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The ANSI-SPARC Architecture, where ANSI-SPARC stands for ''American National Standards Institute, Standards Planning And Requirements Committee'', is an abstract design standard for a Database Management System (DBMS), first proposed in 1975.〔ANSI/X3/SPARC Study Group on Data Base Management Systems: (1975), ''Interim Report. FDT'', ACM SIGMOD bulletin. Volume 7, No. 2〕 Most modern commercial DBMS are based on this system. The ANSI-SPARC model however never became a formal standard. == Three-level architecture == The objective of the three-level architecture is to separate the users’ view, *It allows independent customized user views: Each user should be able to access the same data, but have a different customized view of the data. These should be independent: changes to one view should not affect others. *It hides the physical storage details from users: Users should not have to deal with physical database storage details. * The database administrator should be able to change the database storage structures without affecting the users’ views. * The internal structure of the database should be unaffected by changes to the physical aspects of the storage: For example, a changeover to a new disk. three level are: * External level * Conceptual level * Internal level The Three Level Architecture has the aim of enabling users to access the same data but with a personalised view of it. The distancing of the internal level from the external level means that users do not need to know how the data is physically stored in the database. This level separation also allows the Database Administrator (DBA) to change the database storage structures without affecting the users' views. * External Level (User Views): A user's view of the database describes a part of the database that is relevant to a particular user. It excludes irrelevant data as well as data which the user is not authorised to access. * Conceptual Level: The conceptual level is a way of describing what data is stored within the whole database and how the data is inter-related. The conceptual level does not specify how the data is physically stored. Some important facts about this level are: # DBA works at this level. # Describes the structure of all users. # Only DBA can define this level. # Global view of database. # Independent of hardware and software. * Internal Level: The internal level involves how the database is physically represented on the computer system. It describes how the data is actually stored in the database and on the computer hardware. 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「ANSI-SPARC Architecture」の詳細全文を読む スポンサード リンク
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