基于时间戳的带IF的MySQL SELECT

Question

I have a MySQL database with 3 tables as defined below.

Readings: (_id, value, timestamp, sensorId)
Sensor: (_id)
DeviceSensor: (deviceId, sensorId, timestamp)

Each Reading has a timestamp of when the data was collected. The Sensor is the owner of the Reading . DeviceSensor contains the relationship between Device and Sensor , including the timestamp of when the relationship started.

In my architecture, when a Device is paired with a Sensor , the Sensor ignores former Devices .

What I need is to query all the Readings and associate them with the corresponding Device at each time of reading. For example:

DeviceSensor = [(1,1,1483527932),(1,2,1507058076)]
Sensor = [(1)]
Readings = [(1,5,1483527932,1),(2,3,1493527932,1),(3,7,1507058076,1)]

With this data, my query would return: [(1,5,1483527932,1),(2,3,1493527932,1),(3,7,1507058076,2)] . That is, the Readings that have timestamp between 1483527932 and 1507058076 should return sensorId = 1 and the Readings after 1507058076 should return sensorId = 2 .

Answer 1

SELECT r._id
      ,r.value
      ,r.timestamp
      ,(SELECT max(ds.deviceId) as deviceId
          FROM DeviceSensor as ds
         WHERE ds.timestamp <= r.timestamp
           AND ds.sensorId = r.sensorId
         ORDER BY ds.timestamp DESC
       LIMIT 1) as deviceId
  FROM Readings as r

The subselect selects the records from DeviceSensor which are for the current sensor of the main select, restricted to those with a pairing up to the reading timestamp. The LIMIT together with ORDER BY ds.timestamp DESC selects the latest of these.