题解 | 分组计算练习题

题意分析：

要查询出的字段有gender, university,用户数量，30天的平均天数，发帖数量的平均数。

• 对性别，学校分组group by gender,university
• 分组后，对性别进行计数。为用户数量count(gender) user_num
• 平均天数、发贴平均数avg(active_days_within_30), avg(question_cnt) as avg_question_cnt
• 性别、学校升序排列order by gender, university

select 
    gender, university,
    count(gender) as user_num,
	#count(device_id) as user_num
    avg(active_days_within_30) as avg_active_day,
    avg(question_cnt) as avg_question_cnt
from user_profile
group by gender, university
order by gender, university