题解 | #统计每个学校各难度的用户平均刷题数#
统计每个学校各难度的用户平均刷题数
https://www.nowcoder.com/practice/5400df085a034f88b2e17941ab338ee8
SELECT
university,
difficult_level,
round(count(t1.question_id) / count(distinct t1.device_id),4) as avg_answer_cnt
FROM question_practice_detail t1
left join user_profile t2
on t1.device_id = t2.device_id
left join question_detail t3
on t1.question_id = t3.question_id
GROUP BY university,difficult_level
三表连接的两种方式：
- FROM 表1 left join 表2 on 表1.a = 表2.b
- left join 表3 on 表1.c = 表3.d
- from 表1,(select...from...where...) as 表2, (select...from...where...) as 表3
- where 表1.a=表2.b and 表2.b=表3.c
