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（http://www.my400800.cn）
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; }
}
});