oracle sql 树形菜单

create table test_dept
    (dept_id varchar2(20),dept_name varchar2(40),parent_id varchar2(20));

    insert into test_dept values ('0','dept1',null);
    insert into test_dept values ('1','dept11','0');
    insert into test_dept values ('11','dept11','1');
    insert into test_dept values ('12','dept12','1');
    insert into test_dept values ('2','dept2','0');
    insert into test_dept values ('21','dept21','2');
    insert into test_dept values ('211','dept211','21');
    insert into test_dept values ('212','dept212','21');
    insert into test_dept values ('22','dept22','2');

    select dept_id,dept_name,level
    from test_dept
    start with dept_id='0'
    connect by prior dept_id=parent_id;