mongodb aggregate example. First query counts documents by country. Second, counts documents by date

gistfile1.txt

db.contests_tbbt2013.aggregate(
    { $group: {_id: "$country", total: {$sum: 1} } }
);

db.contests_tbbt2013.aggregate(    
  {$group:      
   {          
     _id:{ $dayOfYear:'$regdate' },
     sum:{$sum:1},
     date:{$first: '$regdate'}
   }
  },
  { $sort : { date:1 } }
);

db.schedules.aggregate(
{$group:
{
_id:{ $dayOfYear:'$start' },
sum:{$sum:1},
date:{$first: '$start'}
}
},
{$match:{date: {$gte: new Date('jan 17,2014')}}},
{ $sort : { date:1 } },
{$limit: 15}
);