create table strings as
  select 'emcpower1' str from dual
  union all 
  select 'emcpower2' from dual 
  union all
  select 'emcpower3' from dual
  union all 
  select 'sda' from dual
  union all
  select 'sdb' from dual
  union all
  select 'sdc' from dual

with rws as ( 
  -- generate rows up to the length of the longest string 
  select rownum r from dual 
  connect by level <= (select max(length(str)) from strings) 
) 
select distinct s  
from ( 
  select str, max(subp) s  
  from ( 
    select str,  
           substr(str, 1, rws.r) subp 
    from   strings s1 
    cross join rws 
    where  r < length(str) 
    and    exists ( 
      -- check whether there's another string matching the first N chars 
      select * from strings s2 
      where  s1.str <> s2.str 
      and    substr(s1.str, 1, rws.r) = substr(s2.str, 1, rws.r) 
    ) 
  )  
  group by str 
)

insert into strings values ('emcdisk')

insert into strings values ('samba')

with rws as (
  -- generate rows up to the length of the longest string
  select rownum r from dual
  connect by level <= (select max(length(str)) from strings)
)
select distinct s 
from (
  select str, max(subp) s 
  from (
    select str, 
           substr(str, 1, rws.r) subp
    from   strings s1
    cross join rws
    where  r < length(str)
    and    exists (
      -- check whether there's another string matching the first N chars
      select * from strings s2
      where  s1.str <> s2.str
      and    substr(s1.str, 1, rws.r) = substr(s2.str, 1, rws.r)
    )
  ) 
  group by str
)