''The Third Manifesto'', to avoid the confusion sometimes arising from the use of the term relation, by the inventor of the relational model, E. F. Codd, for a variable to which a relation is assigned as well as for the relation itself. The term is used in Date's well-known database textbook ''An Introduction to Database Systems'' and in various other books authored or coauthored by him. Relvar is not universally accepted as a term, and it is not used in the context of existing database management system products that support SQL, whose counterpart concept (but not exact equivalent) is the base table, this being something that, like computer language variables in general, has a name and is subject to update (i.e., being assigned different values from time to time). Other database textbooks continue to use the term relation for both the variable and the data it contains. Similarly, texts on SQL tend to use the term ''table'' for both purposes, though the qualified term ''base table'' is used in the standard for the variable. A closely related term often used in academic texts is relation schema, this being a set of attributes paired with a set of constraints, together defining a set of relations for the purpose of some discussion (typically, database normalization). Constraints that mention just one relvar are termed ''relvar constraints'', so relation schema can be regarded as a single term encompassing a relvar and its relvar constraints. ==References== * C.J. Date. ''An Introduction to Database Systems'', 8th Ed. (Addison-Wesley, 2004, ISBN 0-321-19784-4), pp. 65–6. * C.J. Date and Hugh Darwen. ''Databases, Types, and The Relational Model: The Third Manifesto'' (Addison-Wesley, 2007, ISBN 0-321-39942-0), p.85 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「relvar」の詳細全文を読む スポンサード リンク
|