How to select documents with values above the average in MongoDB?

Use aggregate() in MongoDB to select documents with values above the average. To find the average, use $avg in MongoDB.

Let us create a collection with documents −

> db.demo552.insertOne({values:10});{
   "acknowledged" : true, "insertedId" : ObjectId("5e8e3b1c9e5f92834d7f05ea")
}
> db.demo552.insertOne({values:50});{
   "acknowledged" : true, "insertedId" : ObjectId("5e8e3b1f9e5f92834d7f05eb")
}
> db.demo552.insertOne({values:40});{
   "acknowledged" : true, "insertedId" : ObjectId("5e8e3b289e5f92834d7f05ec")
}

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

> db.demo552.find();

This will produce the following output −

{ "_id" : ObjectId("5e8e3b1c9e5f92834d7f05ea"), "values" : 10 }
{ "_id" : ObjectId("5e8e3b1f9e5f92834d7f05eb"), "values" : 50 }
{ "_id" : ObjectId("5e8e3b289e5f92834d7f05ec"), "values" : 40 }

Following is the query to select documents with values above the average −

> var findAvg = db.demo552.aggregate([
...    { "$group": { "_id": "null", Average: { "$avg": "$values"} }}
... ]).toArray()[0]["Average"];
> db.demo552.find({ "values": { "$gt": findAvg } })

This will produce the following output −

{ "_id" : ObjectId("5e8e3b1f9e5f92834d7f05eb"), "values" : 50 }
{ "_id" : ObjectId("5e8e3b289e5f92834d7f05ec"), "values" : 40 }