MongoDB学习--高级查询 [聚合Group]

    //测试的数据
    db.stocks.insert({"day": "2010/10/03", "time": "2010/10/03 03:47:01 GMT-400", "price": 4.23});  
    db.stocks.insert({"day": "2010/10/04", "time": "2010/10/04 11:28:39 GMT-400", "price": 4.27});  
    db.stocks.insert({"day": "2010/10/03", "time": "2010/10/03 05:00:23 GMT-400", "price": 4.10});  
    db.stocks.insert({"day": "2010/10/06", "time": "2010/10/06 05:27:58 GMT-400", "price": 4.30});  
    db.stocks.insert({"day": "2010/10/04", "time": "2010/10/04 08:34:50 GMT-400", "price": 4.01});  

db.runCommand({"group": {  
    "ns": "stocks",  
    "key": "day",  
    "initial": {"time": 0},  
    "$reduce": function(doc, prev) {  
        if (doc.time > prev.time) {  
            prev.price = doc.price;  
            prev.time = doc.time;  
        }  
    }  
}}); 

    {  
        "retval" : [  
            {  
                "time" : 0  
            }  
        ],  
        "count" : 5,  
        "keys" : 1,  
        "ok" : 1  
    }