为每个传感器选择最近的测量值

Question

我在 MySQL 数据库中有两个表 - Sensors 和 Measurements 其中 'sensor_id' 是进行测量的传感器的 ID，'value' 是测量值，'measured' 是 Unix 时间（值是什么时候测量）。我想从 Sensors 表中获取每个传感器的最后测量值，因此在此示例中，从 Measurements 表中获取最后 3 行：

id     sensor_id               value       measured
22     1000000000000001        10.20       1619166847821
23     0100000000000001        67.00       1619166847827
24     0000000000000001        3.05        1619166847830

我尝试过类似SELECT MAX(m.id) as id, m.sensor_id, m.value, MAX(m.measured) as measured FROM Measurements m JOIN Sensors s ON m.sensor_id=s.id GROUP BY s.id; 的方法，它适用于“id”和“measured”，但不适用于“value”：

id     sensor_id               value       measured
22     1000000000000001        23.00       1619166847821
23     0100000000000001        47.00       1619166847827
24     0000000000000001        3.03        1619166847830

有什么想法吗？谢谢！

Answer 1

我想从传感器表中获取每个传感器的最后测量值

我只会使用相关子查询：

select m.*
from Measurements m
where s.measured = (select max(m2.measured)
                    from Measurements m2
                    where m2.sensor_id = m.sensor_id
                   );

我认为查询不需要sensors。

为了提高性能，您需要在Measurements(sensor_id, measured) 上建立索引。

Answer 2

youerquery 失败，因为结果集按照定义是无序的。

所以用例子

CREATE TABLE Measurements  (
  `id` INTEGER,
  `sensor_id` vaRCHAr(16),
  `value` FLOAT,
  `measured` varchar(14)
);

INSERT INTO Measurements 
  (`id`, `sensor_id`, `value`, `measured`)
VALUES
  ('22', '1000000000000001', '10.20', '1619166847821'),
  ('23', '0100000000000001', '67.00', '1619166847827'),
  ('24', '0000000000000001', '3.05', '1619166847830');

SELECT t1.* 
FROM Measurements  t1 
INNER JOIN (SELECT MAX(id) max_id,`sensor_id` FROM Measurements  GROUP BY `sensor_id`) t2
ON t1.`sensor_id` = t2.`sensor_id` AND t1.id = t2.max_id

编号 | sensor_id |价值 |测量的 -: | :--------------- | ----: | :------------ 22 | 1000000000000001 | 10.2 | 1619166847821 23 | 0100000000000001 | 67 | 1619166847827 24 | 0000000000000001 | 3.05 | 1619166847830

db小提琴here

Answer 3

您需要分析或窗口函数。我不确定它们是在哪个版本的 MySQL 中引入的，所以请同时指定您的 MySQL 版本。

SELECT DISTINCT
FIRST_VALUE(id) OVER (PARTITION BY sensor_id ORDER BY measured DESC) as id
,sensor_id
,FIRST_VALUE(value) OVER (PARTITION BY sensor_id ORDER BY measured DESC) as value
,FIRST_VALUE(measured ) OVER (PARTITION BY sensor_id ORDER BY measured DESC) as measured 
FROM Measurements m 
JOIN Sensors s ON m.sensor_id=s.id