mongodb聚合统计

> db.salary.find()
{ "_id" : ObjectId("532be543edb9e06960e550b3"), "name" : "tom", "salary" : 98 }
{ "_id" : ObjectId("532be54eedb9e06960e550b4"), "name" : "tom", "salary" : 96 }
{ "_id" : ObjectId("532be559edb9e06960e550b5"), "name" : "jerry", "salary" : 86 }
{ "_id" : ObjectId("532be55eedb9e06960e550b6"), "name" : "jerry", "salary" : 88 }

db.salary.aggregate(
{$match:{name:"tom"}},{$group:{_id:"$name",total:{$sum:"$salary"}}}
);