6.2 Defining Object Views

Object views allow you to present only data that you want users to see.

The procedure for defining an object view is:

  1. Define an object type, where each attribute of the type corresponds to an existing column in a relational table.
  2. Write a query that specifies how to extract the data from the relational table. Specify the columns in the same order as the attributes in the object type.
  3. Specify a unique value, based on attributes of the underlying data, to serve as an object identifier, enabling you to create pointers (REFs) to the objects in the view. You can often use an existing primary key.

To update an object view where the attributes of the object type do not correspond exactly to columns in existing tables, you may need to do the following:

Write an INSTEAD OF trigger procedure for Oracle to execute whenever an application program tries to update data in the object view. See "Object View Manipulations".

After these steps, you can use an object view just like an object table.

Example 6-1 contains SQL statements to define an object view, where each row in the view is an object of type employee_t:

Example 6-1 Creating an Object View

CREATE TABLE emp_table (
    empnum   NUMBER (5),
    ename    VARCHAR2 (20),
    salary   NUMBER (9,2),
    job      VARCHAR2 (20));

    empno    NUMBER (5),
    ename    VARCHAR2 (20),
    salary   NUMBER (9,2),
    job      VARCHAR2 (20));

CREATE VIEW emp_view1 OF employee_t
        SELECT e.empnum, e.ename, e.salary, e.job
            FROM emp_table e
            WHERE job = 'Developer';

insert into emp_table values(1,'John',1000.00,'Architect');
insert into emp_table values(2,'Robert',900.00,'Developer');
insert into emp_table values(3,'James',2000.00,'Director');

select * from emp_view1;

    EMPNO ENAME                    SALARY JOB
---------- -------------------- ---------- --------------------
         2 Robert                      900 Developer

To access the data from the empnum column of the relational table, access the empno attribute of the object type.