SQL & PL/SQL

Hello gurus,

I am creating this similar structure in my application to hold nested table type in a database table column but i am getting this error , please assist me

create type degree_subjects as object

(

subject1 varchar2(20),

subject2 varchar(20)

);

--create subject table

CREATE TYPE subjects_tt AS TABLE OF degree_subjects;

-- A degree has multiple subjects like maths etc.

create type emp_degre as object

(

degree_type varchar2(20),

degree_name varchar(20),

subjects subjects_tt

);

CREATE TYPE emp_degree_tt AS TABLE OF emp_degre;

-- employee holds degrees

CREATE TYPE emp_rt AS OBJECT

             ( empno NUMBER(4)

             , ename VARCHAR2(10)

             , mgr   NUMBER(4)

             , emp_deg emp_degree_tt

             );

CREATE TYPE emp_tt AS TABLE OF emp_rt;

create table test_dest

(

   key number,

   employees emp_tt

)nested table employees store as employees_tab ;

Here in the last statement i am getting this error.Basically the cloumn employees is a nested table type which is 2 level in depth , it is holding emp_deg_tt table which has subjects_tt nested table.

is it possible to create this table , please assist.