mongodb mapreduce学习

最新推荐文章于 2025-09-13 12:54:57 发布

转载最新推荐文章于 2025-09-13 12:54:57 发布 · 56 阅读

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原文链接：http://www.cnblogs.com/cxd4321/archive/2012/05/18/2507779.html

文章标签：

#大数据 #数据库

本文通过几个具体的示例介绍了如何使用 MongoDB 中的 MapReduce 功能来处理和汇总数据。包括了基本的计数操作，更复杂的聚合操作以及如何使用 finalize 函数进行后期处理等。

for( var i = 1; i < 10001; i++){
db.students.insert({classid:i%400, age:i%100, name:'Tom'+i})
}

db.students.insert({classid:1, age:14, name:'Tom'})
db.students.insert({classid:1, age:12, name:'Jacky'})
db.students.insert({classid:2, age:16, name:'Lily'})
db.students.insert({classid:2, age:9, name:'Tony'})
db.students.insert({classid:2, age:19, name:'Harry'})
db.students.insert({classid:2, age:13, name:'Vincent'})
db.students.insert({classid:1, age:14, name:'Bill'})
db.students.insert({classid:2, age:17, name:'Bruce'})

m = function() {
    emit(this.classid, 1)
}

r = function(key, values) {
     var x = 0;
     values.forEach(
        function(v) { x += v }
     );
    return x;
}

res = db.runCommand({
    mapreduce:"students",
     map:m,
    reduce:r,
    out:"students_res"
});

db.students_res.find()

m = function() {
    emit(this.classid, {cnt:1,name:this.name})
}

r = function(key, values) {
     var x = 0;
     var names="";
     values.forEach(
        function(v)
        {
            x += v.cnt;
            names+=v.name;
        }
     );
    return {classid:key,cnt:x,names:names};
}

res = db.runCommand({
    mapreduce:"students",
     map:m,
    reduce:r,
    out:"students_res"
});

db.students_res.find()

f = function(key, value) { return {classid:key, count:value}; }

res = db.runCommand({
    mapreduce:"students",
    map:m,
    reduce:r,
    out:"students_res",
    finalize:f
});

db.students_res.find()

res = db.runCommand({
     mapreduce:"students",
     map:m,
     reduce:r,
     out:"students_res",
     finalize:f,
     query:{age:{$lt:10}}
});

db.students_res.find()

转载于:https://www.cnblogs.com/cxd4321/archive/2012/05/18/2507779.html