Window Functions

Window functions are used in $setWindowFields to return an aggregated value based on documents in a particular window.

Name	Description
`$addToSet`	Returns an array of all unique values from a particular window. The order of the elements in the returned array is unspecified.
`$avg`	Returns the average of values from a particular window.
`$bottom`	Returns the bottom element within a window according to the specified sort order.
`$bottomN`	Returns the bottom `n` elements within a particular window, according to the specified sort order.
`$concatArrays`	Returns an array of the concatenated array values from documents in a window.
`$count`	Returns the number of documents in a particular window.
`$first`	Returns the value of an expression for the first document in a window.
`$firstN`	Returns an array containing the first `n` elements within a window.
`$last`	Returns the value of an expression for the last document in a window.
`$lastN`	Returns an array containing the last `n` elements within a window.
`$max`	Returns the maximum value from documents in a particular window.
`$maxN`	Returns an array containing the maximum `n` elements within a window.
`$median`	Returns the median (50th percentile) value from documents in a specified window. You can use `$median` in both document-based and range-based windows. New in version 7.0.
`$mergeObjects`	Returns a document created by merging the input documents from a window.
`$min`	Returns the minimum value from documents in a particular window.
`$minN`	Returns an array containing the `n` minimum values within a window.
`$percentile`	Returns an array of percentile values for the specified percentiles within a window.
`$push`	Returns an array of all values, including duplicates, that result from applying an expression to each document in a window.
`$setUnion`	Returns an array of the unique elements in the array values in a window. New in version 8.1.
`$stdDevPop`	Returns the population standard deviation of values from documents in a particular window.
`$stdDevSamp`	Returns the sample standard deviation of values in a window.
`$sum`	Returns the sum of numeric values from documents in a particular window.
`$top`	Returns the top element within a window according to the specified sort order.
`$topN`	Returns the top `n` elements within a particular window, according to the specified sort order.

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$unset

$addToSet