数据库中特定用户的连接 + 计数的 MySQL 查询

Question

我想使用查询从多个表中获取多个实体的总和。 具体来说，我想要用户的设备和消息计数。此外，该用户最后收到的消息的时间戳。 我有 3 张桌子：

用户

id     name     (other fields)
1      Mike     ...
2      John     ...
3      Yay      ...
4      Jim      ...

设备

id     user_id   (other fields)
1      1         ...
2      1         ...
3      1         ...
4      2         ...

消息

id     device_id   message    time                   (other fields)
1      1           Hi         2019-04-07 12:06:44    ...
2      1           Hey        2019-04-06 12:06:44    ...
3      2           Sup        2019-04-05 12:06:44    ...
4      3           Ok         2019-04-04 12:06:44    ...
5      4           Yay        2019-04-08 12:06:44    ...

...例如，对于用户Mike，我想最终得到：

结果

nDevices    nMessages      time 
3           4              2019-04-07 12:06:44

有什么想法吗？ 提前致谢。

Answer 1

加入 3 个表并计算列的不同值：

select 
  count(distinct d.id) devicescounter, 
  count(distinct m.id) messagescounter,
  max(m.time) lastmessagetime
from users u
left join devices d on u.id = d.user_id 
left join nessages m on m.device_id = d.id
where u.name = 'Mike' 

如果您想要所有用户的结果：

select 
  u.id, u.name, 
  count(distinct d.id) devicescounter, 
  count(distinct m.id) messagescounter,
  max(m.time) lastmessagetime
from users u
left join devices d on u.id = d.user_id 
left join nessages m on m.device_id = d.id
group by u.id, u.name

Answer 2

最简单的方法是获取子查询中每个设备的消息数量和最近消息时间。

  SELECT u.id,
         COUNT(d.id) AS num_devices,
         SUM(messages) AS num_messages,
         MAX(most_recent) AS most_recent_message
    FROM users u
    JOIN devices d ON d.user_id = u.id
    JOIN (SELECT device_id,
                 COUNT() AS messages,
                 MAX(TIME) AS most_recent_message
            FROM messages
        GROUP BY device_id) m ON m.device_id = d.id
GROUP BY u.id

Answer 3

您可以尝试使用用户和设备之间的联接以及带有消息和设备子查询的内部联接

select a.name, a.id,  count(d.user_id), t.nDevice, t.max_time 
from Users a 
inner join  device d on a.id = d.user_id 
inner join  (
  select m.device_id, count(m.device_id) nDevice, max(m.time) max_time
  from Messages  m
  group by m.device_id
) t on t.device_id = d.id 
group by a.name, a.id