mongodb在aggregate中怎么统计总数
katsumi[size=medium]在aggregate只能通过$group中的$sum来进⾏统计，如果sales表中有下⾯数据：[/size]丧偶英文
{ "_id" : { "month" : 3, "day" : 15, "year" : 2014 }, "totalPrice" : 50, "averageQuantity" : 10, "count" : 1 }
nur{ "_id" : { "month" : 4, "day" : 4, "year" : 2014 }, "totalPrice" : 200, "averageQuantity" : 15, "count" : 2 }
{ "_id" : { "month" : 3, "day" : 1, "year" : 2014 }, "totalPrice" : 40, "averageQuantity" : 1.5, "count" : 2 }
[size=medium]如果要得到count总记录数该怎么做呢？
我们可以通过：[/size]
db.sales.aggregate(
[
{
$group : {
every day_id : null,
totalPrice: { $sum: { $multiply: [ "$price", "$quantity" ] } },
averageQuantity: { $avg: "$quantity" },鸿门宴词类活用
count: { $sum: 1 }
}
}英语三级试卷
]
)
[size=medium]即可得到，id:null这个⾮常关键
得到如下结果[/size]：
{ "_id" : null, "totalPrice" : 290, "averageQuantity" : 8.6, "count" : 5 }
[size=medium]java代码可以通过如下[/size]
DBObject count = new BasicDBObject("$sum",1);
before he cheatsDBObject groupFileds = new BasicDBObject("_id", null);
groupFileds.put("count", count);
kansas university
DBObject group = new BasicDBObject("$group", groupFileds);
AggregationOutput output = collection.aggregate(Arrays.asList(group));
List<DBObject> dbObjects = (List<DBObject>) sults()；
生活大爆炸13int count = (int) (0).get("count");
[b]
hellrair
这⾥⼲嘛不直接⽤mongodb的count，这是因为如果在这个统计中如果有根据地理空间距离查找的要求，那么就不满⾜要求了，所以改⽤aggregate进⾏统计是个不错的选择 [/b]
