Writting down $subtract in No-SQL MongoDB in a sophisticated way - mongodb

Question

I'm doing MongoDB Academy, and for this question:
What is the difference between the number of people born in 1998 and the number of people born after 1998 in the sample_training.trips collection?
The simplest way is to do this is (the way they expect you to answer):
db.trips.find({"birth year":1988}).count()
and:
db.trips.find({"birth year":{$gt:1988}}).count()
then, manually calculate.
I'm not familiar with programming and code syntax but, wondering about a sophisticated method, something like the code bellow can be improved.
db.trips.aggregate({$subtract:[db.trips.find({"pop":1988}).count(),db.trips.find({"pop":{$gt:1988}}).count()]})
Note: Atlas "free subscriber" don't allow to use $subtract in queries, so I even tested if it will work.

Answer 1

Much Simpler way use:
(query 1) - (query 2)
db.trips.find({"birth year":{"$gt":1998}}).count() - db.trips.find({"birth year":1998}).count()
You'll get the result as an integer
6

Answer 2

You can group the number of people born in the year 1998 and after 1988 using the following approach that too with just one aggregate query.
Aggregate pipeline stages:
Select only those records where the birth year is equal or greater than 1988
Group the number of people by the birth year
Add a new field as born_on to every record using the $cond operator in the $project stage
Group record again by the newly added field born_on, which will now give you only two records with count as on_1988 and after_1988
The query is as following
db.trips.aggregate([
{
"$match":{
"birth year":{
"$gte":1988
}
}
},
{
"$group":{
"_id":"$birth year",
"count":{
"$sum":1
}
}
},
{
"$project":{
"count":"$count",
"born_on":{
"$cond":[
{
"$eq":[
"$_id",
1988
]
},
"on_1988",
"after_1988"
]
}
}
},
{
"$group":{
"_id":"$born_on",
"total":{
"$sum":"$count"
}
}
}
])
** There could be another more optimized way of achieving the same result but this also works

Answer 3

you can use the following queries to get this value:
db.trips.find( { "birth year": { "$gt": 1998 } } ).count()
db.trips.find( { "birth year": 1998 } ).count()
Use the $gt instead of $gte operator to exclude all 1998 births, and then see how many people were born in 1998 by using implicit equality, then subtract the two values to get the difference.