GROUP BY array of document to get the count of repeated Age values

To GROUP BY array of the document, use $group. Let us create a collection with documents −

>db.demo559.insertOne({details:[{Name:"Chris",Age:21},{Name:"Bob",Age:22},{Name:"Carol", Age:21},{Name:"Sam",Age:21}]});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e8f38d954b4472ed3e8e866")
}

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

> db.demo559.find().pretty();

This will produce the following output −

{
   "_id" : ObjectId("5e8f38d954b4472ed3e8e866"),
   "details" : [
      {
         "Name" : "Chris",
         "Age" : 21
      },
      {
         "Name" : "Bob",
         "Age" : 22
      },
      {
         "Name" : "Carol",
         "Age" : 21
      },
      {
         "Name" : "Sam",
         "Age" : 21
      }
   ]
}

Following is the query to group by an array of the document −

> db.demo559.aggregate([
...    {
...       "$unwind": "$details"
...    },
...    {
...       "$group": {
...          "_id": "$details.Age",
...          "Count": { "$sum" : 1 }
...       }
...    },
...    { "$sort": { "_id" : 1 } }
... ])

This will produce the following output −

{ "_id" : 21, "Count" : 3 }
{ "_id" : 22, "Count" : 1 }