mysql - 在 WHERE 中加入 MAX

Question

有 2 个表:

表温度_现在:

machine_id | temperature
------------------------
    1      |     15
    2      |     20
    3      |     13

表温度历史记录:

change_id | machine_id | temperature | controller
-------------------------------------------------
    1           1            6           Carl
    2           2            9           Steve
    3           1            7           John
    4           1            15          Peter
    5           2            20          Peter
    6           3            13          Martin

温度_now.machine_id = 温度_历史.machine_id

change_id 是 auto_increment

我需要获取每台机器的最后一次温度变化:

machine_id | temperature_now | temperature_before | controller
--------------------------------------------------------------
    1              15                   7              John
    2              20                   9              Steve

感谢您的帮助。

Answer 1

您可以使用通用表表达式来完成此操作，如下所示。

已更新

WITH History (MachineID, Temp, Controller)
AS
(
   SELECT machine_id, temperature, controller 
   FROM dbo.temperature_history
   WHERE change_id in ((SELECT MAX(change_id) FROM dbo.temperature_history AS TH
   INNER JOIN dbo.temperature_now AS TN
   ON TN.machine_id = TH.machine_id
   WHERE TN.temperature != TH.temperature 
   GROUP BY TH.machine_id))
)

SELECT N.machine_id, 
N.temperature, 
H.Temp as 'temperature_before',
H.Controller
FROM dbo.temperature_now AS N
INNER JOIN History AS H
ON H.MachineID = N.machine_id
ORDER BY N.machine_id;

然后你会得到如下结果:

machine_id   temperature    temperature_before  Controller
1            15             7                   John
2            20             9                   Steve