选择table1.collumn1 = table2.column2的最后一个不同记录

Question

我有两个来自同一个数据库的表：

表读物：

表传感器：

我正在尝试创建一个将返回的查询，对于在SENSORS中注册的每个传感器，它是在READINGS中注册的最后一行。请注意，同一传感器在READINGS中可以有多个寄存器，但在SENSORS中只有一个寄存器。

结果应该是这样的：

转储数据库：https://dl.dropboxusercontent.com/u/85576999/redeSensores.sql

Answer 1

http://sqlfiddle.com/#!9/95818/5

SELECT readings.*
FROM readings
INNER JOIN sensors
ON sensors.idSensor = readings.sensorid
LEFT JOIN readings r
ON r.sensorid = readings.sensorid
  AND r.`datetime` > readings.`datetime`
WHERE r.id IS NULL

或者，如果您需要来自两个表的信息：

http://sqlfiddle.com/#!9/95818/6

SELECT readings.*, 
       sensors.*
FROM readings
INNER JOIN sensors
ON sensors.idSensor = readings.sensorid
LEFT JOIN readings r
ON r.sensorid = readings.sensorid
  AND r.`datetime` > readings.`datetime`
WHERE r.id IS NULL

更新 AVG最后一天

http://sqlfiddle.com/#!9/95818/10

SELECT t.*, 
   AVG(r_avg.temperature),
   AVG(r_avg.pollution),
   AVG(r_avg.noise),
   AVG(r_avg.humidity)
FROM (SELECT readings.*, sensors.*
FROM readings
INNER JOIN sensors
ON sensors.idSensor = readings.sensorid
LEFT JOIN readings r
ON r.sensorid = readings.sensorid
  AND r.`datetime` > readings.`datetime`
WHERE r.id IS NULL 
) t
LEFT JOIN readings r_avg
ON t.sensorid = r_avg.sensorid
  AND r_avg.`datetime` >= DATE_ADD(t.datetime,INTERVAL -1 DAY)
GROUP BY t.id;

更新2 要过滤汇总记录，您可以使用HAVING：

http://sqlfiddle.com/#!9/95818/11

HAVING AVG(r_avg.temperature)!=42