mysql rownumer

explain select t.n from 
(
select  @n:=(coalesce(@n,0))+1 as n  ,id from (select @n:=0 ) t ,student 
where  id = 342  order by created_at score 

) t where  t.id = 301 ;

   查询学生301 在学生按分数排行中所在的排位