很难的sql语句求教，不知道怎么写这条SQL语句？？

小叶子sy

表a(a1,a2)
a1 a2
1 a
1 b
2 x
2 y
2 z
如何得到
1 ab
2 xyz

谢谢！！！

sshd

create or replace function f(x varchar2) return varchar2
is
    cursor c is select a2 from a where a1 = x;
    str varchar2(20);
begin
    for i in c loop
        str := str || i.a2;
    end loop;
    return (str);
end;
/

col c format a30
select a1, f(a1) c
from a
group by a1;