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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Conjunctive query ： ウィキペディア英語版 Conjunctive query In database theory, a conjunctive query is a restricted form of first-order queries. Many first-order queries can be written as conjunctive queries. In particular, a large part of queries issued on relational databases can be expressed in this way. Conjunctive queries also have a number of desirable theoretical properties that larger classes of queries (e.g., the relational algebra queries) do not share. ==Definition== The conjunctive queries are simply the fragment of (domain independent) first-order logic given by the set of formulae that can be constructed from atomic formulae using conjunction $\backslash wedge$ and existential quantification $\backslash exists$, but not using disjunction $\backslash lor$, negation $\backslash neg$, or universal quantification $\backslash forall$. Each such formula can be rewritten (efficiently) into an equivalent formula in prenex normal form, thus this form is usually simply assumed. Thus conjunctive queries are of the following general form: :$(x\_1,\; \backslash ldots,\; x\_k).\backslash exists\; x\_,\; \backslash ldots\; x\_m.\; A\_1\; \backslash wedge\; \backslash ldots\; \backslash wedge\; A\_r$, with the free variables $x\_1,\; \backslash ldots,\; x\_k$ being called distinguished variables, and the bound variables $x\_,\; \backslash ldots,\; x\_m$ being called undistinguished variables. $A\_1,\; \backslash ldots,\; A\_r$ are atomic formulae. Conjunctive queries without distinguished variables are called boolean conjunctive queries. As an example of why the restriction to domain independent first-order logic is important, consider $x\_1.\backslash exists\; x\_2.\; R(x\_2)$, which is not domain independent; see Codd's theorem. This formula cannot be implemented in the select-project-join fragment of relational algebra, and hence should not be considered a conjunctive query. Conjunctive queries can express a large part of queries, which are frequently issued on relational databases. To give an example, imagine a relational database for storing information about students, their address, the courses they visit and their gender. Finding all male students and their addresses who attend a course that is also attended by a female student is expressed by the following conjunctive query: (student, address) . $\backslash exists$(student2, course) . attends(student, course) $\backslash wedge$ gender(student, 'male') $\backslash wedge$ attends(student2, course) $\backslash wedge$ gender(student2, 'female') $\backslash wedge$ lives(student, address) Note that since the only entity of interest is the male student and his address, these are the only distinguished variables, while the variables course, student2 are only existentially quantified, i.e. undistinguished. 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「Conjunctive query」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.