请教，关于存储过程的问题

qinghua2000

偶有一存储过程，运行：execute procedure substring("112233",1,3);
成功返回值：“112”
可是，运行：select substring("scode",1,3)  from axy;
报“201: A syntax error has occurred.”错误
运行:select substring("112233",1,3) from axy;
也报“201: A syntax error has occurred.”错误，百思不得其解   
附：
CREATE PROCEDURE substring(str VARCHAR(255), pos integer, len INTEGER)
  RETURNING VARCHAR(255);
   DEFINE i INTEGER;
   DEFINE retstr VARCHAR(255);

   LET retstr = '';
   if len is null or pos is null then
                   return retstr;
   end if;

   IF pos >; 1 THEN
      FOR i = 2 TO pos
         LET str = str[2,255];
      END FOR;
   END IF
   FOR i = 1 to len
      LET retstr = retstr || str[1,1];
      LET str = str[2,255];
   END FOR;
-- LET retstr=retstr[2,255];
   RETURN retstr;
END PROCEDURE;

qinghua2000

先谢过各位了，换个名字没问题，substring是informix自带的函数，请问能否将我的存储过程替换informix自带的函数？