MySQL query conversion to MongoDB - mysql

Question

I am new to MongoDB. I have done code to get the highest deaths value country-wise with their reporting date in MySQL. As the initial step, I am trying to get the max value of the deaths column, but it is returning another value that is not the highest. Here is my MySQL code:
SELECT
d.country_name, s.dt, MAX(s.deaths)
FROM
Demographics d
inner JOIN statistics s
where d.country_id=s.country_id
GROUP BY country_name
ORDER BY MAX(s.deaths) DESC;
It is returning:
Germany
2022-01-29
118335
Bangladesh
2022-01-30
34
What will be the equivalent code to MongoDB to get the same result?
To get the max value of the deaths column in MongoDB i used:
db.statistics.aggregate([
{
$group: {
_id: "$country_id",
maxQuantity: {
$max: "$deaths"
}
}
}
])
Here is my sample input:
Demographics
{"country_id":"BGD","country_name":"Bangladesh","population":"164700000","area":"148460","density":"1265"}, {"country_id":"DEU","country_name":"Germany","population":"83200000","area":"357386","density":"232"}
statistics
{"country_id":"DEU","dt":"2022-01 29", "confirmed_cases":"2016684", "deaths":"118335"},
{"country_id":"DEU","dt":"2022-01-17", "confirmed_cases":"53916", "deaths":"143"},
{"country_id":"BGD","dt":"2022-01-30", "confirmed_cases":"12183", "deaths":"34"},
{"country_id":"BGD","dt":"2022-01-29", "confirmed_cases":"10378", "deaths":"21"},

Answer 1

Updated: Post Owner request for the max deaths for each country.
$lookup - Join both statistics and Demographic collections by country_id.
$set - Set death field by converting to integer.
$sort - Sort by death DESC.
$group - Group by country_id. Get first value ($first) as the result was sorted in Stage 3.
$sort - Sort by maxQuantity DESC.
$project - Decorate output document.
Side note: It's weird to store confirmed_cases and deaths as string type.
db.statistics.aggregate([
{
"$lookup": {
"from": "Demographics",
"localField": "country_id",
"foreignField": "country_id",
"as": "demographics"
}
},
{
"$set": {
deaths: {
$toInt: "$deaths"
}
}
},
{
$sort: {
deaths: -1
}
},
{
$group: {
_id: {
country_id: "$country_id"
},
country: {
$first: "$demographics"
},
dt: {
$first: "$dt"
},
maxQuantity: {
$first: "$deaths"
}
}
},
{
$sort: {
maxQuantity: -1
}
},
{
$project: {
_id: 0,
country_name: {
$first: "$country.country_name"
},
dt: "$dt",
maxQuantity: "$maxQuantity"
}
}
])
Sample Mongo Playground
For MySQL query, INNER JOIN should be:
INNER JOIN statistics s ON d.country_id=s.country_id
and without the need of WHERE.