MongoDB query to group by _id

To group by _id in MongoDB, use the $group stage in an aggregation pipeline. This allows you to group documents by the _id field or set _id: null to group all documents together.

Syntax

db.collection.aggregate([
    {
        $group: {
            _id: "$fieldName",
            aggregatedField: { $operator: "expression" }
        }
    }
]);

Sample Data

db.demo529.insertMany([
    { "Score": 10 },
    { "Score": 20 },
    { "Score": 10 },
    { "Score": 10 }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e8b1d5bef4dcbee04fbbbe4"),
        ObjectId("5e8b1d5fef4dcbee04fbbbe5"),
        ObjectId("5e8b1d61ef4dcbee04fbbbe6"),
        ObjectId("5e8b1d62ef4dcbee04fbbbe7")
    ]
}

Display Sample Data

db.demo529.find();

{ "_id": ObjectId("5e8b1d5bef4dcbee04fbbbe4"), "Score": 10 }
{ "_id": ObjectId("5e8b1d5fef4dcbee04fbbbe5"), "Score": 20 }
{ "_id": ObjectId("5e8b1d61ef4dcbee04fbbbe6"), "Score": 10 }
{ "_id": ObjectId("5e8b1d62ef4dcbee04fbbbe7"), "Score": 10 }

Example: Group All Documents

To count all documents by grouping them together, set _id: null ?

db.demo529.aggregate([
    {
        $group: {
            _id: null,
            count: { $sum: 1 }
        }
    }
]);

{ "_id": null, "count": 4 }

Example: Group by Score Field

To group documents by the Score field and count occurrences ?

db.demo529.aggregate([
    {
        $group: {
            _id: "$Score",
            count: { $sum: 1 }
        }
    }
]);

{ "_id": 10, "count": 3 }
{ "_id": 20, "count": 1 }

Conclusion

Use $group in MongoDB aggregation to group documents by any field. Set _id: null to group all documents together, or use _id: "$fieldName" to group by specific field values.