View and Materialized view

What is a view?

Logically represents subset of data from one or more table. In sql, a view is a virtual relation based on result-set of a SELECT

statement. It contains rows and columns just like a real table. Views contains no data of its own but is like a window through

which data from table can be viewed or changed. The table on which view a view is based are called BASE TABLE. View is stored as a SELECT statement in the data dictonary. SYNTAX CREATE VIEW view_name AS

SELECT column_name(s) FROM table_name WHERE condition;

Advantages of views

Views restrict access to data.Views can be used to make simple queries to

retrieve the result of complicated queries.Views provide group of users access to data

according to their particular criteria.

Types of views

Simple viewsComplex views

Simple viewsSimple views Complex viewsComplex views

A simple view is one that : Derives data from one table. Contains no functions or group of

data. Can perform DML operations

through view.

Example:CREATE VIEW emp_view ASSELECT f_name, salary from

Employees WHERE dep_id=90;

A complex view is one that: Derives data from many tables. Contains functions or group of data. Does not always allow DML

operations through view. In particular, complex views can

contain:-join condition-Distinct-Pseudo columns-Group By clause-Having clause

Example:CREATE VIEW comp_view ASSELECT

e.f_name,e.emp_id,e.job_id,e.dep_id,d.dep_id,d.location

WHERE e.dep_id=d.dep_id;

Difference between simple and complex views

Creating a view

You can create a view by embedding a subquery within the CREATE VIEW statement.

The sub query that defines the view can not contain ORDER BY clause. The ORDER BY clause is specified when you retrieve data from the view.

Syntax:CREATE OR REPLACE view view_nameASSubquery;In the syntax:OR REPLACE – re_creates the view if its already exists.View _name – name of the view.Subquery - SELECT statement.Eg:CREATE OR REPLACE VIEW emp_view ASSELECT f_name, salary from Employees WHERE dep_id=90;SELECT * FROM emp_view;

View WITH READ ONLY option

To restrict the data manipulation using views is done using WITH READ ONLY.

CREATE OR REPLACE view v1ASSELECT A from T1Where A=1 WITH READ ONLY; Now you perform a DML operation on this view it will throw

an error.Eg:INSERT into v1 value(3);Output:ORA-01733: virtual column not allowed here

View WITH CHECK OPTION

It specifies that only rows accessible to the view can be inserted or updated.

Here WHERE clause will be checked. If WHERE clause does not satisfy error will be thrownEg:CREATE OR REPLACE view v1ASSELECT A FROM T1WHERE A=1 WITH CHECK OPTION;INSERT into v1 values(3);Output:ORA-01402: view WITH CHECK OPTION where-clause violation

View with FORCE option

Creates the view regardless of whether or not the base table exist. But we will get error.Eg :CREATE OR REPLACE FORCE view v_newASSELECT * FROM GREENS; When we execute this particular query view will be created but with

error.Output: ORA-00942: table or view does not existEg:SELECT * FROM v_new; When we execute this particular view it will throw an error.ORA-04063: view "HR.V1" has errors.

Querying a view

Once your view has been created, you can query the data dictionary view called USER_VIEWS to see the name of the view and the view definition.

The text of the SELECT statement that constitutes your view is stored in a LONG column.

Removing a view

You can remove a view without losing data . The DROP statement removes the view definition from the

DB. Views based on deleted views become invalid. Only the creator or a user with the DROP ANY VIEW

privilege can remove a view. We can remove the error of view by dropping the view or

by creating a table . SELECT * FROM USER_ERROR;

SyntaxDrop view <viewname>;Eg:Drop view v_new;View dropped.

Inline views

An inline view is a subquery with an alias that you can use within a sql statement.

Subquery used in the FROM clause is called inline view.Eg:Select MAX(salary),MIN(salary)FROM (SELECT f_name,salary from emp where d_id=60);

Materialized view

It is a snapshot of a remote table data. It is a database object. Data manipulation cannot be done here. The tables in the query are called master tables or detail table. To store a query view is used and to store a result materialized view is

used. Sysntax:Create materialized view mvASSELECT f_name, salary from employeesWHERE d_id=90;Select * from mv; When you update a table to refresh the materialized view we useDBMS_MVVIEW.REFRESH(‘mv’,’C’);C means complete refresh. To check all the materialized views created we useSELECT * FROM USER_MVVIEW;

Drop materialized view

We can delete a materialized view using syntaxDROP MATERIALIZED VIEW <view_name>;Eg:Drop materialized view mv;

Difference between normal view and materialized view

Normal view It is a stored select statement. It is a virtual component. It allows DESC,DML,SELECT on it. It is stored permanently in “user_views” system table. DML on view are reflected in table and DML on table are

reflected in view. It is used to share “selected rows and columns” with other

rows. It is used for reporting purpose. It will improve the performance while manipulating or

retrieving data through views.

Materialized view It is a static view It holds data in it It will not support DML on it. DML on tale will not reflect in view To create it “create materialized view” permission is

required. It is used to maintain historic data. It is used for data analysis and reporting purpose. It is same as SNAP SHOT.