если WM_CONCAT не работает на вас, можно использовать эту уловку:
select rtrim(xmlagg(xmlelement(e, str)).extract('//text()'), ',') as output
from (select substr(str, level, 1) str, level l,
row_number() over (partition by substr(str, level, 1)
order by level) rn
from (select 'helloexample' str from dual) d
connect by level <= length(str)
order by level)
where rn = 1;
SQL> select rtrim(xmlagg(xmlelement(e, str)).extract('//text()'), ',') as output
2 from (select substr(str, rownum, 1) str, level l,
3 row_number() over (partition by substr(str, rownum, 1) order by level) rn
4 from (select 'helloexample' str from dual) d
5 connect by level <= length(str)
6 order by level)
7 where rn = 1;
OUTPUT
--------------------------------------------------------------------------------
heloxamp