Does updating a view affects the base table? - oracle

Question

When I updated a view which was created using a base table, the updation affected the base table as well. How is that possible? If view is considered as just a 'window' through which we can see a set of data of the base table then how can the base table change when I try to change the data inside a view.

Answer 1

You can make changes to the state of underlying table using the view as long as the you are targeting the change in single table.
View is a security layer on top of table object and allows most of the DML operation as long as you do not violet the base rule.
Example:
CREATE TABLE T1
(ID INT IDENTITY(1,1), [Value] NVARCHAR(50))
CREATE TABLE T2
(ID INT IDENTITY(1,1), [Value] NVARCHAR(50))
--Dummy Insert
INSERT INTO T1 VALUES ('TestT1')
INSERT INTO T2 VALUES ('TestT2')
--Create View
CREATE VIEW V1
AS
SELECT T1.ID AS T1ID, T2.ID AS T2ID, T1.Value AS T1Value, T2.Value AS T2Value FROM T1 INNER JOIN T2
ON T2.ID = T1.ID
--Check the result
SELECT * FROM V1
--Insert is possible via view as long as it affects only one table
INSERT INTO V1 (T1Value) VALUES
('TestT1_T1')
INSERT INTO V1 (T2Value) VALUES
('TestT2_T2')
--Change is possible only if target is only one table
UPDATE V1
SET T1Value = 'Changed'--**
WHERE T2ID = 1
--This is not allowed
INSERT INTO V1 (T1Value, T2Value) VALUES
('TestT1_T1','TestT2_T2')
--Msg 4405, Level 16, State 1, Line 1
--View or function 'V1' is not updatable because the modification affects multiple base tables.
--Check T1 and T2 with each statement to see how it gets affected
--

Answer 2

In some databases it's possible to update the source table(s) for a view if there is a one-to-one relationship between the rows in the view and the rows in the underlying table, that is, you cant have derived columns, aggregate functions or a distinct clause in your view for example.
In Oracle, even if a view is not inherently updatable, updates may be allowed if an INSTEAD OF DML trigger is defined.
If you use mysql, you can read a detailed description about this feature Updatable and insertable views.

Answer 3

" If view is considered as just a 'window' through which we can see a set of data of the base table "
- Where did you get this definition?
What oracle says about views:
A view is a logical representation of another table or combination of
tables. A view derives its data from the tables on which it is based.
These tables are called base tables. Base tables might in turn be
actual tables or might be views themselves. All operations performed
on a view actually affect the base table of the view. You can use
views in almost the same way as tables. You can query, update, insert
into, and delete from views, just as you can standard tables.
Such a view into which you can update or insert are fondly named as "Updatable and Insertable Views". Oracle documentation about them is here.
Also, this is how the purpose of an "insert" statement is defined by Oracle:
Use the INSERT statement to add rows to a table, the base table of a
view, a partition of a partitioned table or a subpartition of a
composite-partitioned table, or an object table or the base table of
an object view.

Answer 4

Yes we can achieve the DML Operation in Views like belows:
Create or replace view emp_dept_join as Select d.department_id,
d.department_name, e.first_name, e.last_name from employees
e, departments d where e.department_id = d.department_id;
SQL>CREATE OR REPLACE TRIGGER insert_emp_dept
INSTEAD OF INSERT ON emp_dept_join DECLARE v_department_id departments.department_id%TYPE;
BEGIN
BEGIN
SELECT department_id INTO v_department_id
FROM departments
WHERE department_id = :new.department_id;
EXCEPTION
WHEN NO_DATA_FOUND THEN
INSERT INTO departments (department_id, department_name)
VALUES (dept_sequence.nextval, :new.department_name)
RETURNING ID INTO v_department_id;
END;
INSERT INTO employees (employee_id, first_name, last_name, department_id)
VALUES(emp_sequence.nextval, :new.first_name, :new.last_name, v_department_id);
END insert_emp_dept;
/

Answer 5

if the viwe is defined through a simple query involving single base relation and either containing primary key or candidate key, so there will be change in base relation if changing the view. ( however there is restriction)
And updates are not allowed through view if there is multiple base relations or grouping operations.