查询某月的每一天

ADDDATE(CURDATE(),INTERVAL 0 MONTH)，当月，或者CURDATE()也行，

ADDDATE(CURDATE(),INTERVAL -1 MONTH)，上个月的每一天

select date_add(ADDDATE(CURDATE(),INTERVAL 0 MONTH), interval(cast(help_topic_id as signed integer) - DATE_FORMAT( DATE_SUB( curdate( ), INTERVAL 1 DAY ), '%d' )) day) day
from mysql.help_topic
where help_topic_id < day(last_day(curdate()))
order by help_topic_id