View (virtual table)

View• A VIEW is a virtual table• A view contains rows and columns, just like a

real table. • The fields in a view are fields from one or more

real tables in the database. • A view is a table whose rows are not explicitly

stored in the database.• A view does not require a disk space

View• Advantages of view• Restrict access to the data because the view can

display selective columns from the table• Make simple query to retrieve the results of

complicated queries ie without writing join statements

• One view can retrieve data from several tables• Views create with a check option, to prevent

updating of other rows and columns• View allow same data to seen by different

users in different ways at the same time

View• Views are two types• Simple Views & Complex Views• Simple View• Derived from only one table• Can perform DML operations through the view

• Complex View• Derived from many tables• Does not always allow DML operations through

the view

View• Syntax:

• CREATE [OR REPLACE] VIEW view_name AS• Subquery• [WITH CHECK OPTION][WITH READ ONLY]

• OR REPLACE:- is used to re-create the view if it already exists without having to drop it first.

• Subquery:- is a complete SELECT statement • WITH CHECK OPTION :- specifies that only those rows

that are accessible to the view can be inserted or updated

• WITH READ ONLY:- ensure that no DML operations can be performed on this view

View• Example

• CREATE VIEW EMP AS SELECT EMP_ID,EMP_NAME,AGE FROM EMPLOYEE WHERE AGE>45;

• To describe the structure of the view by using• DESC EMP;

View• Retrieving data from a View• SELECT * FROM VIEW;

• Rename the column of a view • CREATE VIEW EMP (ID,NAME,EMP_AGE)AS

SELECT EMP_ID,EMP_NAME,AGE FROM EMPLOYEE WHERE AGE>45;

View• Creating a complex view• Contain values from more tables• If a view is created from multiple table, which

where created using reference key then, through the primary key column as well as the NOT NULL column are included in the view

• The view behavior will be as• An insert operation is not allowed• If a delete operation is executed on the view, the

corresponding records, from the detail table will be deleted

• The view can be used to modify the columns of the detail table included in the view

View• Syntax• Create view viewnane as select

table1.column,.. table2.column,, from table1,table2 where expression1=expression2;

View• CUSTOMER CUSTOMER1

• If we want to create a view named comp_view• Create view comp_view select cid,cname,dname from

customer C, customer1 C1 where C.deptno=C1.deptno;

• Select * from comp_view;

CID CNAME DEPTNO101 RAVI 10102 ANI 20103 AJAY 30104 MANOJ 40105 RAJESH 50

DEPTNO DNAME10 CSE30 IT40 EC50 ME

CID CNAME DNAME101 RAVI CSE103 AJAY IT104 MANOJ EC105 RAJESH ME

View• Updatable & No updatable Views• Views on which data manipulation can be done are called

updatable views• All views are not updatable• Conditions for updatable view are• Views must be derived from a single table• For insert operation:- then primary key column and all

NOT NULL columns must be included in the view• Cannot update, if view is derived using GROUP BY clause• Cannot update, if view is derived using DISTINCT• Cannot update, if view is derived using nested Subquery• Cannot update, if view is derived from another view• The view definition must not include union, intersection,

Minus clause

View• Simple View• Insert row in view• Insert into emp_view values(109,’appu’,900,’manager’);• This SQL insert the row in base table and the view both

• Modify view• Update emp_view set salary=7000 where

ename=‘appu’;• This SQL update both base table & view

• Delete• Delete from emp_view where ename=‘appu’;

View• Complex view• Created from more than one table• create table tab1(rollno int, name char(10) not null, age int not

null, primary key(rollno))

• create table tab2(mid int, rollno int references tab1(rollno), mark int, primary key(mid))

• create or replace view v1 as select tab1.rollno,tab1.name,tab2.mark from tab1,tab2 where tab1.rollno=tab2.rollno

ROLLNO NAME AGE1 aji 202 saji 30

MID ROLLNO MARK1 1 1002 1 90

ROLLNO NAME MARK1 aji 1001 aji 90

ViewComplex viewinsert into v1 values(2,'raj',50)Display an errorcannot modify a column which maps to a non key-

preserved table

update v1 set name='kittu' where rollno=1Display an errorcannot modify a column which maps to a non key-

preserved table

View• Read only views• A view that cannot modify• Create view v2 as select name,age from tab1

where rollno=1 with read only

• Removing views• Drop view view name;

View