在Mysql中实现Oracle中的rownum

select x.id,x.newname,rownum as clicks from (select t.id,t.newname,@rownum:=@rownum+1 rownum from(select id,newname,@rownum:=0 from topicnew where topicid = 'xxxxxx' order by id desc)t)x  where x.id='xxxxx'