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mongodb group分组

先插入测试数据:
for(var i=1; i<20; i++){
    var num=i%6;
    db.test.insert({_id:i,name:"user_"+i,age:num});
}
 说明,如果有_id这个,mongodb就不会生成_id,会采用你提供的_id.
 
 
1.普通分组查询
db.test.group({key:{age:true},initial:{num:0},$reduce:function(doc,prev){
prev.num++
}});
 
[        {                "age" : 1,                "num" : 4        },        {                "age" : 2,                "num" : 3        },        {                "age" : 3,                "num" : 3        },        {                "age" : 4,                "num" : 3        },        {                "age" : 5,                "num" : 3        },        {                "age" : 0,                "num" : 3        }]

 

db.runCommand({group:
{
ns:"test",
key:{age:true},
initial:{num:0},
$reduce:function(doc,prev){
prev.num++}
}
});
 
2.筛选后再分组
db.test.group({key:{age:true},initial:{num:0},$reduce:function(doc,prev){
prev.num++
},
condition:{age:{$gt:2}}
});
 
db.runCommand({group:
{
ns:"test",
key:{age:true},
initial:{num:0},
$reduce:function(doc,prev){
prev.num++},
condition:{age:{$gt:2}}
}
});
 
普通的$where查询:
db.test.find({$where:function(){
return this.age>2;
}
    });
 
group联合$where查询
db.test.group({key:{age:true},initial:{num:0},$reduce:function(doc,prev){
prev.num++
},
condition:{$where:function(){
return this.age>2;
}
    }
});
 
3.使用函数返回值分组
//注意,$keyf指定的函数一定要返回一个对象
db.test.group({$keyf:function(doc){return {age:doc.age};},initial:{num:0},$reduce:function(doc,prev){
prev.num++
}
});
 
db.runCommand({group:
{
ns:"test",
$keyf:function(doc){return {age:doc.age};},
initial:{num:0},
$reduce:function(doc,prev){
prev.num++}
}
});
 
 
4.使用终结器
db.test.group({$keyf:function(doc){return {age:doc.age};},initial:{num:0},$reduce:function(doc,prev){
prev.num++
},
finalize: function(doc){ doc.count=doc.num;delete doc.num; }
});
 
db.runCommand({group:
{
ns:"test",
$keyf:function(doc){return {age:doc.age};},
initial:{num:0},
$reduce:function(doc,prev){
prev.num++},
finalize: function(doc){ doc.count=doc.num;delete doc.num; }
}
});

 

 

有关MapReduce

 

//首先插入测试数据
for(var i=1;i<21;i++)
{
    db.test.insert({_id:i,name:‘mm‘+i});
}

//进行mapreduce
db.runCommand(
{
    mapreduce:‘test‘,
    map:function(){emit(this.name.substr(0,3),this);},
    reduce:function(key,vals){return vals[0];},  //注意:vals是一个Object对象而不是数组
    out:‘wq‘
});

注意:
1.mapreduce是根据map函数里调用的emit函数的第一个参数来进行分组的
2.仅当根据分组键分组后一个键匹配多个文档,才会将key和文档集合交由reduce函数处理。例如:
db.runCommand(
{
    mapreduce:‘test‘,
    map:function(){emit(this.name.substr(0,3),this);},
    reduce:function(key,vals){return ‘wq‘;}, 
    out:‘wq‘
});
执行mapreduce命令后,再查看wq表数据:
db.wq.find()

{ "_id" : "mm1", "value" : "wq" }
{ "_id" : "mm2", "value" : "wq" }
{ "_id" : "mm3", "value" : { "_id" : 3, "name" : "mm3" } }
{ "_id" : "mm4", "value" : { "_id" : 4, "name" : "mm4" } }
{ "_id" : "mm5", "value" : { "_id" : 5, "name" : "mm5" } }
{ "_id" : "mm6", "value" : { "_id" : 6, "name" : "mm6" } }
{ "_id" : "mm7", "value" : { "_id" : 7, "name" : "mm7" } }
{ "_id" : "mm8", "value" : { "_id" : 8, "name" : "mm8" } }
{ "_id" : "mm9", "value" : { "_id" : 9, "name" : "mm9" } }
 
 
参考:http://www.cnblogs.com/mxw09/archive/2011/08/31/2161457.html
http://www.cnblogs.com/huangxincheng/archive/2012/02/21/2361205.html
 

mongodb group分组