$Sum in MongoDB

What is $sum Operator in MongoDB?

The $sum in MongoDB operator is an aggregation operator used to calculate the sum of numerical values within a collection. It is primarily used in the aggregation framework to perform various data analysis and reporting tasks. The $sum in MongoDB takes an expression as its argument, which can be a field name or a valid expression that resolves to a numerical value. It then accumulates the values across all documents in the collection and returns the total sum.

Stages in which $sum Operator is Available

The $sum in MongoDB is typically used as part of the aggregation framework. It can be used within various stages of the aggregation pipeline to perform calculations and generate aggregated results. Here are some common stages where the $sum in MongoDB is commonly used:

• $group stage:
  The $sum in MongoDB is frequently used within the $group stage to calculate the sum of values for a specific field or expression across multiple documents. It allows you to group documents based on certain criteria and calculate the sum within each group.
• $project stage:
  The $sum in MongoDB can be used within the $project stage to create new fields that represent the sum of existing fields or expressions. It allows you to add calculated sum fields to the output documents.
• $addFields stage:
  Similar to the $project stage, the $sum in MongoDB can be used within the $addFields stage to add new fields with calculated sum values to the output documents. This stage is particularly useful when you want to preserve the existing fields and add new fields with the sum.
• $facet stage:
  The $facet stage allows you to perform multiple aggregations within a single query. The $sum in MongoDB can be used within each sub-pipeline of the $facet stage to calculate sums independently for different subsets of data.

It's important to note that the availability of the $sum in MongoDB depends on the stage being used within the aggregation pipeline. While it is commonly used in the stages mentioned above, it may not be applicable or make sense in certain stages like $match or $sort, which focus on filtering and sorting documents rather than aggregating values.

Syntax of $sum in MongoDB

The syntax of the $sum in MongoDB aggregation framework is as follows:

The <expression> represents the field or expression whose values will be summed. It can be a field path or a valid expression that evaluates to a numerical value.

Behavior of $sum in MongoDB

The $sum in MongoDB behaves in the following way when used in the aggregation framework:

1. Summing Numeric Values:
   When the $sum in MongoDB is applied to a field or expression that evaluates to a numerical value, it calculates the sum of those values across all documents in the aggregation pipeline.
2. Ignoring Non-Numeric Values:
   If a field or expression being summed contains non-numeric values (e.g., null, strings, or arrays), MongoDB ignores those values and only considers the valid numeric values for the sum calculation.
3. Handling Missing Fields:
   If a field being summed is missing in some documents, MongoDB treats the missing field as 0 (zero) during the sum calculation. It considers the documents with missing fields as having a value of 0 for summing.
4. Aggregating Across Groups:
   When the $sum in MongoDB is used within the $group stage, it calculates the sum separately for each group based on the grouping criteria specified in the $group stage. This allows you to obtain the sum of specific fields or expressions within different groups.
5. Output:
   The result of the $sum operator is typically included in the output documents as a field specified in the $group, $project, or $addFields stage. The output field will contain the calculated sum value.

It's important to note that the $sum in MongoDB is a cumulative operator and is commonly used in conjunction with other operators and stages of the aggregation framework to perform complex calculations and generate aggregated results. The behavior and outcome of the $sum in MongoDB depend on how it is used within the aggregation pipeline and the data it operates on.

Examples of $sum in MongoDB

Here are some examples of how the $sum in MongoDB can be used in MongoDB's aggregation framework:

Example - 1: Summing a Field:

Consider a collection called orders with documents representing orders and their quantities:

To calculate the total quantity across all orders, you can use the $sum in MongoDB within a $group stage:

The result will be,

Example - 2: Here's an example that demonstrates the behavior of the $sum operator in MongoDB when encountering non-numeric values, null values, and missing fields:

Consider a collection called products with documents representing products and their quantities:

To calculate the sum of the "quantity" field using the $sum operator, you can use the following aggregation query:

The result will be:

Explanation:

The $sum in MongoDB calculates the sum of numerical values. It ignores non-numeric values, treats missing fields as 0, and null values have no impact on the sum. In the example provided, the sum of the "quantity" field is 5, considering valid numeric values while ignoring null values, non-numeric values, and arrays.