主要思路

1. 使用内连接，可以用where语句或者inner join
2. 注意where语句要先筛选‘山东大学’
3. 注意 表格列名含义 如device_id指学生的ID，question_id是题目的ID
   注意要和题目里的answer_cnt的区别

SELECT u.university,qd.difficult_level,
count(qp.question_id) / count(distinct qp.device_id) as avg_as
FROM user_profile u,question_practice_detail qp , question_detail qd
WHERE u.device_id=qp.device_id
AND qp.question_id=qd.question_id
AND u.university='山东大学'
GROUP BY u.university,qd.difficult_level;

SELECT    university,d.difficult_level,
    count( q.question_id )/count(DISTINCT q.device_id) avg_ans_cnt
FROM    user_profile u
    INNER JOIN question_practice_detail qp 
            ON u.device_id = qp.device_id
    INNER JOIN question_detail qd 
            ON qd.question_id = qp.question_id 
WHERE    u.university = '山东大学' 
GROUP BY  qd.difficult_level;