select uid,exam_id,round(AVG(score)) as avg_score
from user_info
join exam_record using(uid)
join examination_info using(exam_id)
where submit_time is not null and tag rlike '^(?i)c' and nick_name rlike '^牛客[0-9]+号$' OR nick_name rlike '^[0-9]+$' 
# 这边注意因为AND优先级高，需要把and的条件写在前面
GROUP BY uid,exam_id
order by uid asc,avg_score asc