How count e.g. ice days over month/years - mongodb-query

Question

I have a weather station with data over 14 years every 10min: like this
_id:60fbcf880000000000000000
datum:2021-07-24T08:30:00.000+00:00
temperature:19.5
Now I want to count per year and per month certain days.
Ice days (temperature is a 24h always below 0°),
winter days (temperature is at a time under 0°)
cold days (max temp <10°C)
hot days (max temp 25-30°C)
very hot days (max temp over 30°).
I have no real clue about the best query code for mongo.
I can group for days, but then I have the issue to count those certain days ($buckets?)
I come until the grouping:
{$group: {
_id: [{$year: '$datum'}, <br>
{$month: '$datum'}, <br>
{$dayOfMonth: '$datum'}], <br>
temp_avg: {$avg: '$tempAussen'}, <br>
temp_min: {$min: '$tempAussen'}, <br>
temp_max: {$max: '$tempAussen'}, <br>
}}
Result in a list with elements like:
_id:Array
0:2020
1:3
2:2
temp_avg:6.12
temp_min:0.7
temp_max:9.6
But now starts my problem: How to count the days for (e.g. 2020 month 3) with temperature <0° and the other days?

Answer 1

Your grouping is working fine. You just need one more extra $group by month to sum conditionally for each type of day. For example, you can do
{
$group: {
_id: {
"year": "$_id.year",
"month": "$_id.month",
},
winter_days_count: {
$sum: {
"$cond": {
"if": {
$lt: [
"$temp_min",
0
]
},
"then": 1,
"else": 0
}
}
}
...
to count the winter days in the month.
Here is the Mongo playground for your reference.