Concept of View in DBMS and its advantages

A view is a virtual table. A view has no independent physical existence. The view’s definition exists only in the database’s metadata, but the data comes from the table or tables from which you derive the view.

A view is the result set of a stored query on the data in which the database users can query just as they would in a persistent database collection object.

This pre-established query command is kept in the database dictionary. Unlike ordinary base tables in a relational database, a view does not form part of the physical schema: as a result set, it is a virtual table computed or collated dynamically from data in the database when access to that view is requested. Changes applied to the data in a relevant underlying table are reflected in the data shown in subsequent invocations of the view.

Advantages of View:-

·         Views can represent a subset of the data contained in a table. Consequently, a view can limit the degree of exposure of the underlying tables to the outer world: a given user may have permission to query the view, while denied access to the rest of the base table.

·         Views can join and simplify multiple tables into a single virtual table.

·         Views can act as aggregated tables, where the database engine aggregates data (Sum, Average etc.) and presents the calculated results as part of the data.

·         Views can hide the complexity of data.

·         Views take very little space to store; the database contains only the definition of a view, not a copy of all the data that it presents.

·         Depending on the SQL engine used, views can provide extra security.

Basically there are  two Views regarding to DBMS.

1.     Updatable View

2.     Non-updatable views

Difference between updatable and non-updatable views:

Database practitioners can define views as non-updatable or updatable. If the database system can determine the reverse mapping from the view schema to the schema of the underlying base tables, then the view is updatable.

INSERT, UPDATE, and DELETE operations can be performed on updatable views.

Read-only views do not support such operations because the DBMS cannot map the changes to the underlying base tables. A view update is done by key preservation.