PL / SQL函数错误

declare
BEREITS_VORHANDEN EXCEPTION;
begin
IF (select username from benutzer where username=:P12_USERNAME) IS NOT NULL THEN
raise BEREITS_VORHANDEN;
END IF;
end;

ERROR:
ORA-06550: Zeile 5, Spalte 5:
PLS-00103: Encountered the symbol \"SELECT\" when expecting one of the following:

   ( - + case mod new not null 

   continue avg count current exists max min prior sql stddev
   sum variance execute forall merge time timestamp interval
   date 
   pipe

CREATE FUNCTION some_user_name_stuff(_P12_USERNAME varchar) RETURNS void AS $$
DECLARE
  user_name_not_found boolean;
BEGIN
  SELECT count(*) = 0 INTO user_name_not_found
  FROM benutzer
  WHERE username=_P12_USERNAME;

  IF (user_name_not_found) THEN
    raise EXCEPTION \'BEREITS_VORHANDEN\'; 
  END IF; 
END;
$$ LANGUAGE plpgsql;