Row_Number() over( PARTITION By cno ...)

create table score
(
sno varchar(20) ,
cno varchar(20),
degree int
)

insert into score (sno ,cno ,degree ) values ('001','a',100)
insert into score (sno ,cno ,degree ) values ('002','a',99)
insert into score (sno ,cno ,degree ) values ('003','a',98)
insert into score (sno ,cno ,degree ) values ('004','a',97)
insert into score (sno ,cno ,degree ) values ('001','b',100)
insert into score (sno ,cno ,degree ) values ('002','b',100)
insert into score (sno ,cno ,degree ) values ('003','b',99)
insert into score (sno ,cno ,degree ) values ('004','b',98)
insert into score (sno ,cno ,degree ) values ('001','c',100)
insert into score (sno ,cno ,degree ) values ('002','c',100)
insert into score (sno ,cno ,degree ) values ('003','c',99)
insert into score (sno ,cno ,degree ) values ('004','c',98)

select * from 
(
select *,ROW_NUMBER () over(PARTITION BY cno order by degree desc) as pm from score 
) x
where x.pm <=3

select * from 
(
select *,rank () over(PARTITION BY cno order by degree desc) as pm from score 
) x
where x.pm <=3

select * from 
(
select *,dense_rank  () over(PARTITION BY cno order by degree desc) as pm from score 
) x
where x.pm <=3