SQL> desc t
 Name                            Null?    Type
 ------------------------------- -------- ----
 ID                                       NUMBER
 VAL                                      NUMBER

SQL> select * from t;

        ID        VAL
---------- ----------
         1          1
         1          2
         1          3
         1          7
         2          5
         2          8
         3          1
         3          3
         4          1

9 rows selected.

SQL> ed
Wrote file afiedt.buf

  1  create or replace function concat_val(p_id in number)
  2  return varchar2
  3  is
  4    cursor c_val
  5    is
  6    Select val
  7    From t
  8    Where id = p_id;
  9    v_return varchar2(2000);
 10  begin
 11    for rec in c_val
 12    loop
 13      v_return := v_return||'-'||rec.val;
 14    end loop;
 15    v_return := substr(v_return,2);
 16    return v_return;
 17* end;
SQL> /

Function created.

SQL> Column TEST format a50
SQL> Select Distinct id
  2       , concat_val(id) TEST
  3    From t;

        ID TEST
---------- --------------------------------------------------
         1 1-2-3-7
         2 5-8
         3 1-3
         4 1

SQL>