MongoDB query for counting the distinct values across all documents?

For this, use aggregate() in MongoDB. Let us create a collection with documents −

> db.demo718.insertOne(
...    {
...       "id":101,
...       "details":
...       {
...          "OtherDetails": ["Chris", "Mike", "Sam"], "GroupName": ["Group-1"], "Info": []
...       }
...    }
... );
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5eaae25843417811278f5880")
}
> db.demo718.insertOne(
...    {
...       "id":102,
...       "details":
...       {
...          "OtherDetails": ["Chris", "David"], "GroupName": ["Group-1"], "Info": []
...       }
...    }
... );
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5eaae25943417811278f5881")
}

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

> db.demo718.find();

This will produce the following output −

{ "_id" : ObjectId("5eaae25843417811278f5880"), "id" : 101, "details" : { "OtherDetails" : [ "Chris", "Mike", "Sam" ], "GroupName" : [ "Group-1" ], "Info" : [ ] } }
{ "_id" : ObjectId("5eaae25943417811278f5881"), "id" : 102, "details" : { "OtherDetails" : [ "Chris", "David" ], "GroupName" : [ "Group-1" ], "Info" : [ ] } }

Following is the query to count the distinct values across all documents −

> db.demo718.aggregate([
...    {
...       $unwind: "$details.GroupName"
...    },
...    {
...       $match: {
...          "details.GroupName": "Group-1"
...       }
...    },
... {
...    $unwind: "$details.OtherDetails"
... },
... {
...    $group: {
...       _id: "$details.GroupName",
...       OtherDetailsUnique: {
...          $addToSet: "$details.OtherDetails"
...       }
...    }
... },
... {
...    $project: {
...       _id : 0,
...       GROUP_NAME: "$_id",
...       OtherDetailsUniqueCount: {
...       $size: "$OtherDetailsUnique"
...    }
... }
... }
... ])

This will produce the following output −

{ "GROUP_NAME" : "Group-1", "OtherDetailsUniqueCount" : 4 }