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$max (window function)

Definition

$max

Returns the maximum value from documents in a particular window.

Note

Disambiguation

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

You can also use $max in these other contexts:

$max (accumulator), which returns the maximum value across a group of input documents.
$max (expression), which returns the maximum value from an array or a list of values.

Syntax

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

Behavior

Result Type

$max compares input data following the BSON Comparison order to determine the appropriate output type. When the input data contains multiple data types, the $max output type is the highest in the comparison order.

Null and Missing Values

If some, but not all, documents in the window have a null value for the field or are missing the field, $min ignores them. It considers only the non-null and non-missing values.

If all documents in the window have a null value for the field or are missing the field, $min returns null.

Array Operand

In the $setWindowFields stage, if the expression resolves to an array, $max does not traverse the array and compares the array as a whole.

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 $max in the $setWindowFields stage to output the highest IMDb rating for highly-rated movies in 1999 and 2000. For each movie, the pipeline adds a field containing the highest IMDb rating across all movies in that year:

db.movies.aggregate( [
   {
      $match: {
         year: { $in: [ 1999, 2000 ] },
         "imdb.rating": { $gte: 8.5 }
      }
   },
   {
      $setWindowFields: {
         partitionBy: "$year",
         sortBy: { "imdb.rating": 1 },
         output: {
            highestRatingInYear: {
               $max: "$imdb.rating",
               window: {
                  documents: [ "unbounded", "unbounded" ]
               }
            }
         }
      }
   },
   {
      $project: {
         _id: 0,
         title: 1,
         year: 1,
         "imdb.rating": 1,
         highestRatingInYear: 1
      }
   },
   { $sort: { year: 1, "imdb.rating": 1 } }
] )

[
  {
    imdb: { rating: 8.5 },
    year: 1999,
    title: 'The Green Mile',
    highestRatingInYear: 8.9
  },
  {
    imdb: { rating: 8.7 },
    year: 1999,
    title: 'The Matrix',
    highestRatingInYear: 8.9
  },
  {
    title: 'Eddie Izzard: Dress to Kill',
    year: 1999,
    imdb: { rating: 8.7 },
    highestRatingInYear: 8.9
  },
  {
    imdb: { rating: 8.9 },
    year: 1999,
    title: 'Fight Club',
    highestRatingInYear: 8.9
  },
  {
    imdb: { rating: 8.5 },
    year: 2000,
    title: 'Gladiator',
    highestRatingInYear: 8.5
  },
  {
    imdb: { rating: 8.5 },
    year: 2000,
    title: 'Memento',
    highestRatingInYear: 8.5
  },
  {
    title: 'Hera Pheri',
    year: 2000,
    imdb: { rating: 8.5 },
    highestRatingInYear: 8.5
  },
  {
    title: "A Dog's Will",
    year: 2000,
    imdb: { rating: 8.5 },
    highestRatingInYear: 8.5
  }
]

In the preceding pipeline:

$match filters for movies from 1999 and 2000 with an IMDb rating of 8.5 or higher.
$setWindowFields partitions movies by year and sorts them by imdb.rating in ascending order within each partition. The $max window function uses a documents window with unbounded lower and upper limits, spanning the entire partition. $max returns the highest imdb.rating across all documents in each year's partition.
$project limits the output to title, year, imdb.rating, and the computed highestRatingInYear field.
$sort orders results by year and then by IMDb rating in ascending order.

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