查询按月组合两个表

Question

我尝试通过 join 连接两个表并将它们分组以获取计数。但不幸的是，这两个表没有任何共同的加入价值（或者我误解了解决方案）。

select date_format(check_in.date,'%M') as Month, count(check_in.Id) as checkInCount 
from check_in 
group by month(check_in.date);

Month	checkInCount
July	1
October	2

这是第一张桌子。

select date_format(reservation.date,'%M') as Month, count(reservation.id) as reserveCount 
from reservation
group by month(reservation.date);

Month	reserveCount
July	3
October	5

这是第二张桌子。 我想在一张表中显示这两个表。

Month	checkInCount	reserveCount
July	1	3
October	2	5

感谢您尝试这个，如果这太简单了，我们很抱歉。

Answer 1

您需要通过month 从您的两个子查询中加入结果。 此查询假定所有month（七月、八月、九月...）存在于您的子查询monthCheckInStat、monthCheckOutStat，即使计数为0

SELECT monthCheckInStat.Month, monthCheckInStat.checkInCount, monthCheckOutStat.reserveCount
FROM
(
    select date_format(check_in.date,'%M') as Month, count(check_in.Id) as checkInCount 
    from check_in 
    group by month(check_in.date)
) monthCheckInStat
INNER JOIN
(
    select date_format(reservation.date,'%M') as Month, count(reservation.id) as reserveCount 
    from reservation
    group by month(reservation.date)
) monthCheckOutStat
ON monthCheckInStat.Month = monthCheckOutStat.Month;