compagnie per city

db.parsed_data.aggregate(
     {
	$match : {"Origin.City" : "Alghero"},
    },
    {$unwind: "$Segments"},
    {
		$group : {_id : "$Segments.Carrier" }//_id : "$Origin.Code"
    },
    {
		$group : {_id : "-", "count": {$sum: 1} }//_id : "$Origin.Code"
    }
      );