create table object_types ( object_type_id number primary key, parent_id number, name varchar2(20) unique , description varchar2(20))
alter table object_types add foreign key (parent_id) references object_types (object_type_id);

Create table Objects ( object_id Number primary key , parent_id Number, object_type_id Number NOT NULL, name Varchar2(20));
Alter table Objects add foreign key (parent_id) references Objects (object_id);
Alter table Objects add foreign key (object_type_id) references Object_types (object_type_id);