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Calculate the average value in a MongoDB document grouping by null?
You can use $group operator with _id: null. Following is the syntax −
db.yourCollectionName.aggregate([{$group: {_id:null, "anyFieldName": {$avg:"$yourFieldName"} } }]);Let us first create a collection with documents −
> db.caculateTheAverageValueDemo.insertOne({"Population":100});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd68a197924bb85b3f4895f")
}
> db.caculateTheAverageValueDemo.insertOne({"Population":500});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd68a1c7924bb85b3f48960")
}
> db.caculateTheAverageValueDemo.insertOne({"Population":200});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd68a237924bb85b3f48961")
}
> db.caculateTheAverageValueDemo.insertOne({"Population":100});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd68a297924bb85b3f48962")
}
> db.caculateTheAverageValueDemo.insertOne({"Population":100});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd68a2e7924bb85b3f48963")
}Following is the query to display all documents from a collection with the help of find() method −
> db.caculateTheAverageValueDemo.find().pretty();
This will produce the following output −
{ "_id" : ObjectId("5cd68a197924bb85b3f4895f"), "Population" : 100 }
{ "_id" : ObjectId("5cd68a1c7924bb85b3f48960"), "Population" : 500 }
{ "_id" : ObjectId("5cd68a237924bb85b3f48961"), "Population" : 200 }
{ "_id" : ObjectId("5cd68a297924bb85b3f48962"), "Population" : 100 }
{ "_id" : ObjectId("5cd68a2e7924bb85b3f48963"), "Population" : 100 }Following is the query to average the values of a MongoDB document −
> db.caculateTheAverageValueDemo.aggregate([{$group: {_id:null, "AveragePopulation": {$avg:"$Population"} } }]);This will produce the following output −
{ "_id" : null, "AveragePopulation" : 200 }
Updated on: 30-Jul-2019
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