MongoDB query for counting number of unique fields grouped by another field?

For this, use aggregate() and group with $group. Let us create a collection with documents −

> db.demo354.insertOne({"Name1":"Chris","Name2":"David"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e5685a6f8647eb59e5620c0")
}
> db.demo354.insertOne({"Name1":"Chris","Name2":"David"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e5685a9f8647eb59e5620c1")
}
> db.demo354.insertOne({"Name1":"Chris","Name2":"Bob"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e5685aff8647eb59e5620c2")
}
> db.demo354.insertOne({"Name1":"Carol","Name2":"John"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e5685c4f8647eb59e5620c3")
}
> db.demo354.insertOne({"Name1":"Carol","Name2":"John"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e5685c5f8647eb59e5620c4")
}

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

> db.demo354.find();

This will produce the following output −

{ "_id" : ObjectId("5e5685a6f8647eb59e5620c0"), "Name1" : "Chris", "Name2" : "David" }
{ "_id" : ObjectId("5e5685a9f8647eb59e5620c1"), "Name1" : "Chris", "Name2" : "David" }
{ "_id" : ObjectId("5e5685aff8647eb59e5620c2"), "Name1" : "Chris", "Name2" : "Bob" }
{ "_id" : ObjectId("5e5685c4f8647eb59e5620c3"), "Name1" : "Carol", "Name2" : "John" }
{ "_id" : ObjectId("5e5685c5f8647eb59e5620c4"), "Name1" : "Carol", "Name2" : "John" }

Following is the query for counting number of unique fields grouped by another field −

> db.demo354.aggregate([
... {
...    $group: {
...       _id: {
...          "Name1": "$Name1",
...          "Name2": "$Name2"
...       },
...    }
... },
... {
...    $group: {
...       _id: "$_id.Name1",
...       "Name2": { $sum: 1 }
...    }
... },
... {
...    $project: {
...       _id: 0,
...       Name1: "$_id",
...       Name2: 1
...    }
... }
... ])

This will produce the following output −

{ "Name2" : 2, "Name1" : "Chris" }
{ "Name2" : 1, "Name1" : "Carol" }