SQL按日期分组,但也得到没有记录的日期

假设你有比日期更多的订单这样的东西可以工作:

select date, count(id) as orders
from
(
  SELECT DATE_ADD('2008-01-01', INTERVAL @rn:=@rn+1 DAY) as date from (select @rn:=-1)t, `order` limit 365
) d left outer join `order` using (date)
group by date