Consider that you have a collection called population which has fields state and city.

db.population.aggregate( [{ $group: { _id: { state: " $state " , city: " $city " },pop: { $pop: 1 } } }] )

db.population.aggregate( [{ $group: { _id: { city: " $city " },pop: { $sum: " $pop " } } }] )Multi Document Transaction is not supported by MongoDB

db.population.aggregate( [{ $group: { _id: { state: Estate " , city: " $city " },pop: { $sum: " $pop " } } }] )

db.population.aggregate( [{ $group: { _id: { state: Estate " , city: " $city " },pop: { $sum: 1 > > > ] )