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mongodb使用group by：

我们比较熟悉的group by 的sql语句select key from table groupby key,而mongoDB没提供SQL那样通过Group By就轻松实现数据库的分组功能，我们通过接口来实现的

db.collection.group({key,reduce,initial[,keyf][,cond][,finalize]})

1.MonogoDB数据库中添加订单的数据

/*0*/{"_id":ObjectId("552a330e05c27486b9b9b650"),"_class":"com.mongo.model.Orders","onumber":"002","date":ISODate("2014-01-03T16:03:00Z"),"cname":"zcy","item":{"quantity":1,"price":4.0,"pnumber":"p002"}}/*1*/{"_id":ObjectId("552a331d05c275d8590a550d"),"_class":"com.mongo.model.Orders","onumber":"003","date":ISODate("2014-01-04T16:03:00Z"),"cname":"zcy","item":{"quantity":10,"price":2.0,"pnumber":"p001"}}/*2*/{"_id":ObjectId("552a333105c2f28194045a72"),"_class":"com.mongo.model.Orders","onumber":"003","date":ISODate("2014-01-04T16:03:00Z"),"cname":"zcy","item":{"quantity":30,"price":4.0,"pnumber":"p002"}}/*3*/{"_id":ObjectId("552a333f05c2b62c01cff50e"),"_class":"com.mongo.model.Orders","onumber":"004","date":ISODate("2014-01-05T16:03:00Z"),"cname":"zcy","item":{"quantity":5,"price":4.0,"pnumber":"p002"}}

2.MongoDB实现分组并统计

1）我们要对日期和产品编码进行分组，并计算相同的产品的数量

Sql语句：

Selectdate,pnumber,sum(quantity)astotalfromorders,itemsgroupbydate,pnumber

（少了两张表的关联的条件）

MongoDB：db.orders.group({key:{date:1,'item.pnumber':1},initial:{"total":0},reduce:functionReduce(doc,out){out.total+=doc.item.quantity}});

结果：

2）实现一天卖出了多少个产品，金额是多少，平均价格是多少

db.orders.group({key:{date:1},initial:{"total":0,"money":0},reduce:functionReduce(doc,out){out.total+=doc.item.quantity;out.money+=doc.item.quantity*doc.item.price;},finalize:functionFinalize(out){out.avg=out.money/out.totalreturnout;}});

结果：

3）keyf的使用

keyf 对日期进行处理并以作为key来进来分组

db.orders.group({keyf:function(doc){return{'month':doc.date.getMonth()+1};},initial:{"total":0,"money":0},reduce:functionReduce(doc,out){out.total+=doc.item.quantity;out.money+=doc.item.quantity*doc.item.price;},finalize:functionFinalize(out){out.avg=out.money/out.totalreturnout;}});

结果：

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