declare
  var_count integer;
BEGIN
 INSERT INTO s_hz(k_hz, fk1, fk2, fk3)
 VALUES(............);
 COMMIT;
  return  1 ;
EXCEPTION
  WHEN  others THEN 

    select count(*) into var_count from a1 where id=fk1;
    if var_count= 0  return - 2 ; end if;
    select count(*) into var_count from a2 where id=fk2;
    if var_count= 0  return - 3 ; end if;
    select count(*) into var_count from a3 where id=fk3;
    if var_count= 0  return - 4 ; end if;
    return - 1 ;

END;