题解 | #试卷发布当天作答人数和平均分#

select er.exam_id as exam_id,count(distinct er.uid) as uv,round(avg(score),1) as avg_score
from user_info as uf
right join exam_record as er
on uf.uid=er.uid
left join examination_info as ei
on er.exam_id=ei.exam_id
where ei.tag='SQL' and level>'5' and date_format(submit_time,'yyyy-mm-dd')=date_format(release_time,"yyyy-mm-dd")
group by er.exam_id
order by uv desc,avg_score;

连表（拿到题目要做的事情），条件查询（题目要求），分组（不分组会报错，因为字段用了聚合函数），排序（题目要求），秒了