oracle存储过程截取字符串

declare
CURSOR l_c IS select classid from cjt_class;
Begin
   FOR i IN l_c LOOP
    update cjt_class t set t.orders = substr(i.classid,0,4) where t.classid = i.classid;
   END LOOP;
End;

　　根据截取的字符串更新信息；