6.6 Nested Tables and Varrays Used in Object Views

Collections, both nested tables and VARRAYs, can be columns in views. You can select these collections from underlying collection columns or you can synthesize them using subqueries. The CAST-MULTISET operator provides a way of synthesizing such collections.

This section contains the following topics:

• Single-Level Collections in Object Views

• Multilevel Collections in Object Views

6.6.1 Single-Level Collections in Object Views

You can create an object view with a single level connection.

Using Example 6-1 and Example 6-2 as starting points, each employee in an emp relational table has the structure in Example 6-4. Using this relational table, you can construct a dept_view with the department number, name, address and a collection of employees belonging to the department.

First, define a nested table type for the employee type employee_t. Next, define a department type with a department number, name, address, and a nested table of employees. Finally, define the object view dept_view.

The SELECT subquery inside the CAST-MULTISET block selects the list of employees that belong to the current department. The MULTISET keyword indicates that this is a list as opposed to a singleton value. The CAST operator casts the result set into the appropriate type, in this case to the employee_list_t nested table type.

A query on this view could provide the list of departments, with each department row containing the department number, name, the address object and a collection of employees belonging to the department.

Example 6-4 Creating a View with a Single-Level Collection

-- Requires Ex. 6-1 and Ex. 6-2
CREATE TABLE emp (
   empno    NUMBER PRIMARY KEY,
   empname  VARCHAR2(20),
   salary   NUMBER,
   job      VARCHAR2 (20), 
   deptno   NUMBER REFERENCES dept(deptno));

CREATE TYPE employee_list_t AS TABLE OF employee_t;  -- nested table
/
CREATE TYPE dept_t AS OBJECT (
    deptno     NUMBER,
    deptname   VARCHAR2(20),
    address    address_t,
    emp_list   employee_list_t);
/
CREATE VIEW dept_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.empno, e.empname, e.salary, e.job
                           FROM emp e 
                           WHERE e.deptno = d.deptno) 
                        AS employee_list_t)
                   AS emp_list
   FROM dept d;

insert into dept values(100,'ST','400 Oracle Pkwy','Redwood S','CA',94065);
insert into dept values(200,'Sales','500 Oracle Pkwy','Redwood S','CA',94065);
insert into emp values(1,'John',900,'Developer1',100);
 
insert into emp values(2,'Robert',1000,'Developer2',100);
insert into emp values(3,'Mary', 1000,'Apps1',200);
insert into emp values(4,'Maria',1500,'Developer3',200);
select * from dept_view where deptno = 100;
 
    DEPTNO DEPTNAME
---------- --------------------
ADDRESS(STREET, CITY, STATE, ZIP)
--------------------------------------------------------------------------------
EMP_LIST(EMPNO, ENAME, SALARY, JOB)
--------------------------------------------------------------------------------
       100 ST
ADDRESS_T('400 Oracle Pkwy', 'Redwood S', 'CA', '94065')
EMPLOYEE_LIST_T(EMPLOYEE_T(1, 'John', 900, 'Developer1'), EMPLOYEE_T(2, 'Robert'
, 1000, 'Developer2'))
 
 
select emp_list from dept_view where deptno = 100;
 
EMP_LIST(EMPNO, ENAME, SALARY, JOB)
--------------------------------------------------------------------------------
EMPLOYEE_LIST_T(EMPLOYEE_T(1, 'John', 900, 'Developer1'), EMPLOYEE_T(2, 'Robert'
, 1000, 'Developer2'))

6.6.2 Multilevel Collections in Object Views

You can create to view and query objects.

Multilevel collections and single-level collections are created and used in object views in the same way. The only difference is that, for a multilevel collection, you must create an additional level of collections.

Example 6-5 builds an object view containing a multilevel collection. The view is based on flat relational tables that contain no collections. As a preliminary to building the object view, the example creates the object and collection types it uses. An object type (for example, emp_t) is defined to correspond to each relational table, with attributes whose types correspond to the types of the respective table columns. In addition, the employee type has a nested table (attribute) of projects, and the department type has a nested table (attribute) of employees. The latter nested table is a multilevel collection. The CAST-MULTISET operator is used in the CREATE VIEW statement to build the collections.

Example 6-5 Creating a View with Multilevel Collections

CREATE TABLE depts
  ( deptno     NUMBER,
    deptname   VARCHAR2(20));

CREATE TABLE emps
  ( ename VARCHAR2(20),
    salary     NUMBER,
    deptname   VARCHAR2(20));

CREATE TABLE projects
  ( projname   VARCHAR2(20),
    mgr        VARCHAR2(20));

CREATE TYPE project_t AS OBJECT
  ( projname   VARCHAR2(20),
    mgr        VARCHAR2(20));
/
CREATE TYPE nt_project_t AS TABLE OF project_t;
/
CREATE TYPE emp_t AS OBJECT
(  ename      VARCHAR2(20),
   salary     NUMBER,
   deptname   VARCHAR2(20),
   projects   nt_project_t );
/
CREATE TYPE nt_emp_t AS TABLE OF emp_t;
/
CREATE TYPE depts_t AS OBJECT
  ( deptno     NUMBER,
    deptname   VARCHAR2(20),
    emps       nt_emp_t );
/
CREATE VIEW v_depts OF depts_t WITH OBJECT IDENTIFIER (deptno) AS
  SELECT d.deptno, d.deptname, 
    CAST(MULTISET(SELECT e.ename, e.salary, e.deptname,
        CAST(MULTISET(SELECT p.projname, p.mgr
          FROM projects p
          WHERE p.mgr = e.ename)
        AS nt_project_t)
      FROM emps e
      WHERE e.deptname = d.deptname)
    AS nt_emp_t)
  FROM depts d;