Group Accumulator Operators¶

Changed in version 3.2: Some accumulators are now available in the $project stage. In previous versions of MongoDB , accumulators are available only for the $group stage.

Accumulators, when used in the $group stage, maintain their state (e.g. totals, maximums, minimums, and related data) as documents progress through the pipeline.

When used in the $group stage, accumulators take as input a single expression, evaluating the expression once for each input document, and maintain their stage for the group of documents that share the same group key.

When used in the $project stage, the accumulators do not maintain their state. When used in the $project stage, accumulators take as input either a single argument or multiple arguments.

Name	Description
`$sum`	Returns a sum of numerical values. Ignores non-numeric values. Changed in version 3.2: Available in both `$group` and `$project` stages.
`$avg`	Returns an average of numerical values. Ignores non-numeric values. Changed in version 3.2: Available in both `$group` and `$project` stages.
`$first`	Returns a value from the first document for each group. Order is only defined if the documents are in a defined order. Available in `$group` stage only.
`$last`	Returns a value from the last document for each group. Order is only defined if the documents are in a defined order. Available in `$group` stage only.
`$max`	Returns the highest expression value for each group. Changed in version 3.2: Available in both `$group` and `$project` stages.
`$min`	Returns the lowest expression value for each group. Changed in version 3.2: Available in both `$group` and `$project` stages.
`$push`	Returns an array of expression values for each group. Available in `$group` stage only.
`$addToSet`	Returns an array of unique expression values for each group. Order of the array elements is undefined. Available in `$group` stage only.
`$stdDevPop`	Returns the population standard deviation of the input values. Changed in version 3.2: Available in both `$group` and `$project` stages.
`$stdDevSamp`	Returns the sample standard deviation of the input values. Changed in version 3.2: Available in both `$group` and `$project` stages.

← $ifNull (aggregation) $sum (aggregation) →

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