Editing Data model (section)

== Overview ==
The term '''''data model''''' can refer to two distinct but closely related concepts. Sometimes it refers to an abstract formalization of the [[Object (philosophy)|objects]] and relationships found in a particular application domain: for example the customers, products, and orders found in a manufacturing organization. At other times it refers to the set of concepts used in defining such formalizations: for example concepts such as entities, attributes, relations, or tables. So the "data model" of a banking application may be defined using the entity–relationship "data model". This article uses the term in both senses.

Managing large quantities of structured and [[unstructured data]] is a primary function of [[information system]]s. Data models describe the structure, manipulation, and integrity aspects of the data stored in data management systems such as relational databases. They may also  describe data with a looser structure, such as [[Word processor|word processing]] documents, [[Email|email messages]], pictures, digital audio, and video: [[XQuery and XPath Data Model|XDM]], for example, provides a data model for [[XML]] documents.

=== The role of data models ===
[[File:3-4 Data model roles.svg|thumb|320px|How data models deliver benefit<ref name="MW99">Matthew West and Julian Fowler (1999). [https://sites.google.com/site/drmatthewwest/publications/princ03.pdf Developing High Quality Data Models]. The European Process Industries STEP Technical Liaison Executive (EPISTLE).</ref>]]
The main aim of data models is to support the development of [[information system]]s by providing the definition and format of data. According to West and Fowler (1999) "if this is done consistently across systems then compatibility of data can be achieved. If the same data structures are used to store and access data then different applications can share data. The results of this are indicated above. However, systems and interfaces often cost more than they should, to build, operate, and maintain. They may also constrain the business rather than support it. A major cause is that the quality of the data models implemented in systems and interfaces is poor".<ref name="MW99"/>
* "Business rules, specific to how things are done in a particular place, are often fixed in the structure of a data model. This means that small changes in the way business is conducted lead to large changes in computer systems and interfaces".<ref name="MW99"/>
* "Entity types are often not identified, or incorrectly identified. This can lead to replication of data, data structure, and functionality, together with the attendant costs of that duplication in development and maintenance".<ref name="MW99"/>
* "Data models for different systems are arbitrarily different. The result of this is that complex interfaces are required between systems that share data. These interfaces can account for between 25–70% of the cost of current systems".<ref name="MW99"/>
* "Data cannot be shared electronically with customers and suppliers, because the structure and meaning of data has not been standardized. For example, engineering design data and drawings for process plant are still sometimes exchanged on paper".<ref name="MW99"/>
The reason for these problems is a lack of standards that will ensure that data models will both meet business needs and be consistent.<ref name="MW99"/>

A data model explicitly determines the structure of data. Typical applications of data models include database models, design of information systems, and enabling exchange of data. Usually, data models are specified in a data modeling language.[3]

=== 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.