题解 | 统计每个用户的平均刷题数

select
    up.university,
    dtl.difficult_level,
    round(count(prct.question_id)/count(distinct(prct.question_id)),4) as avg_answer_cnt
from 
    user_profile up,
    question_practice_detail prct,
    question_detail dtl
where
    up.university = '山东大学'
    and up.device_id = prct.device_id
    and prct.question_id = dtl.question_id
group by
    dtl.difficult_level;

内什么内连接外连接左连接右连接…瞎连接都不懂…只会用where这样

听说where效率低，但又有朋友说编译器会自行优化…

我就很好奇…