求一个有关于字段内容长度的SQL语句或存储过程

改用动态的sql
str:='select topicid,title,boardid,locktopic,postusername,postuserid,dateandtime,
                hits,votetotal,istop,lastposttime,isbest,isvote,lastpostusername,
                lastpostuserid,bbscount,isdelete,bbsboard,topicimg,t_color 
     From 
     (
         Select topicid,title,boardid,locktopic,postusername,postuserid,dateandtime,
                hits,votetotal,istop,lastposttime,isbest,isvote,lastpostusername,
                lastpostuserid,bbscount,isdelete,bbsboard,topicimg,t_color
         From bbs_tb_bbsTopic t
         --下面的lengthb(title) > MaxLength不正确         
         Where t.isbest = 1 and lengthb(t.title) >'|| MaxLength||'
         order by t.dateandtime desc
     )  where rownum <= '||nPageSize;
open topicCursor for str;