Database Views

A database view is a stored query accessible as a virtual table. The contents of this virtual table are the results of that query. Views are only logically stored in the database (i.e. the contents are not physically stored, but dynamically computed whenever needed). Materialized views are an exception to this rule. Changing the data in a base table automatically alters any view that references that table.

A view can be used in exactly the same way as a base table. Views can be used for the following reasons:

• Simplify queries by creating views to represnts parts of these queries
• Hiding the complexity of the data, by creating views that reflect on the parts of the data relevant to a particular user
• Providing extra security by giving users access only to the views, not to the base tables, thus restricting the data they can access
• Since the actual contents of the views are not physically stored, they do not require much space

Views can be seen in analogy to modules (or functions) in programming. They can be used to create abstraction. They can be nested (a query can reference a view, and a view can reference other views, etc.)

For example, consider the following two queries:

Query 1:		Query 2:
SELECT name, balance FROM (SELECT name, money_received, money_sent, (money_received - money_sent) AS balance, address FROM table_customers c, accounts_table a WHERE a.customerid = c.customer_id ) WHERE money_sent > 10000		SELECT name, address FROM (SELECT name, money_received, money_sent, (money_received - money_sent) AS balance, address FROM table_customers c, accounts_table a WHERE a.customerid = c.customer_id ) WHERE balance between 10000 and 20000

Both these queries contain the same sub-query. To simplify these queries, it is possible to define a view (v_cust_account) as follows:

CREATE VIEW v_cust_account AS
SELECT name, money_received, money_sent,
(money_received - money_sent) AS balance, address
FROM table_customers c, accounts_table a
WHERE a.customerid = c.customer_id


Now this view can be treated as a table containing the results of the subquery. Now the 2 initial queries can be simplified as follows:

Query 1a:		Query 2a:
SELECT name, balance FROM v_cust_account WHERE money_sent > 10000		SELECT name, address FROM v_cust_account WHERE balance between 10000 and 20000

Views are usually read-only, i.e. one cannot directly update the data in a view, but can only update the underlying tables. However, in some cases, a view can be updatable, but only if the database system is able to determine the reverse mapping from the view schema to the schema of the underlying base tables. In this case, INSERT, UPDATE, and DELETE operations can be performed. Read-only views do not support such operations because the DBMS is not able to map the changes to the underlying base tables.

During query processing, queries that reference views are usually rewritten to reference the base tables. For example, if a user issues Query 1a to the DBMS, the query processor will determine that the query references the view (v_cust_account). It will then proceed to rewrite this query to reference the base tables (thus it will look like the original query (Query 1).