常用sql001_partition by以及row_number()和 dense_rank()和rank()区别

[sql] 
create table  student (  
 sid varchar2(10), --学号  
 sname varchar2(10), --姓名  
 classid varchar2(10), --班级号  
 score  int   --分数  
);  
问题一：   班级中平均分最高的 班级号 以及 分数；
[sql] 
insert into  student values('001','z001','1',80);  
insert into  student values('002','z001','1',90);  
insert into  student values('003','z001','1',70);  
insert into  student values('004','z001','1',90);  
insert into  student values('005','z001','1',80);  
insert into  student values('006','z001','1',70);  
  
insert into  student values('007','z001','2',60);  
insert into  student values('008','z001','2',70);  
insert into  student values('009','z001','2',50);  
insert into  student values('010','z001','2',70);  
insert into  student values('011','z001','2',60);  
insert into  student values('012','z001','2',50);  

[sql] 
--求解sql：  
  
 SELECT * FROM (SELECT  CLASSID,AVG(score) score_ FROM STUDENT GROUP BY CLASSID  ORDER BY score_ DESC)  
 WHERE ROWNUM=1 ;

问题二：求每个班级第二名的学生的学号和分数（如果并列第二，全部求出）

原始数据：

执行如下sql：

[sql] 
select  s.* from (  
select  t.* , dense_rank() over(partition by classid order by score) as cn  from  student   t ) s where s.cn = 2;

执行结果：

分析以上sql：

[sql] 
select  t.* , dense_rank() over(partition by classid order by score) as cn  from  student   t

以上sql的执行结果：

以下是 row_number() 和 dense_rank的区别：区别在于最后一行 cn 上

[sql] 
select  t.* , row_number() over(partition by classid order by score) as cn  from  student   t

rank() 的用法：

[sql] 
select  t.* , RANK() over(partition by classid order by score) as cn  from  student   t