select wm_concat( child_ID ) from t_belong t   
  start with t.father_id=1002   
  connect by prior t.child_id=t.father_id

------解决方案--------------------

SQL code

create table t_belong(
id number(18,0),
father_id number(18,0),
child_id number(18,0)
);

insert into t_belong(id,father_id,child_id) values(1, 1002, 1003);
insert into t_belong(id,father_id,child_id) values(2, 1002, 1004);
insert into t_belong(id,father_id,child_id) values(3, 1003, 1005);
insert into t_belong(id,father_id,child_id) values(4, 1004, 1006);
insert into t_belong(id,father_id,child_id) values(5, 1004, 1007);