select
a.uid,
level,
register_time,
max(score) max_score
from
user_info a,
examination_info b,
exam_record c
where 
a.uid=c.uid
and c.exam_id=b.exam_id
and b.tag='算法'
and a.job='算法'
and date_format(a.register_time,'%Y%m%d')=date_format(c.submit_time,'%Y%m%d')
group by a.uid
order by max_score desc
limit 6,3

把题目读错了，以为是要注册当天完成所有算法类题目