题解 | #0级用户高难度试卷的平均用时和平均得分#

SELECT uid, 
	# 计算均分，当有score的时候，就返回实际的，没有就是0
    ROUND(AVG(IF(score, score, 0)), 0) avg_score,
	# 计算均用时，使用TIMESTAMPDIFF计算时间差，分钟为单位
    ROUND(AVG(IF(score, TIMESTAMPDIFF(MINUTE, start_time, submit_time), duration)), 1) avg_time_took
FROM exam_record c
    JOIN user_info a USING(uid)
    JOIN examination_info b USING(exam_id)
# 限制条件
WHERE level = 0 AND difficulty = 'hard'
GROUP BY uid