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MongoDB query to group by _id
To group by _id in MongoDB, use $group. Let us create a collection with documents −
> db.demo529.insertOne({"Score":10});{
"acknowledged" : true,
"insertedId" : ObjectId("5e8b1d5bef4dcbee04fbbbe4")
}
> db.demo529.insertOne({"Score":20});{
"acknowledged" : true,
"insertedId" : ObjectId("5e8b1d5fef4dcbee04fbbbe5")
}
> db.demo529.insertOne({"Score":10});{
"acknowledged" : true,
"insertedId" : ObjectId("5e8b1d61ef4dcbee04fbbbe6")
}
> db.demo529.insertOne({"Score":10});{
"acknowledged" : true,
"insertedId" : ObjectId("5e8b1d62ef4dcbee04fbbbe7")
}Display all documents from a collection with the help of find() method −
> db.demo529.find();
This will produce the following output −
{ "_id" : ObjectId("5e8b1d5bef4dcbee04fbbbe4"), "Score" : 10 }
{ "_id" : ObjectId("5e8b1d5fef4dcbee04fbbbe5"), "Score" : 20 }
{ "_id" : ObjectId("5e8b1d61ef4dcbee04fbbbe6"), "Score" : 10 }
{ "_id" : ObjectId("5e8b1d62ef4dcbee04fbbbe7"), "Score" : 10 }Following is the query to group by _id −
> db.demo529.aggregate( [
... {
... $group: {
... _id: null,
... count: { $sum: 1 }
... }
... }
... ] )This will produce the following output −
{ "_id" : null, "count" : 4 }
Updated on: 13-May-2020
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