Editing Surrogate key (section)

== Definition ==

There are at least two definitions of a surrogate:

; Surrogate (1) &ndash; 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.<ref>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.
</ref>
; Surrogate (2) &ndash; 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{{specify|date=December 2010}} have argued that a surrogate should have the following characteristics:

* the value is 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.