mongdb group聚合操作

1、数据准备

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[{"goods_id":13,"cat_id":3,"goods_name":"\u8bfa\u57fa\u4e9a5320 XpressMusic","goods_number":8,"click_count":13,"shop_price":1311.00,"add_time":1241967762},{"goods_id":14,"cat_id":4,"goods_name":"\u8bfa\u57fa\u4e9a5800XM","goods_number":1,"click_count":6,"shop_price":2625.00,"add_time":1241968492},{"goods_id":15,"cat_id":3,"goods_name":"\u6469\u6258\u7f57\u62c9A810","goods_number":3,"click_count":8,"shop_price":788.00,"add_time":1241968703},{"goods_id":16,"cat_id":2,"goods_name":"\u6052\u57fa\u4f1f\u4e1aG101","goods_number":0,"click_count":3,"shop_price":823.33,"add_time":1241968949},{"goods_id":17,"cat_id":3,"goods_name":"\u590f\u65b0N7","goods_number":1,"click_count":2,"shop_price":2300.00,"add_time":1241969394},{"goods_id":18,"cat_id":4,"goods_name":"\u590f\u65b0T5","goods_number":1,"click_count":0,"shop_price":2878.00,"add_time":1241969533},{"goods_id":19,"cat_id":3,"goods_name":"\u4e09\u661fSGH-F258","goods_number":12,"click_count":7,"shop_price":858.00,"add_time":1241970139},{"goods_id":20,"cat_id":3,"goods_name":"\u4e09\u661fBC01","goods_number":12,"click_count":14,"shop_price":280.00,"add_time":1241970417},{"goods_id":21,"cat_id":3,"goods_name":"\u91d1\u7acb A30","goods_number":40,"click_count":4,"shop_price":2000.00,"add_time":1241970634},{"goods_id":22,"cat_id":3,"goods_name":"\u591a\u666e\u8fbeTouch HD","goods_number":1,"click_count":15,"shop_price":5999.00,"add_time":1241971076}]

[{"goods_id":23,"cat_id":5,"goods_name":"\u8bfa\u57fa\u4e9aN96","goods_number":8,"click_count":17,"shop_price":3700.00,"add_time":1241971488},{"goods_id":24,"cat_id":3,"goods_name":"P806","goods_number":100,"click_count":35,"shop_price":2000.00,"add_time":1241971981},{"goods_id":25,"cat_id":13,"goods_name":"\u5c0f\u7075\u901a\/\u56fa\u8bdd50\u5143\u5145\u503c\u5361","goods_number":2,"click_count":0,"shop_price":48.00,"add_time":1241972709},{"goods_id":26,"cat_id":13,"goods_name":"\u5c0f\u7075\u901a\/\u56fa\u8bdd20\u5143\u5145\u503c\u5361","goods_number":2,"click_count":0,"shop_price":19.00,"add_time":1241972789},{"goods_id":27,"cat_id":15,"goods_name":"\u8054\u901a100\u5143\u5145\u503c\u5361","goods_number":2,"click_count":0,"shop_price":95.00,"add_time":1241972894},{"goods_id":28,"cat_id":15,"goods_name":"\u8054\u901a50\u5143\u5145\u503c\u5361","goods_number":0,"click_count":0,"shop_price":45.00,"add_time":1241972976},{"goods_id":29,"cat_id":14,"goods_name":"\u79fb\u52a8100\u5143\u5145\u503c\u5361","goods_number":0,"click_count":0,"shop_price":90.00,"add_time":1241973022},{"goods_id":30,"cat_id":14,"goods_name":"\u79fb\u52a820\u5143\u5145\u503c\u5361","goods_number":9,"click_count":1,"shop_price":18.00,"add_time":1241973114},{"goods_id":31,"cat_id":3,"goods_name":"\u6469\u6258\u7f57\u62c9E8 ","goods_number":1,"click_count":5,"shop_price":1337.00,"add_time":1242110412},{"goods_id":32,"cat_id":3,"goods_name":"\u8bfa\u57fa\u4e9aN85","goods_number":4,"click_count":9,"shop_price":3010.00,"add_time":1242110760}]

2、group参数解析

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key
　　reduce：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。

3、执行顺序解析

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

4、执行案例

#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;
}
}

　　

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