聚合函数里的cas when

Question

select stu_id,case when count(*) = 1 then class_id
else max(case when main_class_flag = 'Y' then class_id else NULL END) END as classid
from courses group by stu_id;
不知道为啥要加这个max

Answer 1

select stu_id,
（case when count() = 1 then class_id
else max(case when main_class_flag = 'Y' then class_id else NULL END) END as classid
from courses
group by stu_id;
这个题目的意思 count（）=1,就直接输出class_id（班级id），否则就取class_id最大那个

博主不知道你要表达什么 ，单单 看聚合函数里的cas when ，我能看出啥？