db.zips.aggregate( { $group:
                         { _id: { state: "$state", city: "$city" },
                           zipcount : {$sum : 1},
                           pop: { $sum: "$pop" } } },
                       { $sort: { zipcount: 1 } },
                       { $group:
                         { _id : "$_id.state",
                           biggestCity:  { $last: "$_id.city" },
                           population:   { $last: "$pop" },
                        zipcount : { $last : "$zipcount"}
                            } } ,{ $sort : {population : -1}})