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$sum (Expression)

Definition

$sum

Returns the sum of numeric values in an array.

Note

Other Uses of $sum

This page describes $sum when used as an expression, which returns the sum of numeric values in an array.

You can also use $sum in these other contexts:

$sum (accumulator), which returns the collective sum of numeric values across a group of documents.
$sum (window function), which is used in the $setWindowFields stage to calculate the cumulative sum of values in a window.

Syntax

When used as an expression, $sum has one of two syntaxes:

{ $sum: <expression> }

In the preceding syntax, the operand is a single expression that evaluates to an array. $sum traverses the array and returns the sum of the array's numeric values.

{ $sum: [ <expression1>, <expression2>, ... ] }

In the preceding syntax, each expression is a single numeric value. $sum returns the sum of those values.

Behavior

Result Type

When input types are mixed, $sum promotes the smaller input type to the larger of the two. A type is considered larger when it represents a wider range of values. The order of numeric types from smallest to largest is: integer → long → double → decimal.

The larger of the input types also determines the result type unless the operation overflows and is beyond the range represented by that larger data type. In cases of overflow, $sum promotes the result according to the following order:

If the larger input type is integer, the result type is promoted to long.
If the larger input type is long, the result type is promoted to double.
If the larger type is double or decimal, the overflow result is represented as positive or negative infinity. There is no type promotion of the result.

Non-Numeric or Non-Existent Fields

If used on a field that contains both numeric and non-numeric values, $sum ignores the non-numeric values and returns the sum of the numeric values.

If all operands are non-numeric, $sum returns 0.

Example

The examples on this page use data from the sample_mflix dataset. For details on how to load this dataset into your self-managed MongoDB deployment, see Load the Sample Dataset. If you made any modifications to the sample databases, you may need to drop and recreate the databases to run the examples on this page.

This example uses $sum in the $project stage to calculate the total awards won and nominated for three movies, and store the result in a new field called totalAwards.

db.movies.aggregate( [
   {
      $match: {
         title: {
            $in: [
               "The Godfather",
               "Inception",
               "The Dark Knight"
            ]
         }
      }
   },
   {
      $project: {
         _id: 0,
         title: 1,
         totalAwards: { $sum: [ "$awards.wins", "$awards.nominations" ] }
      }
   },
   {
      $sort: { totalAwards: -1 }
   }
] )

[
  { title: 'Inception', totalAwards: 334 },
  { title: 'The Dark Knight', totalAwards: 250 },
  { title: 'The Godfather', totalAwards: 52 }
]

The preceding pipeline:

Uses $match to filter for three specific movies.
Uses $project with $sum to create the totalAwards field, which holds the sum of awards.wins and awards.nominations for each movie.
Uses $sort to sort results by total awards in descending order.

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