select count(1)
  from (select distinct name, grade from tbl where grade = 100);
select count(1)
  from (select distinct name, grade
          from tbl a
         where a.grade = 80
           and not exists (select name
                  from tbl b
                 where grade = 100
                   and a.name = b.name));
select count(1)
  from (select distinct name, grade
          from tbl a
         where a.grade = 60
           and not exists (select name
                  from tbl b
                 where grade in (100,80)
                   and a.name = b.name));

------解决方案--------------------

select grade,count(1) num
from  
(select name,max(grade) grade from tbl group by name) A
group by grade;

------解决方案--------------------
with tbl as (
   select 'a' name ,100 grade from dual
   union
   select 'a' name ,80 grade from dual
   union
   select 'b' name ,60 grade from dual
  
)select sum(case a.GRADE when 100 then 1 else 0 end) ,sum(case a.GRADE when 80 then 1 else 0 end),sum(case a.GRADE when 60 then 1 else 0 end)
from
(select max(GRADE) GRADE,NAME
  from tbl
 group by NAME

) a

------解决方案--------------------
探讨

select grade,count(1) num
from
(select name,max(grade) grade from tbl group by name) A
group by grade;