题解 | #统计每个学校的答过题的用户的平均答题数#

# SELECT
# university, 
# count(q.question_id) / count(distinct(q.device_id)) as avg_answer_cnt 
# from user_profile u, question_practice_detail q
# WHERE u.device_id = q.device_id
# GROUP BY university


# 运行时间43ms
# 占用内存6516KB
SELECT
university, 
count(question_id) / count(distinct u.device_id) as avg_answer_cnt 
from user_profile u inner join question_practice_detail q
on u.device_id = q.device_id
GROUP BY university

莫名其妙 别人运行时间怎么就是36/37ms呢 我的怎么会多这么多