關於MongoDB的group分組

測試條件：Windows+MongoDB 1.8.2先插入測試資料：for(var i=1; i<20; i++){    var num=i%6;    db.test.insert({_id:i,name:"user_"+i,age:num});}1.普通分組查詢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.篩選後再分組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$where查詢：db.test.find({$where:function(){return this.age>2;}    });group聯合$where查詢db.test.group({key:{age:true},initial:{num:0},$reduce:function(doc,prev){prev.num++},condition:{$where:function(){return this.age>2;}    }});3.使用函數傳回值分組//注意，$keyf指定的函數一定要返回一個對象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.使用終結器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; }}});

 

 

有關MapReduce

//首先插入測試資料
for(var i=1;i<21;i++)
{
    db.test.insert({_id:i,name:'mm'+i});
}

//進行mapreduce
db.runCommand(
{
    mapreduce:'test',
    map:function(){emit(this.name.substr(0,3),this);},
    reduce:function(key,vals){return vals[0];},  //注意：vals是一個Object對象而不是數組
    out:'wq'
});

注意：
1.mapreduce是根據map函數裡調用的emit函數的第一個參數來進行分組的
2.僅當根據分組鍵分組後一個鍵匹配多個文檔，才會將key和文檔集合交由reduce函數處理。例如：
db.runCommand(
{
    mapreduce:'test',
    map:function(){emit(this.name.substr(0,3),this);},
    reduce:function(key,vals){return 'wq';},
    out:'wq'
});
執行mapreduce命令後，再查看wq表資料：
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" } }