Select last distinct records where table1.column1 = table2.column2

Question

I have two tables from the same database:

Table READINGS:

Table SENSORS:

I'm trying to create a query that will return, for each sensor registered in SENSORS, it's last row registered in READINGS. Note that the same sensor can have many registers in READINGS, but ONLY ONE in SENSORS.

The result should be something like:

Dump of the database: 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

Or if you need info from both tables:

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

UPDATE AVG last day

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;

UPDATE 2 To filter aggregated records you can use HAVING :

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

HAVING AVG(r_avg.temperature)!=42