Remove repeating letters

Question

11gR2 DB
We have a string token with all small leters (Sample - abcdaeebfghiejklc)
Need to remove all the repeating letters in this tring. Output should be abcdefghijkl
I know how to do it by splitting the string into multiple rows, then use listagg to make it a string.
But can we do it use without using any kind of row generation. By regexp or something?

Answer 1

And if your version supports recursive subquery factoring:
SQL> select  *
  2    from  tbl
  3  /
STR
abcdaeebfghiejklc
abcdaeebfghiejk
SQL> Now:
with r(
       str,
       new_str,
       l
      ) as (
             select  str,
                     cast(substr(str,1,1) as varchar2(4000)) new_str,
                     1 l
               from  tbl
            union all
             select  str,
                     case instr(new_str,substr(str,l + 1,1))
                       when 0 then new_str || substr(str,l + 1,1)
                       else new_str
                     end new_str,
                     l + 1 l
               from  r
  where l < length(str)
select  str,
        new_str
  from  r
  where l = length(str)
STR               NEW_STR
abcdaeebfghiejk   abcdefghijk
abcdaeebfghiejklc abcdefghijkl
SQL> SY.