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在本文章中，会演示MongoDB使用聚集函数对文档进行分组的用法。

1.测试数据

网站主机域名列表的JSON格式文件

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website.json
{ "_id" : 1, "domainName" : "test1.com", "hosting" : "hostgator.com" }
{ "_id" : 2, "domainName" : "test2.com", "hosting" : "aws.amazon.com"}
{ "_id" : 3, "domainName" : "test3.com", "hosting" : "aws.amazon.com" }
{ "_id" : 4, "domainName" : "test4.com", "hosting" : "hostgator.com" }
{ "_id" : 5, "domainName" : "test5.com", "hosting" : "aws.amazon.com" }
{ "_id" : 6, "domainName" : "test6.com", "hosting" : "cloud.google.com" }
{ "_id" : 7, "domainName" : "test7.com", "hosting" : "aws.amazon.com" }
{ "_id" : 8, "domainName" : "test8.com", "hosting" : "hostgator.com" }
{ "_id" : 9, "domainName" : "test9.com", "hosting" : "cloud.google.com" }
{ "_id" : 10, "domainName" : "test10.com", "hosting" : "godaddy.com" }

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导入website文档

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> mongoimport -d testdb -c website --file website.json
connected to: 127.0.0.1
Mon Jan 13 14:30:22.662 imported 10 objects

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注意
如果是已存在的文档, 加 --upsert 选项来覆盖数据.
> mongoimport -d testdb -c website --file website.json --upsert

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2.分组例子

使用db.collection.aggregate和$group函数进行文档分组

? 2.1 下面的例子通过“hosting”字段分组并显示每个主机总数

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> db.website.aggregate(
    { 
	$group : {_id : "$hosting", total : { $sum : 1 }}
    }
  );

? 结果

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{
        "result" : [
                {
                        "_id" : "godaddy.com",
                        "total" : 1
                },
                {
                        "_id" : "cloud.google.com",
                        "total" : 2
                },
                {
                        "_id" : "aws.amazon.com",
                        "total" : 4
                },
                {
                        "_id" : "hostgator.com",
                        "total" : 3
                }
        ],
        "ok" : 1
}

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用SQL可以表示为：

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SELECT hosting, SUM(hosting) AS total
       FROM website
       GROUP BY hosting

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?2.2 使用$sort函数进行排序