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官方给出参数结构：

　　{
　　　　group:
　　　　　　{
　　　　　　　　ns: <namespace>,
　　　　　　　　key: <key>,
　　　　　　　　$reduce: <reduce function>,
　　　　　　　　$keyf: <key function>,
　　　　　　　　cond: <query>,
　　　　　　　　finalize: <finalize function>
　　　　　　}
　　}

重要参数解析：　　key：作为分组的keyreduce：这些函数可以返回一个sum或count。该函数接受两个参数(curr,result)：curr(当前文档)和result(分组聚集的结果文档)。initial：初始化聚合结果文档变量，为空时自动为每列提供初始变量。keyf：可选。替代的key 字段。指定一个函数创建一个“key object”作为分组的key。使用keyf而是通过group by领域而不是现有的文档域键组。cond：过滤条件finalize：在db.collection.group()返回最终结果之前，此功能可以修改的结果文档或替换的结果文档作为一个整体。

1、key-->2、initial-->3、cond-->4、reduce   将本组信息存储，然后其他组循环2，3，4执行

#key：表示分组的字段
#cond：分组后查询的条件
#reduce：聚合操作，其中curr表示当前文档的指针，result表示分组聚集的文档
#initial:参数初始化操作
#finalize:最终执行函数，类似java中的finally代码块#查询每个栏目下的商品数量
{
key:{cat_id:1},
cond:{},
reduce:function(curr,result) {result.cnt += 1;
},
initial:{cnt:0}
}#查询每个栏目下价格高于50元的商品数量
{
key:{cat_id:1},
cond:{shop_price:{$gt:50}},
reduce:function(curr,res){res.cnt +=1;
},
initial:{cnt:0}
}#每个栏目下的商品库存量 sum()操作
{
key:{cat_id:1},
cond:{},
reduce:function(curr,res){res.num +=curr.goods_number;
},
initial:{num:0}
}#查询每个栏目最贵的商品价格, max()操作
{
key:{cat_id:1},
cond:{},
reduce:function(curr,res){if(res.price<curr.shop_price){res.price = curr.shop_price;}
},
initial:{price:0}
}#查询每个栏目下商品的平均价格
{
key:{cat_id:1},
cond:{},
reduce:function(curr,res){res.cnt +=1;res.sum += curr.shop_price;},
initial:{sum:0,cnt:0},
finalize:function(res){res.avg = res.sum/res.cnt;
}
}