select id,mer_no,mer_level from t_test a start with a.id='111111111111' connect by prior id=parent_mer_id;------向下遍历
结果类似：
0级
1级
2级
...
select distinct id,mer_no,mer_level from t_test a start with a.id='111111111111' connect by prior parent_mer_id=id;------向上遍历
结果类似：
...
2级
1级
0级