select user_info.uid, user_info.level, user_info.register_time,max(exam_record.score) as max_score
from exam_record
join user_info on user_info.uid = exam_record.uid
join examination_info on examination_info.exam_id = exam_record.exam_id
where user_info.job = '算法'
and DATE_FORMAT(user_info.register_time, '%Y%m%d') = if(examination_info.tag = '算法', DATE_FORMAT(exam_record.submit_time, '%Y%m%d'), 0)
group by user_info.uid
order by max_score desc
# limit用法：【select * from tableName limit i,n 】;i : 为查询结果的索引值（默认从0开始）；n : 为查询结果返回的数量
limit 6, 3;