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with temp as(
      select 1 a,0 b from dual
      union all
            select 2 a,1 b from dual
      union all
            select 3 a,1 b from dual
      union all
            select 4 a,2 b from dual
      union all
            select 5 a,3 b from dual     
      union all
            select 6 a,4 b from dual     
)
select * from temp connect by prior a=b start with a=2

 A    B

 2    1
 4    2
 6    4