Editing Data model (section)

=== Three perspectives ===
[[File:4-2 ANSI-SPARC three level architecture.svg|thumb|320px|The ANSI/SPARC [[Three schema approach|three level architecture]]. This shows that a data model can be an external model (or view), a conceptual model, or a physical model. This is not the only way to look at data models, but it is a useful way, particularly when comparing models.<ref name="MW99"/>]]
A data model ''instance'' may be one of three kinds according to [[ANSI]] in 1975:<ref>American National Standards Institute. 1975. ''ANSI/X3/SPARC Study Group on Data Base Management Systems; Interim Report''. FDT (Bulletin of ACM SIGMOD) 7:2.</ref>
# [[Conceptual data model]]: describes the semantics of a domain, being the scope of the model. For example, it may be a model of the interest area of an organization or industry. This consists of entity classes, representing kinds of things of significance in the domain, and relationship assertions about associations between pairs of entity classes. A conceptual schema specifies the kinds of facts or propositions that can be expressed using the model. In that sense, it defines the allowed expressions in an artificial 'language' with a scope that is limited by the scope of the model.
# [[Logical data model]]: describes the semantics, as represented by a particular data manipulation technology.  This consists of descriptions of tables and columns, object oriented classes, and XML tags, among other things.
# [[Physical data model]]: describes the physical means by which data are stored.  This is concerned with partitions, CPUs, tablespaces, and the like.

The significance of this approach, according to ANSI, is that it allows the three perspectives to be relatively independent of each other. Storage technology can change without affecting either the logical or the conceptual model. The table/column structure can change without (necessarily) affecting the conceptual model.  In each case, of course, the structures must remain consistent with the other model.  The table/column structure may be different from a direct translation of the entity classes and attributes, but it must ultimately carry out the objectives of the conceptual entity class structure.  Early phases of many software development projects emphasize the design of a [[Conceptual schema|conceptual data model]]. Such a design can be detailed into a [[logical data model]]. In later stages, this model may be translated into [[physical data model]]. However, it is also possible to implement a conceptual model directly.