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$stdDevPop (window function)

Definition

$stdDevPop

Returns the population standard deviation of values from documents in a particular window.

Note

Disambiguation

This page describes $stdDevPop when used as a window function.

You can also use $stdDevPop in these other contexts:

$stdDevPop (accumulator), which returns the population standard deviation of values across a group of documents.
$stdDevPop (expression), which returns the population standard deviation of numeric values in an array.

Syntax

{
   $setWindowFields: {
      ...
      output: {
         <field>: {
            $stdDevPop: <expression>
         }
      }
   }
}

Behavior

Result Type

$stdDevPop returns the population standard deviation of the input values as a double.

Non-Numeric Values

The $stdDevPop window function ignores non-numeric values. If the window contains no numeric values, $stdDevPop returns null.

Single Value

If the input consists of a single numeric value, $stdDevPop 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 $setWindowFields to add to each document a field that contains the running population standard deviation of IMDb ratings for Musicals from 2009 and 2010.

db.movies.aggregate( [
   {
      $match: {
         genres: "Musical",
         year: { $in: [ 2009, 2010 ] },
         "imdb.rating": { $type: "double" }
      }
   },
   {
      $setWindowFields: {
         partitionBy: "$year",
         sortBy: { title: 1 },
         output: {
            stdDevPopRatingInYear: {
               $stdDevPop: "$imdb.rating",
               window: {
                  documents: [ "unbounded", "current" ]
               }
            },
            rankInYear: {
               $documentNumber: {}
            }
         }
      }
   },
   {
      $project: {
         _id: 0,
         title: 1,
         year: 1,
         "imdb.rating": 1,
         stdDevPopRatingInYear: 1,
         rankInYear: 1
      }
   },
   { $sort: { year: 1, title: 1 } }
] )

[
  {
    imdb: {
      rating: 6.3
    },
    year: 2009,
    title: 'Bran Nue Dae',
    stdDevPopRatingInYear: 0,
    rankInYear: 1
  },
  {
    title: 'Face',
    year: 2009,
    imdb: {
      rating: 5.9
    },
    stdDevPopRatingInYear: 0.1999999999999995,
    rankInYear: 2
  },
  {
    imdb: {
      rating: 5
    },
    year: 2009,
    title: 'Fame',
    stdDevPopRatingInYear: 0.5436502143433362,
    rankInYear: 3
  },
  {
    title: 'Guy and Madeline on a Park Bench',
    year: 2009,
    imdb: {
      rating: 6.1
    },
    stdDevPopRatingInYear: 0.4968651728587945,
    rankInYear: 4
  },
  {
    title: 'Lo',
    year: 2009,
    imdb: {
      rating: 6.6
    },
    stdDevPopRatingInYear: 0.5418486873657623,
    rankInYear: 5
  },
  {
    imdb: {
      rating: 5.9
    },
    year: 2009,
    title: 'Nine',
    stdDevPopRatingInYear: 0.4955356249106166,
    rankInYear: 6
  },
  {
    title: 'Oxygen',
    year: 2009,
    imdb: {
      rating: 6.8
    },
    stdDevPopRatingInYear: 0.5436085025840125,
    rankInYear: 7
  },
  {
    title: 'Ricky Rapper and the Bicycle Thief',
    year: 2009,
    imdb: {
      rating: 5.3
    },
    stdDevPopRatingInYear: 0.5710461890250208,
    rankInYear: 8
  },
  {
    title: 'Stingray Sam',
    year: 2009,
    imdb: {
      rating: 7.2
    },
    stdDevPopRatingInYear: 0.659592092429148,
    rankInYear: 9
  },
  {
    title: 'Anjaana Anjaani',
    year: 2010,
    imdb: {
      rating: 5.8
    },
    stdDevPopRatingInYear: 0,
    rankInYear: 1
  },
  {
    title: 'Basilicata Coast to Coast',
    year: 2010,
    imdb: {
      rating: 6.7
    },
    stdDevPopRatingInYear: 0.4500000000000002,
    rankInYear: 2
  },
  {
    title: 'Camp Rock 2: The Final Jam',
    year: 2010,
    imdb: {
      rating: 5.1
    },
    stdDevPopRatingInYear: 0.6548960901462835,
    rankInYear: 3
  },
  {
    title: 'Pure Country 2: The Gift',
    year: 2010,
    imdb: {
      rating: 4.9
    },
    stdDevPopRatingInYear: 0.7048936089935841,
    rankInYear: 4
  },
  {
    title: 'Score: A Hockey Musical',
    year: 2010,
    imdb: {
      rating: 4.8
    },
    stdDevPopRatingInYear: 0.7116178749862878,
    rankInYear: 5
  },
  {
    title: 'StarStruck',
    year: 2010,
    imdb: {
      rating: 6.2
    },
    stdDevPopRatingInYear: 0.7057305119913376,
    rankInYear: 6
  },
  {
    title: 'The Ugly Duckling',
    year: 2010,
    imdb: {
      rating: 6.7
    },
    stdDevPopRatingInYear: 0.7613092911128131,
    rankInYear: 7
  }
]

The preceding pipeline:

Uses $match to filter for Musicals from 2009 and 2010 with a numeric IMDb rating.
Uses $setWindowFields to partition movies by year and sort them by title. The stage uses $stdDevPop with a documents window of [ "unbounded", "current" ] to create stdDevPopRatingInYear. The stdDevPopRatingInYear field holds the running population standard deviation of imdb.rating from the start of the partition to the current document. $documentNumber assigns a per-partition row number (rankInYear) to each document.
Uses $project to include title, year, imdb, stdDevPopRatingInYear, and rankInYear in the output.
Uses $sort to sort the results by year and title.

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$setUnion

$stdDevSamp