select 
    device_id,
    date_format (event_date, '%Y年%m月') as ym,
    count(question_id) as cnt
from
    question_practice_detail
where
    datediff(last_day(event_date), event_date) < 7
group by device_id,ym
order by device_id asc,ym asc

得到月份最后一天使用last_day()函数，日期进行减法，使用datediff()函数。