sum是对内容的数量进行相加，count 对表行数 对象进行统计
在使用 case 时，如
select subject,
count(case when score>80 then score else null end) 优,
count(case when score<80 and score>59 then score else null end) 优,
count(case when score<60 then score else null end) 优
from stuscore group by [subject]
select * from stuscore
select subject,
sum(case when score>80 then score else 1 end) 优,
sum(case when score<80 and score>59 then score else 1end) 优,
sum(case when score<60 then score else 1end) 优
from stuscore group by [subject]
select * from stuscore
这个时候 对sum 进行返回的是1，而对count 返回的是一列（null），或者说是一个行对象进行对数量的统计
