Aggregations

/* Match for numbers in zip  */


 db.zips.aggregate([     
	 {$project:      {     first_char: {$substr : ["$city",0,1]},  population: "$pop"    }  },      	
	 {$group:{ _id: "$first_char", summation: {$sum:"$population"}}},
	 {$match: { _id: { $in: [ "0", "1", "2", "3", "4", "5", "6", "7", "8", "9" ] } } } ,
	 {$group:{ _id: null, total: {$sum:"$summation"}}}
 
 
 ])


/* I don't want any quizzes */
db.grades.aggregate (

     [ 
    
      { $unwind:"$scores" }, 
      { $match: {"scores.type": {$ne: "quiz"}}},  
      { $group: 
         {
            _id: 
                { 
                   class: "$class_id"  
               }  , 
            
            AvgScore: 
               { 
               $avg: "$scores.score" 
               } 
          } 
        },
      
      { $sort: { "AvgScore": 1 } }
         
   ],
   { allowDiskUse: true }
   
  )

Memory all alone in the moonlight ...