Group results from two different tables on same hours I have two simple tables:

indoor

id | timestamp | temp | humi


outdoor

id | timestamp | temp


And two selects which give me time and average temperature grouped by same hour for the last 24 hours:

SELECT DATE_FORMAT(timestamp, '%H:00') AS time, round(avg(temp), 1) as avg_out_temp
FROM outdoor
WHERE timestamp &gt;= now() - INTERVAL 1 DAY
GROUP BY DATE_FORMAT(timestamp, '%Y-%m-%d %H')
ORDER BY timestamp ASC;

SELECT DATE_FORMAT(timestamp, '%H:00') AS time, round(avg(temp), 1) as avg_in_temp
FROM indoor
WHERE timestamp &gt;= now() - INTERVAL 1 DAY
GROUP BY DATE_FORMAT(timestamp, '%Y-%m-%d %H')
ORDER BY timestamp ASC;


And now what I need to do is to group those two results by same hour, with respect to possibility that there can be no records in indoor or outdoor table for whole hour, so I need to get:

time | avg_out_temp | avg_in_temp
11:00 | 12.5 | 21.4
12:00 | 13.9 | null
13:00 | null | 22.4
14:00 | 14.0 | 22.5


I'm working with MariaDB: 

mysql  Ver 15.1 Distrib 10.1.41-MariaDB, for debian-linux-gnueabihf (armv7l) using readline 5.2
Because both your queries return the same number (and type) of fields, the can be combinied with UNION ALL. The ORDER BY is only needed once at then end of the query.

To see the difference between indoor and outdoor this could be added to your query.

SELECT 'out' as door,DATE_FORMAT(timestamp, '%H:00') AS time, round(avg(temp), 1) as avg_out_temp
FROM outdoor
WHERE timestamp &gt;= now() - INTERVAL 1 DAY
GROUP BY DATE_FORMAT(timestamp, '%Y-%m-%d %H')
UNION ALL
SELECT 'in' as door,DATE_FORMAT(timestamp, '%H:00') AS time, round(avg(temp), 1) as avg_in_temp
FROM indoor
WHERE timestamp &gt;= now() - INTERVAL 1 DAY
GROUP BY DATE_FORMAT(timestamp, '%Y-%m-%d %H')
ORDER BY timestamp ASC;