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2-1-3 Definition of View

View refers to the table that is virtually set from the real tables and corresponds to external schema. (View is also referred to as virtual table, and table is also referred to as base table.) View can record attributes of a single table and new attributes that are created from multiple tables. Users can operate the defined views in the same way as normal tables, and therefore, restrict the scope of use of a database, which can help in data security protection and maintenance. CREATE VIEW statement is used for defining a view.

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ex03: Extract StudentNumber and Name from the “Student” table and define the view “Name”.

View defines only a virtual table and does not create any new tables. (It can simply be considered that the shaded portion in Figure 4-9 is made invisible.) Therefore, updating a view would also reflect the updated results in the original table. Because of that, there is a constraint that the views where tuples and the original table are not in a 1:1 relationship cannot be updated. Such a view (e.g. view that uses the set function) is described later.