题解 | #商品交易(网易校招笔试真题)#

```WITH t AS (
	SELECT
		a.device_id,
		a.university,
		a.answer_cnt,
		b.question_id 
	FROM
		user_profile a
		LEFT JOIN question_practice_detail b ON a.device_id = b.device_id 
	WHERE
		a.university = "山东大学" 
	) SELECT
	university,
	difficult_level,
	ROUND( count( t.question_id ) / count( DISTINCT t.device_id ), 4 ) 
FROM
	t
	LEFT JOIN question_detail c ON t.question_id = c.question_id 
GROUP BY
	c.difficult_level,
	t.university;