The actual tables in the schema for a relational database, the logical-level tables, are called base tables. In the standard three-level architecture, the top level consists of external views. An external view is the structure of the database as it appears to a particular user. In the relational model, the word view has a slightly different meaning. Rather than being the entire external model of a user, a relational view is a virtual table that is either a subset of a base table or is constructed by performing operations such as relational algebra selection, projection, join, or other calculations on base tables. Thus, an external model can comprise both actual base tables and views derived from tables.

The view mechanism is desirable ...

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