题解 | #试卷发布当天作答人数和平均分#

解题思路=找出user_info表和examination_info符合条件的数据-嵌套exam_record表
注意，
1、where要在join之后编写
2、date(submit_time)=date(release_time)也可以用date_format函数代替
3、uid非唯一，要指定表格

select exam_id,
count(distinct e.uid) uv,
round(avg(score) ,1) avg_score
from exam_record e

join user_info 
using(uid)


join examination_info i
using(exam_id)

where level>5
and tag="SQL"
and date(submit_time)=date(release_time)

group by exam_id
order by uv desc,avg_score asc