SQL LIMIT dynamic N - mysql

Question

I want to get the (last row) average air_temperature from all stations that have the specified county_number.
Therefor, my solution would be something like
SELECT AVG(air_temperature)
FROM weather
WHERE station_id IN (
SELECT station_id
FROM stations
WHERE county_number = 25
)
ORDER
BY id DESC
LIMIT 1;
Clearly, this does not give the correct row as it returns the average air_temperature based on all air_temperature ever recorded of one station.
Back to the problem, I want to get the average air_temperature over the last inserted row from each station that have the specified county_number.
Table weather
+------------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| station_id | char(20) | YES | MUL | NULL | |
| timestamp | timestamp | YES | | NULL | |
| air_temperature | float | YES | | NULL | |
+------------------+-------------+------+-----+---------+----------------+
Table stations
+---------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------+-------------+------+-----+---------+-------+
| station_id | char(20) | NO | PRI | NULL | |
| county_number | int(10) | YES | | NULL | |
+---------------+-------------+------+-----+---------+-------+
Tables are minimized

Answer 1

I would recommend doing this with a join and some filtering:
select avg(w.air_temperature)
from weather w join
stations s
on w.station_id = s.station_id
where s.county_number = 25 and
w.timestamp = (select max(w2.timestamp) from weather w2 where w2.station_id = w.station_id)

Answer 2

You can get the last inserted row by checking the max(timestamp):
SELECT
AVG(w.air_temperature)
FROM weather w
INNER JOIN (
SELECT station_id, max(timestamp) maxtimestamp FROM weather GROUP BY station_id
) t
ON w.station_id = t.station_id AND w.timestamp = t.maxtimestamp
WHERE
w.station_id IN (SELECT station_id FROM stations WHERE county_number = 25)

Answer 3

UPDATE: I just noticed that your timestamp column is nullable and you are talking about the "last inserted row". That is the one with the greatest ID. Hence:
As of MySQL 8 you can use window functions in order to read the table only once:
select avg(air_temperature)
from
(
select air_temperature, id, max(id) over (partition by station_id) as max_id
from weather
where station_id in (select station_id from stations where county_number = 25)
) analyzed
where id = max_id;
In older versions you must read the table twice:
select avg(air_temperature)
from weather
where (station_id, id) in
(
select station_id, max(id)
from weather
where station_id in (select station_id from stations where county_number = 25)
group by station_id
);