*)View is a database Object that keeps reference of data in the
form of SELECT statement.*)It presents itself just like a table,but
the basic difference between a table and view is data.table keeps
data while view keeps query.*)A view is used for following
purposes:-->Restricted data from one or more tables.(selected
records)-->Projection of data from one or more tables.(selected
columns).-->Hiding the actual tables from users.--> To avoid
data manipulation on the tables.*)View means stored query within
database.Syntax:CREATE [FORCE] VIEW AS ;Note:[FORCE]:word is used
to create a view eventhe objects required by the
query(table,columns) does not exist physically.such views does not
work until objects are provided.Example: CREATE VIEW VW1ASSELECT
E.empno,E.ename,E.sal,D.deptno,D.dnameFROM emp E,dept DWHERE
E.deptno=D.deptno;--> SELECT * FROM VW1;--> SELECT * FROM VW1
WHERE sal>2000;--> DELETE FROM VW1; error: cannot
delete--> UPDATE VW1 SET ename='aaa'; error: manipulations are
not allowed on multi table views.Q) How to see the content of a
view?-----------------------------------Oracle Provides a view
under data dictionary that providescontent of views. This view name
is USER_VIEWS. It can beaccessed by any user.--> DESC
USER_VIEWS--> SELECT VIEW_NAME,text FROM USER_VIEWS;Note:-----If
view is based on single table then if we delete the viewthen table
content is also deleted.example:DELETE FROM
VW4;===============================================SQL> create
view d4 2 as 3 select * from dept2;View created.SQL> select *
from d4;DEPTNO DNAME LOC------ -------------- ------------- 10
ACCOUNTING NEW YORK 20 RESEARCH DALLAS 30 SALES CHICAGO 40
OPERATIONS BOSTONSQL> delete from d4;4 rows deleted.SQL>
select * from d4;no rows selectedSQL> select * from dept2;no
rows selectedSQL> drop table dept2;Table dropped.SQL> select
* from d4;select * from d4 *ERROR at line 1:ORA-04063: view
"SIVA1.D4" has
errors======================================================================================================================================================================================================SYNONYMS:=========*)Synonyms
are used :--> To keep short cut names for objects.--> To keep
meaningful name Objects.--> To keep the reference of remote
object.--> To hide the actual place of objects.Syntax:CREATE
[PUBLIC] SYNONYM FOR ;example: CREATE SYNONYM s1 FOR VW4;-->
DESC VW4;--> DESC s1--> DESC USER_SYNONYMS;--> SELECT
Synonym_name,table_name,table_owner FROM user_sysnonyms;Q) How to
know whether it is a synonym or view ?Make the following
trials:--> DELETE FROM s1;--> SELECT * FROM tab;--> DESC
s1;error: invalid object for describe.Note:--> View is only for
table(It works only on query.)--> Synonym is for any type of
Object.(Works on
Object).===================================================================SQL>
create view d4 2 as 3 select * from dept2;View created.SQL>
select * from d4;DEPTNO DNAME LOC------ --------------
------------- 10 ACCOUNTING NEW YORK 20 RESEARCH DALLAS 30 SALES
CHICAGO 40 OPERATIONS BOSTONSQL> delete from d4;4 rows
deleted.SQL> select * from d4;no rows selectedSQL> select *
from dept2;no rows selectedSQL> drop table dept2;Table
dropped.SQL> select * from d4;select * from d4 *ERROR at line
1:ORA-04063: view "SIVA1.D4" has errorsSQL> create table dept2
as select * from dept;Table created.SQL> create view VW4 2 as 3
select * from dept2;View created.SQL> select * from vw4;DEPTNO
DNAME LOC------ -------------- ------------- 10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTONSQL>
create synonym s1 2 from vw4;from vw4*ERROR at line 2:ORA-00905:
missing keywordSQL> create synonym s1 2 for vw4;Synonym
created.SQL> select * from s1;DEPTNO DNAME LOC------
-------------- ------------- 10 ACCOUNTING NEW YORK 20 RESEARCH
DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTONSQL> desc vw4 Name
Null? Type ----------------------------------------- --------
---------------------------- DEPTNO NUMBER(2) DNAME VARCHAR2(14)
LOC VARCHAR2(13)SQL> desc s1 Name Null? Type
----------------------------------------- --------
---------------------------- DEPTNO NUMBER(2) DNAME VARCHAR2(14)
LOC VARCHAR2(13)SQL> desc user_synonyms; Name Null? Type
----------------------------------------- --------
---------------------------- SYNONYM_NAME NOT NULL VARCHAR2(30)
TABLE_OWNER VARCHAR2(30) TABLE_NAME NOT NULL VARCHAR2(30) DB_LINK
VARCHAR2(128)SQL> select synonym_name,table_name,table_owner 2
from user_synonyms;SYNONYM_NAME
TABLE_NAME------------------------------
------------------------------TABLE_OWNER------------------------------S1
VW4SIVA1SQL> delete from s1;4 rows deleted.SQL> select * from
vw4;no rows selectedSQL> select * from dept2;no rows
selectedSQL>
=============================================================SQL>
select * from s2;select * from s2 *ERROR at line 1:ORA-00942: table
or view does not existSQL> select * from s1;DEPTNO DNAME
LOC------ -------------- ------------- 10 ACCOUNTING NEW YORK 20
RESEARCH DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTONSQL> delete
from s1;4 rows deleted.SQL> select * from vw4;no rows
selectedSQL> select * from dept2;no rows selectedSQL>
rollback;Rollback complete.SQL> create view vw5 2 as 3 select *
from s1;View created.SQL> select * from vw5;DEPTNO DNAME
LOC------ -------------- ------------- 10 ACCOUNTING NEW YORK 20
RESEARCH DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTONSQL> create
view vw6 2 as select * from vw4;View created.SQL> delete from s1
where deptno=10;1 row deleted.SQL> select * from vw6;DEPTNO
DNAME LOC------ -------------- ------------- 20 RESEARCH DALLAS 30
SALES CHICAGO 40 OPERATIONS BOSTONSQL> select * from vw4;DEPTNO
DNAME LOC------ -------------- ------------- 20 RESEARCH DALLAS 30
SALES CHICAGO 40 OPERATIONS BOSTONSQL> select * from
dept2;DEPTNO DNAME LOC------ -------------- ------------- 20
RESEARCH DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTONSQL>
rollback;Rollback complete.SQL> delete from vw5 2 where
depno=10;where depno=10 *ERROR at line 2:ORA-00904: invalid column
nameSQL> edWrote file afiedt.buf 1 delete from vw5 2* where
deptno=20SQL> /1 row deleted.SQL> select * from dept2;DEPTNO
DNAME LOC------ -------------- ------------- 10 ACCOUNTING NEW YORK
30 SALES CHICAGO 40 OPERATIONS BOSTONSQL> select * from
vw4;DEPTNO DNAME LOC------ -------------- ------------- 10
ACCOUNTING NEW YORK 30 SALES CHICAGO 40 OPERATIONS BOSTONSQL>
select * from vw6;DEPTNO DNAME LOC------ --------------
------------- 10 ACCOUNTING NEW YORK 30 SALES CHICAGO 40 OPERATIONS
BOSTONSQL> drop synonym s1;Synonym dropped.SQL> select * from
vw5;select * from vw5 *ERROR at line 1:ORA-04063: view "SIVA1.VW5"
has errorsSQL> select * from vw4;DEPTNO DNAME LOC------
-------------- ------------- 10 ACCOUNTING NEW YORK 30 SALES
CHICAGO 40 OPERATIONS BOSTONSQL> select * from vw6;DEPTNO DNAME
LOC------ -------------- ------------- 10 ACCOUNTING NEW YORK 30
SALES CHICAGO 40 OPERATIONS BOSTONSQL> rollback;Rollback
complete.SQL> select * from vw2;select * from vw2 *ERROR at line
1:ORA-00942: table or view does not existSQL> edWrote file
afiedt.buf 1* select * from vw4SQL> /DEPTNO DNAME LOC------
-------------- ------------- 10 ACCOUNTING NEW YORK 30 SALES
CHICAGO 40 OPERATIONS BOSTONSQL>
==================================================
