MongoDB Group query to get the count of repeated marks in documents?

For group query, use MongoDB $group and get the count with $sum. Let us create a collection with documents −

> db.demo676.insertOne({"Marks":87});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea41eed04263e90dac943f2")
}
> db.demo676.insertOne({"Marks":75});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea41ef304263e90dac943f3")
}
> db.demo676.insertOne({"Marks":87});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea41ef404263e90dac943f4")
}
> db.demo676.insertOne({"Marks":65});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea41ef704263e90dac943f5")
}
> db.demo676.insertOne({"Marks":65});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea41ef804263e90dac943f6")
}

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

> db.demo676.find();

This will produce the following output −

{ "_id" : ObjectId("5ea41eed04263e90dac943f2"), "Marks" : 87 }
{ "_id" : ObjectId("5ea41ef304263e90dac943f3"), "Marks" : 75 }
{ "_id" : ObjectId("5ea41ef404263e90dac943f4"), "Marks" : 87 }
{ "_id" : ObjectId("5ea41ef704263e90dac943f5"), "Marks" : 65 }
{ "_id" : ObjectId("5ea41ef804263e90dac943f6"), "Marks" : 65 }

Following is the query to get the count of repeated marks −

> db.demo676.aggregate( { $group: {
...    _id: {Marks: "$Marks" },
...    'Count': { $sum :1 }
... }})

This will produce the following output −

{ "_id" : { "Marks" : 75 }, "Count" : 1 }
{ "_id" : { "Marks" : 65 }, "Count" : 2 }
{ "_id" : { "Marks" : 87 }, "Count" : 2 }