题解 | #SQL35 浙大不同难度题目的正确率#

SELECT qd.difficult_level,

# 这里分子用 sum，求该列上所有值的和；分母用 count，求有多少个非空行
sum(if(qpd.result = 'right', 1, 0)) / count(qpd.question_id) correct_rate

# AVG(IF(result='right',1,0)) AS correct_rate
from user_profile u
	join question_practice_detail qpd
	on u.device_id = qpd.device_id
	join question_detail qd
	on qpd.question_id = qd.question_id
    
where u.university = '浙江大学'
group by difficult_level
order by correct_rate;