题解 | #查找入职员工时间排名倒数第三的员工所有信息#

	
SELECT
	tmp1.difficult_level,
	IFNULL(( right_num / num ),0) AS correct_rate 
FROM
	(
SELECT
	det.difficult_level,
	count( 1 ) AS num 
FROM
	`user_profile` pro
	LEFT JOIN question_practice_detail pra ON pro.device_id = pra.device_id
	LEFT JOIN question_detail det ON pra.question_id = det.question_id 
WHERE
	university = '浙江大学' 
GROUP BY
	difficult_level 
	) tmp1
	LEFT JOIN (
SELECT
	det.difficult_level,
	count( 1 ) AS right_num 
FROM
	`user_profile` pro
	LEFT JOIN question_practice_detail pra ON pro.device_id = pra.device_id
	LEFT JOIN question_detail det ON pra.question_id = det.question_id 
WHERE
	university = '浙江大学' 
	AND result = 'right' 
GROUP BY
	difficult_level 
	) tmp2 ON tmp1.difficult_level = tmp2.difficult_level where tmp1.difficult_level > ""
ORDER BY
	correct_rate ASC;
	
	

效率不行，写法也不好看，但是能通过题目测试。