Abstract

Relational database systems provide a powerful abstraction mechanism: any query, since it returns a relation, can be used to define a view, that becomes a derived (or virtual) relation. Views are defined by a statement such as "define view <name> as <query>." Views can be used to tailor the global database schema. . Query formulation is simplified, if frequent subexpressions are predefined. . Application programs are insulated from changes to the underlying schema. . Information can be restructured to better suit an application programs' requirements. . Derived information is kept consistent with base data. . Access restrictions (for authorization) can be enforced by hiding data. In contrast to base relations, views are typically not stored permanently, but rather computed on demand. Queries to view relations are modified by query substitution so as to operate on the underlying b

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