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The three-schema approach, or the ''Three Schema Concept'', in software engineering is an approach to building information systems and systems information management from the 1970s. It proposes to use three different views in systems development, in which conceptual modelling is considered to be the key to achieving data integration.〔 (STRAP SECTION 2 APPROACH ). Retrieved 30 September 2008.〕 == Overview == The three-schema approach offers three types of schemas with schema techniques based on formal language descriptions:〔Mary E.S. Loomis (1987). ''The Database Book''. p. 26.〕 * External schema for user views * Conceptual schema integrates external schemata * Internal schema that defines physical storage structures At the center, the conceptual schema defines the ontology of the concepts as the users think of them and talk about them. The physical schema according to Sowa (2004) "describes the internal formats of the data stored in the database, and the external schema defines the view of the data presented to the application programs".〔 John F. Sowa (2004). ("The Challenge of Knowledge Soup" ). published in: ''Research Trends in Science, Technology and Mathematics Education''. Edited by J. Ramadas & S. Chunawala, Homi Bhabha Centre, Mumbai, 2006.〕 The framework attempted to permit multiple data models to be used for external schemata.〔Gad Ariav & James Clifford (1986). ''New Directions for Database Systems: Revised Versions of the Papers''. New York University Graduate School of Business Administration. Center for Research on Information Systems, 1986.〕 Over the years, the skill and interest in building information systems has grown tremendously. However, for the most part, the traditional approach to building systems has only focused on defining data from two distinct views, the "user view" and the "computer view". From the user view, which will be referred to as the “external schema,” the definition of data is in the context of reports and screens designed to aid individuals in doing their specific jobs. The required structure of data from a usage view changes with the business environment and the individual preferences of the user. From the computer view, which will be referred to as the “internal schema,” data is defined in terms of file structures for storage and retrieval. The required structure of data for computer storage depends upon the specific computer technology employed and the need for efficient processing of data.〔 itl.nist.gov (1993) (''Integration Definition for Information Modeling (IDEFIX)'' ). 21 Dec 1993.〕 These two traditional views of data have been defined by analysts over the years on an application by application basis as specific business needs were addressed, see Figure 1. Typically, the internal schema defined for an initial application cannot be readily used for subsequent applications, resulting in the creation of redundant and often inconsistent definition of the same data. Data was defined by the layout of physical records and processed sequentially in early information systems. The need for flexibility, however, led to the introduction of Database Management Systems (DBMSs), which allow for random access of logically connected pieces of data. The logical data structures within a DBMS are typically defined as either hierarchies, networks or relations. Although DBMSs have greatly improved the shareability of data, the use of a DBMS alone does not guarantee a consistent definition of data. Furthermore, most large companies have had to develop multiple databases which are often under the control of different DBMSs and still have the problems of redundancy and inconsistency.〔 The recognition of this problem led the ANSI/X3/SPARC Study Group on Database Management Systems to conclude that in an ideal data management environment a third view of data is needed. This view, referred to as a “conceptual schema” is a single integrated definition of the data within an enterprise which is unbiased toward any single application of data and is independent of how the data is physically stored or accessed, see Figure 2. The primary objective of this conceptual schema is to provide a consistent definition of the meanings and interrelationship of data which can be used to integrate, share, and manage the integrity of data.〔 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「three schema approach」の詳細全文を読む スポンサード リンク
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