Select * from StudentAchievement A
Where Not Exists(Select 1 from StudentAchievement Where CourseID=A.CourseID And Achievement>A.Achievement Having Count(*)>1)
Order by CourseID,Achievement Desc
select *
from StudentAchievement a
where (select count(*) from (select CourseID, Achievement from StudentAchievement group by CourseID, Achievement ) b where b.CourseID=a.CourseID and b.Achievement>=a.Achievement)<3