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$sum (Accumulator Operator)

Definition

$sum

Returns the collective sum of numeric values across a group of documents.

The $sum accumulator is available in these stages:

Note

Other Uses of $sum

This page describes $sum when used as an accumulator. Accumulators return an aggregated value like sum, maximum, or minimum across a group of input documents.

You can also use $sum in these other contexts:

$sum (expression), which returns the sum of numeric values in an array.
$sum (window function), which is used in the $setWindowFields stage to calculate the cumulative sum of values in a window.

Syntax

{ $sum: <expression> }

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

When you use $sum 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.

When you use $sum on a field that does not exist in any document in the collection, $sum returns 0 for that field.

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.

The following example groups movies by content rating and returns the total number of IMDb votes for each group.

db.movies.aggregate( [
   {
      $match: {
         rated: { $in: [ "G", "PG", "PG-13", "R" ] },
         "imdb.votes": { $type: "int" }
      }
   },
   {
      $group: {
         _id: "$rated",
         totalVotes: { $sum: "$imdb.votes" }
      }
   },
   {
      $sort: { _id: 1 }
   }
] )

[
  { _id: 'G', totalVotes: 12773015 },
  { _id: 'PG', totalVotes: 57205811 },
  { _id: 'PG-13', totalVotes: 152323029 },
  { _id: 'R', totalVotes: 209762435 }
]

The preceding pipeline:

Uses $match to filter for movies with a rated value of G, PG, PG-13, or R and an integer IMDb vote count.
Uses $group to group movies by the rated field and computes totalVotes using $sum on imdb.votes.
Uses $sort to return results in alphabetical order by content rating.

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