Since MongoDB 3.4
The fields to include. The resulting objects will also contain these fields.
Represents one of the group/accumulator operators,
The $stdDevPop group accumulator (since MongoDB 3.2)
The $stdDevPop for a single field (since MongoDB 3.2)
The $stdDevSamp group accumulator (since MongoDB 3.2)
The $stdDevSamp for a single field (since MongoDB 3.2)
(Since version 0.12.0) Use SumAll
Factory to declare custom call to a group function.
Sum operation of the form