Group across two columns in MongoDB?

To group across two columns, use $lookup. Let us create a collection with documents −

> db.demo132.insertOne({"CountryName1":"US","CountryName2":"UK",Value:50});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e31950468e7f832db1a7f75")
}
> db.demo132.insertOne({"CountryName1":"UK","CountryName2":"AUS",Value:10});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e31951d68e7f832db1a7f76")
}
> db.demo132.insertOne({"CountryName1":"AUS","CountryName2":"US",Value:40});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e31952c68e7f832db1a7f77")
}

Display all documents from a collection with the help of find() method −

> db.demo132.find();

This will produce the following output −

{ "_id" : ObjectId("5e31950468e7f832db1a7f75"), "CountryName1" : "US", "CountryName2" : "UK", "Value" : 50 }
{ "_id" : ObjectId("5e31951d68e7f832db1a7f76"), "CountryName1" : "UK", "CountryName2" : "AUS", "Value" : 10 }
{ "_id" : ObjectId("5e31952c68e7f832db1a7f77"), "CountryName1" : "AUS", "CountryName2" : "US", "Value" : 40 }

Following is the query to group across two columns in MongoDB −

> db.demo132.aggregate( [
...    {
...       "$lookup" : {
...          "from" : "demo132",
...          "localField" : "CountryName1",
...          "foreignField" : "CountryName2",
...          "as" : "out"
...       }
...    },
... {
...    "$unwind" : "$out"
... },
... {
...    "$project" : {
...       "_id" : 0,
...       "CountryName1" : 1,
...       "total" : { "$sum" : [ "$Value", "$out.Value"]}
...       }
...    }
... ])

This will produce the following output −

{ "CountryName1" : "US", "total" : 90 }
{ "CountryName1" : "UK", "total" : 60 }
{ "CountryName1" : "AUS", "total" : 50 }