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关于MongoDB的group分组

时间:2021-07-01 10:21:17 帮助过:4人阅读

关于MongoDB的group分组 先插入测试数据: for(var i=1; i20; i++){ var num=i%6; db.test.insert({_id:i,name:user_+i,age:num}); } 1.普通分组查询 db.test.group({key:{age:true},initial:{num:0},$reduce:function(doc,prev){ prev.num++ }}); db.runCom

关于MongoDB的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});

}

1.普通分组查询

db.test.group({key:{age:true},initial:{num:0},$reduce:function(doc,prev){

prev.num++

}});

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" } }

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