MongoDB aggregation

MongoDB aggregation examples: db.posts.aggregate({$unwind:"$comments"},{$group:{_id:"$comments.author",count:{$sum:1}}},{$sort:{count:1}}) db.zip.aggregate([{$match:{$or:[{state:"CA"},{state:"NY"}]}}, {$group:{_id:{city:"$city",state:"$state"}, pop:{$sum:"$pop"}}}, {$match:{pop:{$gt:25000}}}, {$group:{_id:"$state", average:{$avg:"$pop"}}}]) db.grades.aggregate({$unwind:"$scores"} , {$match:{"scores.type":{$nin:["quiz"]}}},{$group:{_id:{student_id:"$student_id",class_id:"$class_id"},avg:{$avg:"$scores.score"}}},{$group:{_id:"$_id.class_id",average:{$avg:"$avg"}}}) db.zip.aggregate( [ {$project : {city:{$substr:["$city",0,1]},pop:1}}, {$sort : {city : 1}}, {$match: {city: /^\d.*$/}} , {$group:{_id:1,pop:{$sum:"$pop"}}}])