Probeer te groeperen op null.
db.murders.aggregate([{$group: {_id:null, pop: {$avg:"$murders"} } }])
Meer informatie:mongo-average-aggregation-query-with-no-group
Probeer te groeperen op null.
db.murders.aggregate([{$group: {_id:null, pop: {$avg:"$murders"} } }])
Meer informatie:mongo-average-aggregation-query-with-no-group