sql使用中的一些固定语法

SELECT * from ( SELECT  
 uid,
 day,
 @ROW := case when @cid=uid then @ROW+1 else 1 END rn, 
 @cid := uid
from 
(
　　SELECT uid,day  from 
　　ods_app_action_login_h 
　　where day>="2019-09-01" and day <="2019-09-30" and uid <>‘‘  
      GROUP BY day,uid ORDER BY uid,day desc
) l1,(SELECT @ROW := 0,@cid :=0) l2) l5
where rn <=7

--第二种
SELECT uid,day 
from ods_app_action_login_h as o1 
where day>="2019-09-27" and day<="2019-09-30" and 
　(
　　　　SELECT count(*) from 
　　　　(
　　　　　 　SELECT uid,day from ods_app_action_login_h where 
    　　day>="2019-09-27" and day<="2019-09-30" GROUP BY uid,day
　　    ) as o 
    where  o1.uid= o.uid and o.day <=o1.day   --分组排序
　) <= 7