MongoDb sum query

As llovet suggested, the aggregation framework is the way to go. Here's what your query would look like:

db.CollectionNameGoesHere.aggregate({ $match: {
    $and: [
        { hour: { $gte: 11 } },
        { hour: { $lte: 12 } }
    ]
} },
{ $group: { _id : null, sum : { $sum: "$incoming" } } });

You can also shape the resulting document to only contain the sum by adding a $project operator at the end of the pipeline, like so:

{ $project: { _id: 0, sum: 1 } }

Some examples if you use mongoose:

1. Calculate total sum of product prices :

Products.aggregate([ { $match: {} }, { $group:
  { _id : null, sum : { $sum: "$Price" } }
}])
.then(res => console.log(res[0].sum));

( { $match: {} }, can be removed. )

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2. Sum of Product's prices in each category:

Products.aggregate([{ $group:
  { _id : '$Category', sum : { $sum: "$Price" } }
}])
.then(res => console.log(res));