/ /

$maxN (window function)

Definition

$maxN

Returns an array containing the maximum n elements within a window.

Note

Other Uses of $maxN

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

You can also use $maxN in these other contexts:

$maxN (accumulator), which returns the maximum n values within a group.
$maxN (expression), which returns the largest n values from an array.

Syntax

{
   $setWindowFields: {
      ...
      output: {
         <field>: {
            $maxN: {
               input: <expression>,
               n: <expression>
            },
            window: {
               documents: [ <lower boundary>, <upper boundary> ]
            }
         }
      }
   }
}

Options

Field	Type	Description
`input`	Expression	The expression evaluated over each document in the window. `$maxN` returns the maximum `n` values.
`n`	Expression	The number of elements that `$maxN` returns from the window. Must resolve to a positive integer. If the window contains fewer than `n` elements, `$maxN` returns all elements in the window.

Behavior

Null and Missing Values

$maxN filters out null and missing values.

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 $maxN in $setWindowFields to return the three highest-rated movies for each year. The window spans the entire partition, so every document in the same year receives the same topThreeRatedInYear value.

db.movies.aggregate( [
   {
      $match: {
         year: { $in: [ 1999, 2001 ] },
         "imdb.rating": { $gte: 8.5 }
      }
   },
   {
      $setWindowFields: {
         partitionBy: "$year",
         sortBy: { "imdb.rating": -1 },
         output: {
            topThreeRatedInYear: {
               $maxN: {
                  input: [ "$imdb.rating", "$title" ],
                  n: 3
               },
               window: {
                  documents: [ "unbounded", "unbounded" ]
               }
            }
         }
      }
   },
   {
      $project: {
         _id: 0,
         title: 1,
         year: 1,
         "imdb.rating": 1,
         topThreeRatedInYear: 1
      }
   },
   {
      $sort: { year: 1, "imdb.rating": -1 }
   }
] )

[
  {
    imdb: { rating: 8.9 },
    year: 1999,
    title: 'Fight Club',
    topThreeRatedInYear: [
      [ 8.9, 'Fight Club' ],
      [ 8.7, 'The Matrix' ],
      [ 8.7, 'Eddie Izzard: Dress to Kill' ]
    ]
  },
  {
    imdb: { rating: 8.7 },
    year: 1999,
    title: 'The Matrix',
    topThreeRatedInYear: [
      [ 8.9, 'Fight Club' ],
      [ 8.7, 'The Matrix' ],
      [ 8.7, 'Eddie Izzard: Dress to Kill' ]
    ]
  },
  {
    title: 'Eddie Izzard: Dress to Kill',
    year: 1999,
    imdb: { rating: 8.7 },
    topThreeRatedInYear: [
      [ 8.9, 'Fight Club' ],
      [ 8.7, 'The Matrix' ],
      [ 8.7, 'Eddie Izzard: Dress to Kill' ]
    ]
  },
  {
    imdb: { rating: 8.5 },
    year: 1999,
    title: 'The Green Mile',
    topThreeRatedInYear: [
      [ 8.9, 'Fight Club' ],
      [ 8.7, 'The Matrix' ],
      [ 8.7, 'Eddie Izzard: Dress to Kill' ]
    ]
  },
  {
    title: 'Band of Brothers',
    year: 2001,
    imdb: { rating: 9.6 },
    topThreeRatedInYear: [
      [ 9.6, 'Band of Brothers' ],
      [ 9.2, 'The Blue Planet' ],
      [
        8.8,
        'The Lord of the Rings: The Fellowship of the Ring'
      ]
    ]
  },
  {
    title: 'The Blue Planet',
    year: 2001,
    imdb: { rating: 9.2 },
    topThreeRatedInYear: [
      [ 9.6, 'Band of Brothers' ],
      [ 9.2, 'The Blue Planet' ],
      [
        8.8,
        'The Lord of the Rings: The Fellowship of the Ring'
      ]
    ]
  },
  {
    imdb: { rating: 8.8 },
    year: 2001,
    title: 'The Lord of the Rings: The Fellowship of the Ring',
    topThreeRatedInYear: [
      [ 9.6, 'Band of Brothers' ],
      [ 9.2, 'The Blue Planet' ],
      [
        8.8,
        'The Lord of the Rings: The Fellowship of the Ring'
      ]
    ]
  },
  {
    imdb: { rating: 8.6 },
    year: 2001,
    title: 'Spirited Away',
    topThreeRatedInYear: [
      [ 9.6, 'Band of Brothers' ],
      [ 9.2, 'The Blue Planet' ],
      [
        8.8,
        'The Lord of the Rings: The Fellowship of the Ring'
      ]
    ]
  },
  {
    title: 'Promises',
    year: 2001,
    imdb: { rating: 8.5 },
    topThreeRatedInYear: [
      [ 9.6, 'Band of Brothers' ],
      [ 9.2, 'The Blue Planet' ],
      [
        8.8,
        'The Lord of the Rings: The Fellowship of the Ring'
      ]
    ]
  }
]

The preceding pipeline:

Uses $match to filter for movies released in 1999 or 2001 with an IMDb rating of 8.5 or higher.
Uses $setWindowFields to partition movies by year and sort them by imdb.rating in descending order. The stage uses $maxN with n: 3 to create topThreeRatedInYear from a window spanning the entire partition (documents: ["unbounded", "unbounded"]).
Uses $project to include only the title, year, imdb.rating, and topThreeRatedInYear fields in the output.
Uses $sort to sort the results by year ascending and rating descending.

Learn More

Aggregation Pipeline Limits

Back

$max

$median