create table contracts(
	contract_code integer not null,
	conclusion_date date not null,
	contract_term integer not null check(contract_term <= 5),
	position_code integer not null,
	faculty_code integer not null,
	rate integer not null check(rate <= 5),
	employee_code integer not null,
	constraint contracts_positions foreign key(position_code) references positions,
	constraint contracts_chairs foreign key(faculty_code) references chairs,
	constraint contracts_employees foreign key(employee_code) references employees,
	primary key(contract_code)
);

create or replace type contracts_object as object(
	contract_code integer,
	conclusion_date date,
	contract_term integer,
	position_code integer,
	faculty_code integer,
	rate integer,
	employee_code integer);

create or replace type contracts_collection as table of contracts_object;

create table new_tablex as (select * from chairs);

ALTER TABLE new_tablex ADD chcontracts_collection contracts_collection  NESTED TABLE contracts STORE AS contracts_tab;

SQL> ALTER TABLE new_tablex ADD chcontracts_collection contracts_collection
  2  NESTED TABLE contracts STORE AS contracts_tab
  3  /
NESTED TABLE contracts STORE AS contracts_tab
             *
ERROR at line 2:
ORA-00904: : invalid identifier


SQL> ALTER TABLE new_tablex ADD chcontracts_collection contracts_collection
  2  NESTED TABLE chcontracts_collection STORE AS contracts_tab
  3  /

Table altered.

SQL>