mysql inner join get only the last 3 equals - php

Question

I Hope someone could help me.
My Query.
SELECT * FROM `tbl_device`
INNER JOIN `tbl_temperature` ON tbl_device.ID = tbl_temperature.DevID
Result.
How can get something like this as shown in the photo on the link.
I need to get only the last 3 result of the tbl_device.DevID
I apologize for my bad english, it's really hard to explain. Any help is realy much appreciated.
Thanks a lot!

Answer 1

It should be something like
SELECT * FROM `tbl_device`
INNER JOIN `tbl_temperature` ON tbl_device.ID = tbl_temperature.DevID
ORDER BY tbl_temperature.DevID DESC
LIMIT 3 OFFSET 0
The order column can be not the same as in my example. Google MySQL LIMIT OFFSET
Or if you need to get only three last results from one of your tables then join it to another then it will be something like
SELECT * FROM tbl_device INNER JOIN
(SELECT .... FROM other_table ORDER BY your_column DESC LIMIT 3 OFFSET 0) as
T1 ON T1.id = tbl_device.ID

Answer 2

The simplest way is to use ANSI-standard window functions (row_number() in particular). But MySQL does not support those (yet).
In MySQL, probably the best way is to use variables:
SELECT . . .
FROM tbl_device d JOIN
(SELECT t.*,
(#rn := if(#d = t.devid, #rn + 1,
if(#d := t.devid, 1, 1)
)
) as rn
FROM (SELECT t.*
FROM tbl_temperature t
ORDER BY DevID, id DESC
) t CROSS JOIN
(SELECT #d := -1, #rn := 0) params
) t
ON tbl_device.ID = tbl_temperature.DevID
WHERE rn <= 3;
EDIT:
Here is a simpler way to express the logic. It might be performant with the right indexes:
SELECT d.*, t.*
FROM tbl_device d INNER JOIN
tbl_temperature t
ON d.ID = t.DevID
WHERE t.ID >= (SELECT t2.ID
FROM tbl_temperature t2
WHERE t2.DevId = t.DevId
ORDER BY t2.ID DESC
OFFSET 2 LIMIT 1
);
For performance, this can use an index on tbl_temperature(devid, id).