Views

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 the CREATE 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 the ACCOUNTS_STAFF view to correct the department number specified in the WHERE 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 the EMP 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, a GROUP 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 the ACCOUNTS_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 the ACCOUNTS_STAFF view because the view does not let you alter the DEPTNO field. 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).

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 = 'SHAHRYAR' WHERE Empno = 109;

then the EMP base table that underlies the view changes, and employee 109's name changes from ASHI to SHAHRYAR 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, or VARIANCE
• 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 the EMP_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 the EMP_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 OPTION clause, 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 CHECK OPTION, 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 EMP table 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 valid DEPTNO 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 the EMP 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 OPTION clause, 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.

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.