No Oracle 11g, você pode usar o LISTAGG() função:
select id,
number,
listagg(letter, ', ') within group(order by id, number) as letter
from yourtable
group by id, number;
Consulte SQL Fiddle with Demo
select id,
number,
listagg(letter, ', ') within group(order by id, number) as letter
from yourtable
group by id, number;
