牛客网sql第五十题

Question

select round(avg(gpa),2)) avg_gpa
case when graduate_year=2022 then '大四'
when graduate_year=2023 then '大三'
when graduate_year=2024 then '大二'
else '大一' end as grade
from user_profile
group by grade
order by avg(gpa) desc
我是真不懂为啥要加一个having grade is not null
原数据里面也没有2021年毕业的学生啊，那case when不都刚好分配了嘛
l