mysql - MySQL 选择信息

Question

所以我需要帮助来搜索一些信息。我正在寻找房间里有空闲空间的房间。我添加了表格的图片，它在之前和之后的样子。我希望那是足够的信息:)

Answer 1

首先找出每个房间里有多少人：

SELECT door_nr
     , Count(*) As number_of_occupants
FROM   people
GROUP
    BY door_nr

rooms然后我们可以使用此查询并根据匹配door_nr值将其连接回表。

SELECT rooms.door_nr
     , rooms.space As capacity
     , room_usage.number_of_occupants
     , rooms.space - room_usage.number_of_occupants As free_space
FROM   rooms
 LEFT
  JOIN (
        SELECT door_nr
             , Count(*) As number_of_occupants
        FROM   people
        GROUP
            BY door_nr
       ) As room_usage
    ON room_usage.door_nr = rooms.door_nr

Answer 2

SELECT Door_Nr, COUNT(*) FROM People GROUP BY Door_Nr

会给你房间里的人数。

有了（My）SQL的基本工作知识，您应该能够弄清楚其余的吗？否则，请提出具体问题。

Answer 3

select r.door_nr, 
       r.space - sum(p.name is not null) as free
from rooms r
left join people p on p.door_nr = r.door_nr
group by r.door_nr, r.space
having free > 0

SQLFiddle 演示