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- 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. |