Question

I want to join two tables in a special way, first table is devices which has a list of devices.

The second table is datalog which is where abit of data is stored for everytime a device in devices gets polled.

Devices Table:

+----------+------------+----------------------------+---------------------+
| deviceId | deviceName | deviceDescription          | timeCreated         |
+----------+------------+----------------------------+---------------------+
|        1 | System 1   | Main System in Server Room | 2010-01-01 00:00:00 |
|        2 | System 2   | Outdoor System             | 2010-01-01 00:00:00 |
+----------+------------+----------------------------+---------------------+

DataLog Table:

+----+---------------------+----------+-----------+---------+
| id | time_stamp          | DeviceId | FuelLevel | Voltage |
+----+---------------------+----------+-----------+---------+
|  1 | 2010-01-01 00:00:00 |        1 |        60 |     220 |
|  2 | 2010-01-01 00:00:00 |        2 |        20 |     221 |
|  3 | 2010-01-02 00:00:00 |        1 |       100 |     219 |
|  4 | 2010-01-02 00:00:00 |        2 |       100 |     222 |
|  5 | 2010-01-03 00:00:00 |        1 |        80 |     219 |
|  6 | 2010-01-03 00:00:00 |        2 |        99 |     220 |
+----+---------------------+----------+-----------+---------+

Currently I am getting the latest data for each device using a query on the DataLog table with:

 Where DeviceId = 1 ORDER BY timestamp DESC LIMIT 1

What I would like is one query to return a list of all devices, with the columns joined with the latest data for each device like this:

+----------+------------+----------------------------+---------------------+-----------+---------+
| deviceId | deviceName | deviceDescription          | time_stamp          |FuelLevel  | Voltage |
+----------+------------+----------------------------+---------------------+-----------+---------+
|        1 | System 1   | Main System in Server Room | 2010-01-03 00:00:00 |        80 |     219 |
|        2 | System 2   | Outdoor System             | 2010-01-03 00:00:00 |       99  |     220 |
+----------+------------+----------------------------+---------------------+-----------+---------+

Answer 1

You can’t do the “limit 1” at the outer level, you loose what you are looking for… ALL devices last entry. Use a pre-query for the last ID of each device, then join back…

select 
        Devices.*,
        DataLog.Time_Stamp,
        DataLog.FuelLevel,
        DataLog.Voltage
    from
        ( select DeviceID,
                 max( ID ) LastActionID
              from
                 DataLog
              group by 
                 1 ) LastInstance
        join DataLog
            on LastInstance.LastActionID = DataLog.ID
        join Devices 
            on LastInstance.DeviceID = Devices.DeviceID
   order by 
       Devices.DeviceName

Per your last comment, I would actually change to something like…

Update your device table with a “LastLogID”. Then, via a trigger an insert into your DataLog table, update the Device table immediately with that new ID… This way, you never need to pre-query the data log directly.. You’ll already HAVE the last ID and run from that directly to the data log joined by that ID.