Accumulator Operators

Operator	Description	Example
$count	Performs a count of a grouped documents or, if the property is an array, a count of the number of elements.	{“numInGroup”: {“$count”: “_id”}}
$sum	Calculates the sum of a number property or expression in the group.	{“sumOfDays”: {“$sum”: {“$dayOfMonth”: “created”}}}
$avg	Calculates the average of a number property or expression in the group.	{“averageHeartRate”: {“$avg”: “c_bpm”}}
$first	Returns the first value from the group. The result depends on the sort field(s).	{“oldest”: {“$first”: “created”}}
$last	Returns the last value from the group. The result depends on the sort field(s).	{“latest”: {“$last”: “created”}}
$min	Returns the smallest value from the group.	{“smallest”: {“$min”: “c_size”}}
$max	Returns the largest value from the group.	{“largestArray”: {“$max: {”$size": “c_arr”}}}
$pushAll	Returns the array of pushed values, with NULL included in the array where there are missing values	{"combinedArray": {"$pushAll": "c_value"}}
$push	Returns the array of pushed values	{"combinedArray": {"$push": "c_value"}}
$addToSet	Returns array of unique pushed values	{"uniqueArray": {"$addToSet": "c_value"}}
$stdDevPop	Returns the population standard deviation of input values	{"stdDeviation": { "$stdDevPop": "c_heartrate"}}
$stdDevSamp	Returns the sample standard deviation of input values	{"stdDeviation": { "$stdDevSamp": "c_heartrate"}}