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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 Selection (relational algebra) ： ウィキペディア英語版 Selection (relational algebra) In relational algebra, a selection (sometimes called a restriction to avoid confusion with SQL's use of SELECT) is a unary operation written as $\backslash sigma\_(\; R\; )$ or $\backslash sigma\_(\; R\; )$ where: * $a$ and $b$ are attribute names * $\backslash theta$ is a binary operation in the set $\backslash$ * $v$ is a value constant * $R$ is a relation The selection $\backslash sigma\_(\; R\; )$ selects all those tuples in $R$ for which $\backslash theta$ holds between the $a$ and the $b$ attribute. The selection $\backslash sigma\_(\; R\; )$ selects all those tuples in $R$ for which $\backslash theta$ holds between the $a$ attribute and the value $v$. For an example, consider the following tables where the first table gives the relation $Person$, the second table gives the result of $\backslash sigma\_(\; Person\; )$ and the third table gives the result of $\backslash sigma\_(\; Person\; )$. ( Person ) |- style="vertical-align: top" | | | |} More formally the semantics of the selection is defined as follows: : $\backslash sigma\_(\; R\; )\; =\; \backslash$ : $\backslash sigma\_(\; R\; )\; =\; \backslash$ The result of the selection is only defined if the attribute names that it mentions are in the heading of the relation that it operates upon. In computer languages it is expected that any truth-valued expression be permitted as the selection condition rather than restricting it to be a simple comparison. In SQL, selections are performed by using WHERE definitions in SELECT, UPDATE, and DELETE statements, but note that the selection condition can result in any of three truth values (''true'', ''false'' and ''unknown'') instead of the usual two. ==See also== * Generalized selection 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「Selection (relational algebra)」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.