$sum:

 In MongoDB, the $sum operator is used in the MongoDB Shell (mongoshell) as part of the Aggregation Framework to calculate the sum of numeric values within a group during the aggregation process.


The $sum operator is typically used within a $group stage to perform summation calculations on a specific field or expression. It can be used to calculate the sum of values for each group or the total sum across all documents in the collection.


The basic syntax of the $sum operator in the MongoDB Shell is as follows:

{
  $group: {
    _id: <grouping expression>,
    sumField: { $sum: <expression> }
  }
}

Let's break down the components of the $sum operator:


$group: This is the stage of the aggregation pipeline where you define the grouping operation.


_id: This specifies the grouping expression to determine how documents are grouped. The $sum operator performs the summation calculation within each group.


sumField: This is the field name or expression where you want to calculate the sum. It can refer to an existing field in the documents or a computed expression.


<expression>: This represents the field or expression from which you want to calculate the sum. It can be a field name, a constant value, or an arithmetic expression.

Here's an example to illustrate the usage of the $sum operator in the MongoDB Shell:

db.collection.aggregate([
  {
    $group: {
      _id: "$category",
      totalSales: { $sum: "$sales" }
    }
  }
])

In this example,

 the documents in the collection are grouped by the "category" field, and the $sum operator is used to calculate the total sum of the "sales" field within each group. 

The result will include the _id field, which represents the grouped category, and the totalSales field, which contains the sum of sales for each category.


The $sum operator can also be used without the $group stage to calculate the total sum across all documents in the collection.

 In such cases, you can omit the _id field and use the $sum operator directly on the desired field or expression.


javascript
db.collection.aggregate([ 
 {
    $group:
 {
      _id: null,
totalSales: {
$sum: "$sales" }
    }
  }
])

In this example, all documents in the collection are grouped into a single group (using _id: null), and the $sum operator calculates the total sum of the "sales" field across all documents.


The $sum operator is a useful tool in the MongoDB Shell for performing summation calculations during the aggregation process. 

It allows you to aggregate and analyze numerical data within groups or across the entire collection.