题解 | #操作符混合运用#

考点1 多表链接：

#多表链接：连用两个join 
question_practice_detail qp LEFT JOIN user_profile u ON
qp.device_id = u.device_id
left join question_detail q on
qp.question_id = q.question_id

考点2 分组：

#分组 group by
不同难度下即
group by difficult_level

考点3 限定条件：

where university = '山东大学'

考点4 小数电点保留四位

#round函数
round(COUNT(qp.question_id)/ count(DISTINCT qp.device_id),4)

完整答案：

select university,
difficult_level,
round(COUNT(qp.question_id)/ count(DISTINCT qp.device_id),4) AS avg_answer_cnt
FROM
question_practice_detail qp LEFT JOIN user_profile u ON
qp.device_id = u.device_id
left join question_detail q on
qp.question_id = q.question_id
where university = '山东大学'
group by  difficult_level