Mongodb是针对大数据量环境下诞生的用于保存大数据量的非关系型数据库,针对大量的数据,如何进行统计操作至关重要,那么如何从Mongodb中统计一些数据呢? 在Mongodb中,给我们提供了三种用于数据聚合的方式: (1)简单的用户聚合函数; (2)使用aggregate
Mongodb是针对大数据量环境下诞生的用于保存大数据量的非关系型数据库,针对大量的数据,如何进行统计操作至关重要,那么如何从Mongodb中统计一些数据呢?
在Mongodb中,给我们提供了三种用于数据聚合的方式:
(1)简单的用户聚合函数;
(2)使用aggregate进行统计;
(3)使用mapReduce进行统计;
今天我们首先来讲讲mapReduce是如何统计,在后续的文章中,将另起文章进行相关说明。
MapReduce是啥呢?以我的理解,其实就是对集合中的各个满足条件的文档进行预处理,整理出想要的数据然后进行统计得到最终的统计结果。其中map函数用于对集合中的各个满足条件的文档进行预处理,整理出想要的数据。Reduce函数用于对整理出的数据进行处理得到统计结果。Map函数和Reduce函数都是JavaScript函数。
首先,我们先构造一个测试数据集test,使用js脚本往集合中随机插入一组数据,每条记录是哪个人花了多少钱买了什么东西。具体脚本test1.js如下:
<span style="font-size:18px;">for( var i=0; i=3 && rID=5 && rID</span>
接下来我们通过在控制台执行脚本来向数据库插入具体的数据,具体执行指令如下:
<span style="font-size:18px;">mongo 127.0.0.1:27017/test J:/test1.js</span>
执行之后,通过MongoVUE来查看下具体的数据,如下所示,数据已经插入到集合中了:
接下来,我们可以做几个简单的统计操作了。
(1)统计不同用户都买了多少个商品?编写js脚本test2.js,将结果保存到statis1集合中。
<span style="font-size:18px;"><span style="font-size:18px;">map=function(){ emit(this.user,1); } reduce=function(key, values){ var count = 0; values.forEach(function(val){count += val}); return count; } db.test.mapReduce(map, reduce, {out:"statics1"});</span></span>
按照刚才执行脚本的方式执行test2.js,并查看数据:

从数据库就可以直观看到统计数据了,若想查看某个人如majing购买了多少个商品,直接使用
<span style="font-size:18px;"><span style="font-size:18px;"><span style="font-family:KaiTi_GB2312;font-size:18px;">db.statics1.find({"_id":"majing"});</span></span></span>
脚本test3.js如下所示:
<span style="font-size:18px;"><span style="font-size:18px;">map=function(){ emit({user:this.user,sku:this.sku},1); } reduce=function(key, values){ var count = 0; values.forEach(function(val){count += val}); return count; } db.test.mapReduce(map, reduce, {out:"statics2"});</span></span>
总共返回了10条记录。此时如果我们想查找某个用户购买商品的情况,可以使用下面的查询方法:
<span style="font-size:18px;"><span style="font-size:18px;">db.statics2.find({"_id.user":"majing"});</span></span>

如果我们想查找某个用户购买某个商品的情况,可以使用下面的查询方法:
(3)统计每个用户购买商品的总量及花费的总金额
脚本test4.js如下所示:
<span style="font-size:18px;"><span style="font-size:18px;">map=function(){ emit({user:this.user},{totalprice:this.price,count:1}); } reduce=function(key, values){ var res = {totalprice:0.00,count:1}; values.forEach(function(val){res.totalprice += val.totalprice;res.count+=val.count;}); return res; } db.test.mapReduce(map, reduce, {out:"statics3"});</span></span>
按照刚才执行脚本的方式执行test4.js,并查看数据:

(4)统计每个用户购买商品的平均价钱
在这个情景下,我们需要用到说道mapReduce里的另一个参数finalize,该参数是一个javascript脚本函数,用于对reduce后的集合进行一个后期处理操作。
执行脚本test5.js,具体如下所示:
<span style="font-size:18px;"><span style="font-size:18px;">map=function(){ emit({user:this.user},{totalprice:this.price,count:1}); } reduce=function(key, values){ var res = {totalprice:0.00,count:1,average:0}; values.forEach(function(val){res.totalprice += val.totalprice;res.count+=val.count;}); return res; } finalizeFunc=function(key,reduceResult){ reduceResult.totalprice=(reduceResult.totalprice).toFixed(2); reduceResult.average=(reduceResult.totalprice/reduceResult.count).toFixed(2); return reduceResult; } db.test.mapReduce(map, reduce, {out:"statics4",finalize:finalizeFunc});</span></span>
执行之后查看得到的数据,具体如下所示,显示了总价钱,商品数量和商品单价。
如果想查找某个人的,可以和上面的查询方法一样,使用find()方法进行查询:
<span style="font-size:18px;"><span style="font-size:18px;">db.statics4.find({"_id.user":"majing"});</span></span>
以上通过4个简单的例子对Mongodb中的MapReduce进行了简单的说明,当然MapReduce功能很强大,大家如果想知道其他高级的使用方法,可以到Mongodb的官网进行查阅和学习,网址为 https://docs.mongodb.com/manual/reference/method/db.collection.mapReduce/ ,谢谢。

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