6.12 Applying the Object Model to Remote Tables

Although you cannot directly access remote tables as object tables, object views let you access remote tables as if they were object tables.

Consider a company with two branches; one in Washington D.C. and another in Chicago. Each site has an employee table. The headquarters in Washington has a department table with a list of all the departments. To get a total view of the entire organization, you can create views over the individual remote tables and then a overall view of the organization.

To this requires the following:

• Update the entry in listener.ora, such as: (ADDRESS=(PROTOCOL=tcp) (HOST=stadv07.us.example.com)(PORT=1640))

• Add entries to tnsnames.ora, such as: chicago=(DESCRIPTION= (ADDRESS=(PROTOCOL=ipc)(KEY=linux)) (CONNECT_DATA=(SERVICE_NAME=linux.regress.rdbms.dev.us.example.com)))

• Provide CREATE DATABASE LINK code as shown in Example 6-9

Example 6-9 begins by creating an object view for each employee table and then creates the global view.

Example 6-9 Creating an Object View to Access Remote Tables

-- Requires Ex. 6-2, Ex. 6-4, and Ex. 6-6 
-- Example requires DB links, such as these, modify for your use and uncomment
-- CREATE DATABASE LINK chicago CONNECT TO hr IDENTIFIED BY hr USING 'inst1';
-- CREATE DATABASE LINK washington CONNECT TO hr IDENTIFIED BY hr USING 'inst1';
CREATE VIEW emp_washington_view (eno, ename, salary, job)
   AS SELECT e.empno, e.empname, e.salary, e.job
          FROM emp@washington e;

CREATE VIEW emp_chicago_view (eno, ename, salary, job)
   AS SELECT e.empno, e.empname, e.salary, e.job
          FROM emp@chicago e;

CREATE VIEW orgnzn_view OF dept_t WITH OBJECT IDENTIFIER (deptno) 
    AS SELECT d.deptno, d.deptname,
          address_t(d.deptstreet,d.deptcity,d.deptstate,d.deptzip) AS deptaddr,
          CAST( MULTISET (
                      SELECT e.eno, e.ename, e.salary, e.job
                      FROM emp_washington_view e) 
                   AS employee_list_t) AS emp_list
       FROM dept d
       WHERE d.deptcity = 'Washington'
   UNION ALL
       SELECT d.deptno, d.deptname,
           address_t(d.deptstreet,d.deptcity,d.deptstate,d.deptzip) AS deptaddr,
              CAST( MULTISET (
                       SELECT e.eno, e.ename, e.salary, e.job
                       FROM emp_chicago_view e) 
                    AS employee_list_t) AS emp_list
       FROM dept d
       WHERE d.deptcity = 'Chicago';

This view has a list of all employees for each department. The UNION ALL clause is used because employees cannot work in more than one department.