由 dawei 在实际应用中，我们有时候会遇到“MongoDB中group by能用于什么，有哪些应用”这样的问题，我们该怎样来处理呢？下文给大家介绍了解决方法，希望这篇“MongoDB中group by能用于什么，有哪些应用”文章能帮助大家解决问题。
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：
Select date, pnumber,sum(quantity) as total from orders,items group by date, 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 : function Reduce(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 : function Reduce(doc, out) {
out.total+=doc.item.quantity;
out.money+=doc.item.quantity*doc.item.price;
},
finalize : function Finalize(out) {
out.avg=out.money/out.total
returnout;
}
});
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 : function Reduce(doc, out) {
out.total+=doc.item.quantity;
out.money+=doc.item.quantity*doc.item.price;
},
finalize : function Finalize(out) {
out.avg=out.money/out.total
returnout;
}
});
以上就是关于“MongoDB中group by能用于什么，有哪些应用”的介绍了，感谢各位的阅读，希望这篇文章能帮助大家解决问题。