Views are known as logical tables. They represent the data of one of more tables. A view derives its data from the tables on which it is based. These tables are called base tables. Views can be based on actual tables or another view also.
Whatever DML operations you performed on a view they actually affect the base table of the view. You can treat views same as any other table. You can Query, Insert, Update and delete from views, just as any other table.
Views are very powerful and handy since they can be treated just like any other table but do not occupy the space of a table.
The following sections explain how to create, replace, and drop views using SQL commands.
Creating Views
Suppose we have EMP and DEPT table. To see the empno, ename, sal, deptno, department name and location we have to give a join query like this.
select e.empno,e.ename,e.sal,e.deptno,d.dname,d.loc
From emp e, dept d where e.deptno=d.deptno;
So everytime we want to see emp details and department names where they are working we have to give a long join query. Instead of giving this join query again and again, we can create a view on these table by using a CREATE VIEW command given below
create view emp_det as select e.empno,
e.ename,e.sal,e.deptno,d.dname,d.loc
from emp e, dept d where e.deptno=d.deptno;
Now to see the employee details and department names we don’t have to give a join query, we can just type the following simple query.
select * from emp_det;
This will show same result as you have type the long join query. Now you can treat this EMP_DET view same as any other table.
For example, suppose all the employee working in Department No. 10 belongs to accounts department and most of the time you deal with these people. So every time you have to give a DML or Select statement you have to give a WHERE condition like …..WHERE DEPTNO=10. To avoid this, you can create a view as given below
CREATE VIEW accounts_staff AS
SELECT Empno, Ename, Deptno
FROM Emp
WHERE Deptno = 10
WITH CHECK OPTION CONSTRAINT ica_Accounts_cnst;
Now to see the account people you don’t have to give a query with where condition you can just type the following query.
Select * from accounts_staff;
Select sum(sal) from accounst_staff;
Select max(sal) from accounts_staff;
As you can see how views make things easier.
The query that defines the ACCOUNTS_STAFF view references only rows in department 10. Furthermore, WITH CHECK OPTION creates the view with the constraint that INSERT and UPDATE statements issued against the view are not allowed to create or result in rows that the view cannot select.
Considering the example above, the following INSERT statement successfully inserts a row into the EMP table through the ACCOUNTS_STAFF view:
INSERT INTO Accounts_staff VALUES (110, 'ASHI', 10);
However, the following INSERT statement is rolled back and returns an error because it attempts to insert a row for department number 30, which could not be selected using the ACCOUNTS_STAFF view:
INSERT INTO Accounts_staff VALUES (111, 'SAMI', 30);
Creating FORCE VIEWS
A view can be created even if the defining query of the view cannot be executed, as long as the CREATE VIEW command has no syntax errors. We call such a view a view with errors. For example, if a view refers to a non-existent table or an invalid column of an existing table, or if the owner of the view does not have the required privileges, then the view can still be created and entered into the data dictionary.
You can only create a view with errors by using the FORCE option of theCREATE VIEW command:
CREATE FORCE VIEW AS ...;
When a view is created with errors, Oracle returns a message and leaves the status of the view as INVALID. If conditions later change so that the query of an invalid view can be executed, then the view can be recompiled and become valid. Oracle dynamically compiles the invalid view if you attempt to use it.
Replacing/Altering Views
To alter the definition of a view, you must replace the view using one of the following methods:
- A view can be dropped and then re-created. When a view is dropped, all grants of corresponding view privileges are revoked from roles and users. After the view is re-created, necessary privileges must be regranted.
- A view can be replaced by redefining it with a CREATE VIEW statement that contains the OR REPLACE option. This option replaces the current definition of a view, but preserves the present security authorizations.
For example, assume that you create the ACCOUNTS_STAFF view, as given in a previous example. You also grant several object privileges to roles and other users. However, now you realize that you must redefine theACCOUNTS_STAFF view to correct the department number specified in theWHERE clause of the defining query, because it should have been 30.
To preserve the grants of object privileges that you have made, you can replace the current version of the ACCOUNTS_STAFF view with the following statement:
CREATE OR REPLACE VIEW Accounts_staff AS
SELECT Empno, Ename, Deptno
FROM Emp
WHERE Deptno = 30
WITH CHECK OPTION CONSTRAINT ica_Accounts_cnst;
Replacing a view has the following effects:
⦁ Replacing a view replaces the view’s definition in the data dictionary. All underlying objects referenced by the view are not affected.
⦁ If previously defined but not included in the new view definition, then the constraint associated with the WITH CHECK OPTION for a view’s definition is dropped.
⦁ All views and PL/SQL program units dependent on a replaced view become invalid.
With some restrictions, rows can be inserted into, updated in, or deleted from a base table using a view. The following statement inserts a new row into theEMP table using the ACCOUNTS_STAFF view:
INSERT INTO Accounts_staff
VALUES (199, 'ABID', 30);
Restrictions on DML operations for views use the following criteria in the order listed:
1. If a view is defined by a query that contains SET or DISTINCT operators, aGROUP BY clause, or a group function, then rows cannot be inserted into, updated in, or deleted from the base tables using the view.
2. If a view is defined with WITH CHECK OPTION, then a row cannot be inserted into, or updated in, the base table (using the view), if the view cannot select the row from the base table.
3. If a NOT NULL column that does not have a DEFAULT clause is omitted from the view, then a row cannot be inserted into the base table using the view.
4. If the view was created by using an expression, such as DECODE(deptno, 10, “SALES“, …), then rows cannot be inserted into or updated in the base table using the view.
The constraint created by WITH CHECK OPTION of the ACCOUNTS_STAFF view only allows rows that have a department number of 10 to be inserted into, or updated in, the EMP table. Alternatively, assume that theACCOUNTS_STAFF view is defined by the following statement (that is, excluding the DEPTNO column):
CREATE VIEW Accounts_staff AS
SELECT Empno, Ename
FROM Emp
WHERE Deptno = 10
WITH CHECK OPTION CONSTRAINT ica_Accounts_cnst;
Considering this view definition, you can update the EMPNO or ENAME fields of existing records, but you cannot insert rows into the EMP table through theACCOUNTS_STAFF view because the view does not let you alter the DEPTNOfield. If you had defined a DEFAULT value of 10 on the DEPTNO field, then you could perform inserts.
If you don’t want any DML operations to be performed on views, create them WITH READ ONLY option. Then no DML operations are allowed on views.
Referencing Invalid Views
When a user attempts to reference an invalid view, Oracle returns an error message to the user:
ORA-04063: view 'view_name' has errors
This error message is returned when a view exists but is unusable due to errors in its query (whether it had errors when originally created or it was created successfully but became unusable later because underlying objects were altered or dropped).
Users can only run SELECT and DESC statements against read only views. Examples:
Read-only views
READ ONLY clause on a simple view:
CREATE VIEW clerk (id_number, person, department, position)
AS SELECT empno, ename, deptno, job
FROM emp
WHERE job = 'CLERK'
WITH READ ONLY;
WITH CHECK OPTION
The WITH CHECK OPTION clause specifies the level of checking to be done when doing DML against the view. If specified, every row that is inserted, updated or deleted through the view must conform to the definition of the view.
The problem:
SQL> CREATE VIEW d20 AS SELECT ename, sal, deptno FROM emp2 WHERE deptno = 20;
View created.
SQL> UPDATE d20 SET deptno = 10;
3 rows updated.
The solution:
SQL> CREATE VIEW d20 AS SELECT ename, sal, deptno FROM emp2 WHERE deptno = 20
2 WITH CHECK OPTION;
View created.
SQL> UPDATE d20 SET deptno = 10;
UPDATE d20 SET deptno = 10
*
ERROR at line 1:
ORA-01402: view WITH CHECK OPTION where-clause violation
Dropping Views
Use the SQL command DROP VIEW to drop a view. For example:
DROP VIEW Accounts_staff;
Modifying a Join View
Oracle allows you, with some restrictions, to modify views that involve joins. Consider the following simple view:
CREATE VIEW Emp_view AS
SELECT Ename, Empno, deptno FROM Emp;
This view does not involve a join operation. If you issue the SQL statement:
UPDATE Emp_view SET Ename = 'SEKHAR' WHERE Empno = 109;
then the EMP base table that underlies the view changes, and employee 109’s name changes from ASHI to SEKHAR in the EMP table
However, if you create a view that involves a join operation, such as:
CREATE VIEW Emp_dept_view AS
SELECT e.Empno, e.Ename, e.Deptno, e.Sal, d.Dname, d.Loc
FROM Emp e, Dept d /* JOIN operation */
WHERE e.Deptno = d.Deptno
AND d.Loc IN ('HYD', 'BOM', 'DEL');
then there are restrictions on modifying either the EMP or the DEPT base table through this view.
A modifiable join view is a view that contains more than one table in the top-level FROM clause of the SELECT statement, and that does not contain any of the following:
⦁ DISTINCT operator
⦁ Aggregate functions: AVG, COUNT, GLB, MAX, MIN, STDDEV, SUM, orVARIANCE
⦁ Set operations: UNION, UNION ALL, INTERSECT, MINUS
⦁ GROUP BY or HAVING clauses
⦁ START WITH or CONNECT BY clauses
⦁ ROWNUM pseudocolumn
Any UPDATE, INSERT, or DELETE statement on a join view can modify only one underlying base table.
The following example shows an UPDATE statement that successfully modifies the EMP_DEPT_VIEW view:
UPDATE Emp_dept_view
SET Sal = Sal * 1.10
WHERE Deptno = 10;
The following UPDATE statement would be disallowed on theEMP_DEPT_VIEW view:
UPDATE Emp_dept_view
SET Loc = 'BOM'
WHERE Ename = 'SAMI';
This statement fails with an ORA-01779 error (“cannot modify a column which maps to a non key-preserved table”), because it attempts to modify the underlying DEPT table, and the DEPT table is not key preserved in theEMP_DEPT view.
In general, all modifiable columns of a join view must map to columns of a key-preserved table. If the view is defined using the WITH CHECK OPTIONclause, then all join columns and all columns of repeated tables are not modifiable.
So, for example, if the EMP_DEPT view were defined using WITH CHECKOPTION, then the following UPDATE statement would fail:
UPDATE Emp_dept_view
SET Deptno = 10
WHERE Ename = 'SAMI';
The statement fails because it is trying to update a join column.
Deleting from a Join View
You can delete from a join view provided there is one and only one key-preserved table in the join.
The following DELETE statement works on the EMP_DEPT view:
DELETE FROM Emp_dept_view
WHERE Ename = 'SMITH';
This DELETE statement on the EMP_DEPT view is legal because it can be translated to a DELETE operation on the base EMP table, and because the EMPtable is the only key-preserved table in the join.
In the following view, a DELETE operation cannot be performed on the view because both E1 and E2 are key-preserved tables:
CREATE VIEW emp_emp AS
SELECT e1.Ename, e2.Empno, e1.Deptno
FROM Emp e1, Emp e2
WHERE e1.Empno = e2.Empno;
If a view is defined using the WITH CHECK OPTION clause and the key-preserved table is repeated, then rows cannot be deleted from such a view. For example:
CREATE VIEW Emp_mgr AS
SELECT e1.Ename, e2.Ename Mname
FROM Emp e1, Emp e2
WHERE e1.mgr = e2.Empno
WITH CHECK OPTION;
No deletion can be performed on this view because the view involves a self-join of the table that is key preserved.
Inserting into a Join View
The following INSERT statement on the EMP_DEPT view succeeds, because only one key-preserved base table is being modified (EMP), and 40 is a validDEPTNO in the DEPT table (thus satisfying the FOREIGN KEY integrity constraint on the EMP table).
INSERT INTO Emp_dept (Ename, Empno, Deptno)
VALUES ('ASHU', 119, 40);
The following INSERT statement fails for the same reason: This UPDATE on the base EMP table would fail: the FOREIGN KEY integrity constraint on theEMP table is violated.
INSERT INTO Emp_dept (Ename, Empno, Deptno)
VALUES ('ASHU', 110, 77);
The following INSERT statement fails with an ORA-01776 error (“cannot modify more than one base table through a view”).
INSERT INTO Emp_dept (Ename, Empno, Deptno)
VALUES (110, 'TANNU’, 'BOMBAY');
An INSERT cannot, implicitly or explicitly, refer to columns of a non-key-preserved table. If the join view is defined using the WITH CHECK OPTIONclause, then you cannot perform an INSERT to it.
Listing Information about VIEWS.
To see how many views are there in your schema. Give the following query.
Select * from user_views;
To see which columns are updatable in join views.
Data Dictionaries which shows which columns are updatable.
View Name Description
USER_UPDATABLE_COLUMNS Shows all columns in all tables and views in the user’s schema that are modifiable
DBA_UPDATABLE_COLUMNS Shows all columns in all tables and views in the DBA schema that are modifiable
ALL_UPDATABLE_VIEWS Shows all columns in all tables and views that are modifiable
If you are in doubt whether a view is modifiable, then you can SELECT from the view USER_UPDATABLE_COLUMNS to see if it is. For example:
SELECT * FROM USER_UPDATABLE_COLUMNS WHERE TABLE_NAME = 'EMP_DEPT_VIEW';
This might return:
OWNER TABLE_NAME COLUMN_NAM UPD
---------- ---------- ---------- ---
SCOTT EMP_DEPT EMPNO NO
SCOTT EMP_DEPT ENAME NO
SCOTT EMP_DEPT DEPTNO NO
SCOTT EMP_DEPT DNAME NO
SCOTT EMP_DEPT LOC NO
5 rows selected.
Things You Should Know About Oracle Views
1. A view is like a virtual table. It takes the output of a query and treats it like a table.
2. A view can be based on one or more tables or other views. These tables/views are called base tables.
3. A view takes up no storage space other than for the definition of the view in the data dictionary.
4. A view contains no data. All the data it shows comes from the base tables.
5. A view can provide an additional level of table security by restricting access to a set of rows or columns of a table.
6. A view hides implementation complexity. The user can select from the view with a simple SQL, unaware that the view is based internally on a join between multiple tables.
7. A view lets you change the data you can access, applying operators, aggregation functions, filters etc. on the base table.
8. A view isolates applications from changes in definitions of base tables. Suppose a view uses two columns of a base table, it makes no difference to the view if other columns are added, modified or removed from the base table.
9. Using views encourages the use of shared SQL, which improves efficiency of frequently invoked SQL.
10. An updatable view allows you to insert, update, and delete rows by propagating the changes to the base table. A view can be updatable provided its definition does not contain any of the following constructs: SET or DISTINCT operators, an aggregate or analytic function, a GROUP BY, ORDER BY, CONNECT BY, or START WITH clause, a subquery (or collection expression) in a SELECT list.
11. The data dictionary views ALL_UPDATABLE_COLUMNS, DBA_UPDATABLE_COLUMNS,and USER_UPDATABLE_COLUMNS indicate which view columns are updatable.
12. Views that are not updatable can be modified using an INSTEAD OF trigger.
13. A view can be created even if the defining query of the view cannot be executed, using the CREATE FORCE VIEW command. Such a view is called aview with errors. This option can be useful for import/installation tools to create a view before the underlying objects are present.
14. A view can be replaced with a CREATE OR REPLACE VIEW statement. The REPLACE option updates the current view definition but preserves the present security authorizations.
15. A view lets you reorder columns easily with a CREATE OR REPLACE, rather than going into a messy drop column for the base table with data.
16. To know about the views in your own schema, look up user_views.
17. The underlying SQL definition of the view can be read via select text from user_views for the view.
18. Oracle does not enforce constraints on views. Instead, views are subject to the constraints of their base tables.
19. Be careful when you define views of views. Don’t do it just because it is easy to code – it may not be the optimal query. Check if you would be better off using the base tables directly instead.
20. To create a view in your own schema, you must have the CREATE VIEW system privilege. To create a view in another user’s schema, you must have the CREATE ANY VIEW system privilege.
