Group Sum Document For Free

N = count( conn , collection ) returns the total number of documents in a collection by using the MongoDB® C++ interface connection. n = count( conn , collection ,Query= mongoquery ) returns the total number of documents in an executed MongoDB query on a collection.

The $sum in MongoDB is used to calculate the sum of numerical values within collections. The $sum in MongoDB can be applied in stages like $group, $project, and $addFields to perform aggregations. The $sum in MongoDB handles missing fields by treating them as 0 and ignoring non-numeric values.

An aggregate is a collection of people who happen to be at the same place at the same time but who have no other connection to one another. Example: The people gathered in a restaurant on a particular evening are an example of an aggregate, not a group.

Examples. For this example, consider a container with multiple items that may contain a quantity field. The SUM function is used to sum the values of the quantity field, when it exists, into a single aggregated value.

The $sum in MongoDB is used to calculate the sum of numerical values within collections. The $sum in MongoDB can be applied in stages like $group, $project, and $addFields to perform aggregations. The $sum in MongoDB handles missing fields by treating them as 0 and ignoring non-numeric values.

1 Answer. In MongoDB, the $sum will add up the value of the expression for every row and in your case, you are having 3 rows so it will be 1+1+1 =3.

Use the _id field in the $group pipeline stage to set the group key. See below for usage examples. In the $group stage output, the _id field is set to the group key for that document. The output documents can also contain additional fields that are set using accumulator expressions.