Difference between multiple values in one measurements with InfluxDB


I am currently trying to display difference of results from 2 sensors in a Grafana panel.
In my InfluxDB database I have a measurement that holds values of both sensors:

> SELECT * FROM "Vergleich" WHERE time>now()-2m
name: Vergleich
time                rh_dht22 rh_tfa temp_dht22 temp_tfa           tp_dht22 tp_tfa
----                -------- ------ ---------- --------           -------- ------
1611407400000000000          45                21.883333333333333          9.40611175608533
1611407433000000000 47              21.3                          9.5
1611407460000000000          45                21.833333333333332          9.360802312324173
1611407493000000000 47              21.2                          9.4

For both sensors (DHT22 and some TFA device) I have stored 3 values each. They are not provided with identical timestamp but that should not be a problem.
I want to calculate the difference to see how well the various sensors provide similar data.

I am able to get some useful data out of influx:

> SELECT mean("temp_dht22") - mean("temp_tfa") AS "d_temp_dht22" FROM "Vergleich" WHERE time>now()-5m GROUP BY time(60s) fill(null)
name: Vergleich
time                d_temp_dht22
----                ------------
1611407280000000000 -0.6000000000000014
1611407340000000000 -0.6000000000000014
1611407400000000000 -0.5833333333333321
1611407460000000000 -0.6333333333333329

I got this query string from Calculate difference between two measurements and [VERY NEWBIE ] Show difference between two graphs values

Unfortunately I am not able to add this to my Grafana panel.
If I add a query and enter this code
SELECT mean("temp_dht22") - mean("temp_tfa") AS "d_temp_dht22" FROM "Vergleich" WHERE $timeFilter GROUP BY time($__interval) fill(null)

it is automatically changed to
SELECT "temp_dht22" -, "temp_tfa" AS "d_temp_dht22" FROM "Vergleich" WHERE $timeFilter GROUP BY time($__interval) fill(null)

and the graph only shows “No data”.

My Grafana version is “v7.3.7 (1e261642f4)” and I run it on a Raspberry 3+ with RaspiOS Buster (Image from 2020-12-02).

Any hints how to insert the query into the fields of my panel is welcome.