关于ORDER BY的位置 | #月总刷题数和日均刷题数#

select date_format(submit_time, '%Y%m') submit_month, count(1) month_q_cnt,
round(count(1)/max(day(last_day(submit_time))), 3) avg_day_q_cnt
from practice_record
where year(submit_time) = 2021
group by submit_month
union
select '2021汇总' submit_month, count(1) month_q_cnt,
round(count(1)/31, 3) avg_day_q_cnt
from practice_record
where year(submit_time) = 2021
group by date_format(submit_time, '%Y')
order by submit_month;

两个表联合时为什么要将order by放在最后？第一个表使用order by再和第二个表联合最终结果却没有正确排序？