一，問題描述

【使用 unwind unpack Document 裏面的Array中的每個元素，然後使用 group 分組統計，最後使用 sort 對分組結果排序】

從 images.json 文件中導入數據到MongoDB服務器

mongoimport --drop -d test -c images images.json

其中Document的示例如下：

> db.images.find()
{ "_id" : 3, "height" : 480, "width" : 640, "tags" : [ "kittens", "travel" ] }
{ "_id" : 1, "height" : 480, "width" : 640, "tags" : [ "cats", "sunrises", "kittens", "travel", "vacation", "work" ] }
{ "_id" : 0, "height" : 480, "width" : 640, "tags" : [ "dogs", "work" ] }
{ "_id" : 6, "height" : 480, "width" : 640, "tags" : [ "work" ] }
{ "_id" : 4, "height" : 480, "width" : 640, "tags" : [ "dogs", "sunrises", "kittens", "travel" ] }
{ "_id" : 5, "height" : 480, "width" : 640, "tags" : [ "dogs", "cats", "sunrises", "kittens", "work" ] }
{ "_id" : 7, "height" : 480, "width" : 640, "tags" : [ "dogs", "sunrises" ] }
{ "_id" : 8, "height" : 480, "width" : 640, "tags" : [ "dogs", "cats", "sunrises", "kittens", "travel" ] }
 

現在要統計： 所有Document中的 tags 數組裏面的每個元素 出現的次數。即："kittens"出現了多少次？"travel"出現了多少次？"dogs"出現了多少次？……

二，實現步驟

使用MongoDB的Aggregate操作進行實現

①使用 unwind 分解 tags 數組，得到的結果如下：

> db.images.aggregate(
... [
... {$unwind:"$tags"}
... ])

{ "_id" : 3, "height" : 480, "width" : 640, "tags" : "kittens" }
{ "_id" : 3, "height" : 480, "width" : 640, "tags" : "travel" }
{ "_id" : 1, "height" : 480, "width" : 640, "tags" : "cats" }
{ "_id" : 1, "height" : 480, "width" : 640, "tags" : "sunrises" }
{ "_id" : 1, "height" : 480, "width" : 640, "tags" : "kittens" }
{ "_id" : 1, "height" : 480, "width" : 640, "tags" : "travel" }
{ "_id" : 1, "height" : 480, "width" : 640, "tags" : "vacation" }
{ "_id" : 1, "height" : 480, "width" : 640, "tags" : "work" }
{ "_id" : 0, "height" : 480, "width" : 640, "tags" : "dogs" }
{ "_id" : 0, "height" : 480, "width" : 640, "tags" : "work" }
{ "_id" : 6, "height" : 480, "width" : 640, "tags" : "work" }
{ "_id" : 4, "height" : 480, "width" : 640, "tags" : "dogs" }
{ "_id" : 4, "height" : 480, "width" : 640, "tags" : "sunrises" }
.....
.....
 

②將分解後的每個 tag 進行 group 操作

對於group操作而言，_id 指定了 分組 的字段(對哪個字段進行 group by 操作)，分組操作之後生成的結果由 num_of_tag 字段標識

> db.images.aggregate(
... [
... {$unwind:"$tags"},
... {$group:{_id:"$tags",num_of_tag:{$sum:1}}}
... ]
... )

{ "_id" : "dogs", "num_of_tag" : 49921 }
{ "_id" : "work", "num_of_tag" : 50070 }
{ "_id" : "vacation", "num_of_tag" : 50036 }
{ "_id" : "travel", "num_of_tag" : 49977 }
{ "_id" : "kittens", "num_of_tag" : 49932 }
{ "_id" : "sunrises", "num_of_tag" : 49887 }
{ "_id" : "cats", "num_of_tag" : 49772 }
 

③使用 project 去掉不感興趣的 _id 字段（其實這裏是將 _id 字段名 替換為 tags 字段名）（這一步可忽略）

project操作，_id:0 表示去掉_id 字段；tags:"$_id"，將 _id 字段值 使用tags 字段標識；num_of_tag:1 保留 num_of_tag 字段

> db.images.aggregate( [ {$unwind:"$tags"},{$group:{_id:"$tags",num_of_tag:{$sum:1}}},{$project:{_id:0,tags:"$_id",num_of_tag:1}} ])
{ "num_of_tag" : 49921, "tags" : "dogs" }
{ "num_of_tag" : 50070, "tags" : "work" }
{ "num_of_tag" : 50036, "tags" : "vacation" }
{ "num_of_tag" : 49977, "tags" : "travel" }
{ "num_of_tag" : 49932, "tags" : "kittens" }
{ "num_of_tag" : 49887, "tags" : "sunrises" }
{ "num_of_tag" : 49772, "tags" : "cats" }

④使用 sort 對 num_of_tag 字段排序

> db.images.aggregate( [ {$unwind:"$tags"},{$group:{_id:"$tags",num_of_tag:{$sum:1}}},{$project:{_id:0,tags:"$_id",num_of_tag:1}},{$sort:{num_of_tag:-1}} ])
{ "num_of_tag" : 50070, "tags" : "work" }
{ "num_of_tag" : 50036, "tags" : "vacation" }
{ "num_of_tag" : 49977, "tags" : "travel" }
{ "num_of_tag" : 49932, "tags" : "kittens" }
{ "num_of_tag" : 49921, "tags" : "dogs" }
{ "num_of_tag" : 49887, "tags" : "sunrises" }
{ "num_of_tag" : 49772, "tags" : "cats" }

三，總結

本文是MongoDB University M101課程 For Java Developers中的一次作業。結合Google搜索和MongoDB的官方文檔，很容易就能實現MongoDB的各種組合查詢。

原文：http://www.cnblogs.com/hapjin/p/7944404.html

MongoDB統計文檔(Document)的數組(Array)中的各個元素出現的次數