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. The following table provides a brief overview of the features of the MongoDB aggregation commands. Chapter 7. Aggregation 474 Accumulators MongoDB Documentation, Release 3.0.4 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. 7.4.2 Aggregation Commands Comparison