1. 聚合管道
使用聚合管道可对集合中的文档进行变换和组合。
应用场景：表关联查询、数据的统计。
MongoDB 中使用 db.COLLECTION_NAME.aggregate([{< stage >},…]) 方法 来构建和使用聚合管道。
先看下官网给的实例，感受一下聚合管道的用法。
2. 管道操作符与表达式
管道表达式：
管道操作符作为“键”，所对应的“值”叫做管道表达式。
{ $match:{status:"A"}}，$match 称为管道操作符，而 status:"A" 称为管道表达式， 是管道操作符的操作数(Operand)。 每个管道表达式是一个文档结构，它是由字段名、字段值、和一些表达式操作符组成的。
| 常用表达式操作符 | 描述 |
| $addToSet | 将文档指定字段的值去重 |
| $max | 文档指定字段的最大值 |
| $min | 文档指定字段的最小值 |
| $sum | 文档指定字段求和 |
| $avg | 文档指定字段求平均 |
| $gt | 大于给定值 |
| $lt | 小于给定值 |
| $eq | 等于给定值 |
3. 数据模拟
db.order.insert({"order_id":"1","uid":10,"trade_no":"111","all_price":100,"all_num":2}) db.order.insert({"order_id":"2","uid":7,"trade_no":"222","all_price":90,"all_num":2}) db.order.insert({"order_id":"3","uid":9,"trade_no":"333","all_price":20,"all_num":6}) db.order_item.insert({"order_id":"1","title":"商品鼠标 1","price":50,num:1}) db.order_item.insert({"order_id":"1","title":"商品键盘 2","price":50,num:1}) db.order_item.insert({"order_id":"1","title":"商品键盘 3","price":0,num:1}) db.order_item.insert({"order_id":"2","title":"牛奶","price":50,num:1}) db.order_item.insert({"order_id":"2","title":"酸奶","price":40,num:1}) db.order_item.insert({"order_id":"3","title":"矿泉水","price":2,num:5}) db.order_item.insert({"order_id":"3","title":"毛巾","price":10,num:1})
$project
作用：修改文档的结构，可以用来重命名、增加或删除文档中的字段；
要求查找 order 只返回文档中 trade_no 和 all_price 字段
db.order.aggregate([ { $project:{ trade_no:1, all_price:1 } } ])
$match
作用：用于过滤文档。用法类似于 find() 方法中的参数。
db.order.aggregate([ { $project:{ trade_no:1, all_price:1 } }, { $match:{"all_price":{$gte:90}} } ])
$group
作用：将集合中的文档进行分组，可用于统计结果
统计每个订单的订单数量，按照订单号分组
db.order_item.aggregate( [ { $group: {_id: "$order_id", total: {$sum: "$num"}} } ])
$sort
作用：将集合中的文档进行排序
db.order.aggregate([ { $project:{ trade_no:1, all_price:1 } }, { $match:{"all_price":{$gte:90}} }, { $sort:{"all_price":-1} } ])
$limit
作用：限制结果的数量
db.order.aggregate([ { $project:{ trade_no:1, all_price:1 } }, { $match:{"all_price":{$gte:90}} }, { $sort:{"all_price":-1} }, { $limit:1 } ])
$skip
作用：跳过文档的数量
db.order.aggregate([ { $project:{ trade_no:1, all_price:1 } }, { $match:{"all_price":{$gte:90}} }, { $sort:{"all_price":-1} }, { $skip:1 } ])
$lookup
作用：表关联
eg1：
db.order.aggregate([ { $lookup: { from: "order_item", localField: "order_id", # 主表 foreignField: "order_id", as: "items" } } ])
结果：
{ "_id": ObjectId("5b743d8c2c327f8d1b360540"), "order_id": "1", "uid": 10, "trade_no": "111", "all_price": 100, "all_num": 2, "items": [ { "_id": ObjectId("5b743d9c2c327f8d1b360543"), "order_id": "1", "title": "商品鼠标 1", "price": 50, "num": 1 }, { "_id": ObjectId("5b743da12c327f8d1b360544"), "order_id": "1", "title": "商品键盘 2", "price": 50, "num": 1 }, { "_id": ObjectId("5b74f457089f78dc8f0a4f3b"), "order_id": "1", "title": "商品键盘 3", "price": 0, "num": 1 } ] } ,{ "_id": ObjectId("5b743d902c327f8d1b360541"), "order_id": "2", "uid": 7, "trade_no": "222", "all_price": 90, "all_num": 2, "items": [ { "_id": ObjectId("5b743da52c327f8d1b360545"), "order_id": "2", "title": "牛奶", "price": 50, "num": 1 }, { "_id": ObjectId("5b743da92c327f8d1b360546"), "order_id": "2","title": "酸奶", "price": 40, "num": 1 } ] }, { "_id": ObjectId("5b743d962c327f8d1b360542"), "order_id": "3", "uid": 9, "trade_no": "333", "all_price": 20, "all_num": 6, "items": [ { "_id": ObjectId("5b743dad2c327f8d1b360547"), "order_id": "3", "title": "矿泉水", "price": 2, "num": 5 }, { "_id": ObjectId("5b743dff2c327f8d1b360548"), "order_id": "3", "title": "毛巾", "price": 10, "num": 1 } ] }
eg2
db.order.aggregate([ { $lookup: { from: "order_item", localField: "order_id", foreignField: "order_id", as: "items" } }, { $project:{ trade_no:1, all_price:1,items:1 } }, { $match:{"all_price":{$gte:90}} }, { $sort:{"all_price":-1} }, ])
结果：
{ "_id": ObjectId("5b743d8c2c327f8d1b360540"), "trade_no": "111", "all_price": 100, "items": [ { "_id": ObjectId("5b743d9c2c327f8d1b360543"), "order_id": "1", "title": "商品鼠标 1", "price": 50, "num": 1 }, {"_id": ObjectId("5b743da12c327f8d1b360544"), "order_id": "1", "title": "商品键盘 2", "price": 50, "num": 1 }, {"_id": ObjectId("5b74f457089f78dc8f0a4f3b"), "order_id": "1", "title": "商 品键盘 3", "price": 0, "num": 1 } ] }, { "_id": ObjectId("5b743d902c327f8d1b360541"), "trade_no": "222", "all_price": 90, "items": [ { "_id": ObjectId("5b743da52c327f8d1b360545"), "order_id": "2", "title": "牛奶", "price": 50, "num": 1 }, {"_id": ObjectId("5b743da92c327f8d1b360546"), "order_id": "2", "title": "酸奶", "price": 40, "num": 1} ] }
