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- Aggregation Pipeline Operators >
- Group Accumulator Operators
Group Accumulator Operators¶
Accumulators, available only for the $group stage, compute values by combining documents that share the same group key. Accumulators take as input a single expression, evaluating the expression once for each input document, and maintain their state for the group of documents.
Name | Description |
---|---|
$sum | Returns a sum for each group. Ignores non-numeric values. |
$avg | Returns an average for each group. Ignores non-numeric values. |
$first | Returns a value from the first document for each group. Order is only defined if the documents are in a defined order. |
$last | Returns a value from the last document for each group. Order is only defined if the documents are in a defined order. |
$max | Returns the highest expression value for each group. |
$min | Returns the lowest expression value for each group. |
$push | Returns an array of expression values for each group. |
$addToSet | Returns an array of unique expression values for each group. Order of the array elements is undefined. |