Editing Data modeling (section)

=== Conceptual, logical and physical schemas ===
[[File:4-2 ANSI-SPARC three level architecture.svg|thumb|320px|The ANSI/SPARC 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"/>]]

In 1975 [[American National Standards Institute|ANSI]] described three kinds of data-model ''instance'':<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 schema]]: describes the semantics of a domain (the scope of the model). For example, it may be a model of the interest area of an organization or of an 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. Simply described, a conceptual schema is the first step in organizing the data requirements.
* [[Logical schema]]: describes the structure of some domain of information.  This consists of descriptions of (for example) tables, columns, object-oriented classes, and XML tags. The logical schema and conceptual schema are sometimes implemented as one and the same.<ref name="RS001"/>
* [[Physical schema]]: describes the physical means used to store data.  This is concerned with partitions, CPUs, [[tablespace]]s, and the like.

According to ANSI, this approach allows the three perspectives to be relatively independent of each other. Storage technology can change without affecting either the logical or the conceptual schema. The table/column structure can change without (necessarily) affecting the conceptual schema.  In each case, of course, the structures must remain consistent across all schemas of the same data model.