MongoDB之聚合管道下

####$group#### 先分组，再合并

1.例子

{ "_id" : { "month" : 3, "day" : 15, "year" : 2014 }, 
      "totalPrice" : 50, "averageQuantity" : 10, "count" : 1 }

{ "_id" : { "month" : 4, "day" : 4, "year" : 2014 }, 
      "totalPrice" : 200, "averageQuantity" : 15, "count" : 2 }

{ "_id" : { "month" : 3, "day" : 1, "year" : 2014 }, 
      "totalPrice" : 40, "averageQuantity" : 1.5, "count" : 2 }

_id 为分组依据，_id 为null，及不分组，直接合并。

合并依据：

• 键 totalPrice 保存 键 price 和 键 quantity 值 的乘积 的和

• 键averageQuantity 保存 键 quantity 的值的平均值

• 键 count 作统计

  db.sales.aggregate( [ { $group : { _id : null, totalPrice: { $sum: { $multiply: [ "$price", "$quantity" ] } }, averageQuantity: { $avg: "$quantity" }, count: { $sum: 1 } } } ] )

结果

 { "_id" : null, "totalPrice" : 290, "averageQuantity" : 8.6, "count" : 5 }

2.再看一例

 { "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, 
            "date" : ISODate("2014-03-01T08:00:00Z") }

 { "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, 
            "date" : ISODate("2014-03-01T09:00:00Z") }

 { "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 10, 
            "date" : ISODate("2014-03-15T09:00:00Z") }

 { "_id" : 4, "item" : "xyz", "price" : 5, "quantity" : 20, 
            "date" : ISODate("2014-04-04T11:21:39.736Z") }

 { "_id" : 5, "item" : "abc", "price" : 10, "quantity" : 10, 
            "date" : ISODate("2014-04-04T21:23:13.331Z") }

咱们依据 键 item 分组

  db.sales.aggregate( [ { $group : { _id : "$item" } } ] )

结果

 { "_id" : "xyz" }
 { "_id" : "jkl" }
 { "_id" : "abc" }

####$sort#### 排序

1.例子

db.users.aggregate(
  [
     { $sort : { age : -1, posts: 1 } }
  ]
)

对键age 顺序排序，对键 posts 逆序排序

####$out#### 创建指定副本集合

1.例子

   { "_id" : 8751, "title" : "The Banquet", "author" : "Dante", "copies" : 2 }

   { "_id" : 8752, "title" : "Divine Comedy", "author" : "Dante", "copies" : 1 }

   { "_id" : 8645, "title" : "Eclogues", "author" : "Dante", "copies" : 2 }

   { "_id" : 7000, "title" : "The Odyssey", "author" : "Homer", "copies" : 10 }

   { "_id" : 7020, "title" : "Iliad", "author" : "Homer", "copies" : 10 }

对其按author分组，然后out一个新集合authors

  db.books.aggregate( [
                  { $group : { _id : "$author", books: { $push: "$title" } } },
                  { $out : "authors" }
  ] )

结果

 { "_id" : "Homer", "books" : [ "The Odyssey", "Iliad" ] }
 { "_id" : "Dante", "books" : [ "The Banquet", "Divine Comedy", "Eclogues" ] }

我们现在看到的只是数据映射，不是实体文档，

但是在authors里，有映射副本存成的文档。

也就是说 $out 可以创建新的集合，存储聚合后的文档映射。