Views are virtual relations without independent existence in a database. The contents of the instance of a view are determined by the result of a query on a set of database tables or other views in the system.
Over the years, there have been several definitions and uses for the term view. From one perspective, a view is a query (or a macro) that generates data every time it is invoked. The term view is also used in association with the derived data that is produced by the execution of the view query. Views have also been used as indexes, summary tables, or combinations of both. They provide physical data independence, by decoupling the logical schema from the physical design, which is usually driven by performance reasons. By controlling access to the views, one can control access to different parts of the data and implement different security policies. Views are also used in integration systems in order to provide unified access to...
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