题解 | 浙大不同难度题目的正确率，小丑

#解法1左连接考虑空值
select c.difficult_level,count(if(result = "right",1,null))/count(result)correct_rate

#左连接需要考虑空值，会加一行，加的一行difficult_level自动赋值为null，所以需要去掉空值
from user_profile a  left join question_practice_detail b on (a.device_id=b.device_id) left join question_detail c on (b.question_id=c.question_id)

where university = "浙江大学"
and c.difficult_level!='None'

group by difficult_level
order by correct_rate



# #解法2直接join
# select c.difficult_level,count(if(result = "right",1,null))/count(result)correct_rate

# from user_profile a  join question_practice_detail b  using(device_id) join question_detail c using(question_id)

# where university = "浙江大学"

# group by difficult_level
# order by correct_rate