SQL & PL/SQL

Hello there,

I tried to build my own string aggregate function but I've a troubles when I'm using it into my query. When I'm using it, it writes ORA904, whereas this idenfifier exists. Do you know what's wrong ?

here my aggregate function :

create or replace FUNCTION aggme (query_in in VARCHAR2) RETURN VARCHAR2 IS
    incoming    varchar2(4000);
    hold_result varchar2(4000);
    c sys_refcursor;
Begin
    open c for query_in;
    loop
        fetch c into incoming;
        exit when c%notfound;
        hold_result := hold_result||','||incoming;
    end loop;
    return ltrim(hold_result,',');
END;

and here my query by using my aggregate function aggme:

select
  RES.LASTNAME as "LASTNAME",
  RES.FIRSTNAME as "FIRSTNAME",
  aggme('select NAME from PATHOLOGY where ID= RES.PATHOLOGY_ID') as "PATHOLOGY"
from
  TBK_RESOURCE RES