|
A surrogate key in a database is a unique identifier for either an ''entity'' in the modeled world or an ''object'' in the database. The surrogate key is ''not'' derived from application data, unlike a ''natural'' (or ''business'') key which is derived from application data. == Definition == There are at least two definitions of a surrogate: ; Surrogate (1) – Hall, Owlett and Todd (1976): A surrogate represents an ''entity'' in the outside world. The surrogate is internally generated by the system but is nevertheless visible to the user or application. 〔P A V Hall, J Owlett, S J P Todd, "Relations and Entities", Modelling in Data Base Management Systems (ed GM Nijssen), North Holland 1976. 〕 ; Surrogate (2) – Wieringa and De Jonge (1991): A surrogate represents an ''object'' in the database itself. The surrogate is internally generated by the system and is invisible to the user or application. The ''Surrogate (1)'' definition relates to a data model rather than a storage model and is used throughout this article. See Date (1998). An important distinction between a surrogate and a primary key depends on whether the database is a current database or a temporal database. Since a ''current database'' stores only ''currently'' valid data, there is a one-to-one correspondence between a surrogate in the modeled world and the primary key of the database. In this case the surrogate may be used as a primary key, resulting in the term ''surrogate key''. In a temporal database, however, there is a many-to-one relationship between primary keys and the surrogate. Since there may be several objects in the database corresponding to a single surrogate, we cannot use the surrogate as a primary key; another attribute is required, in addition to the surrogate, to uniquely identify each object. Although Hall ''et al.'' (1976) say nothing about this, others have argued that a surrogate should have the following characteristics: * the value is unique system-wide, hence never reused * the value is system generated * the value is not manipulable by the user or application * the value contains no semantic meaning * the value is not visible to the user or application * the value is not composed of several values from different domains. 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「Surrogate key」の詳細全文を読む スポンサード リンク
|