一、MongoDB 聚合管道（Aggregation Pipeline）
使用聚合管道可以对集合中的文档进行变换和组合。
实际项目：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。
二、MongoDB Aggregation 管道操作符与表达式
$project 增加、删除、重命名字段
$match 条件匹配。只满足条件的文档才能进入下一阶段
$limit 限制结果的数量
$skip 跳过文档的数量
$sort 条件排序。
$group 条件组合结果 统计
$lookup 操作符 用以引入其它集合的数据 （表关联查询）
SQL 和 NOSQL 对比:
管道表达式:
管道操作符作为“键”,所对应的“值”叫做管道表达式。
例如{$match:{status:"A"}}，$match 称为管道操作符，而 status:"A"称为管道表达式，
是管道操作符的操作数(Operand)。
每个管道表达式是一个文档结构，它是由字段名、字段值、和一些表达式操作符组成的。
WHERE----$match
GROUP BY---- $group
HAVING ----$match
SELECT ----$project
ORDER BY ----$sort
LIMIT ----$limit
SUM() ----$sum
COUNT() ----$sum
join ----$lookup
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。
例如{$match:{status:"A"}}，$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。
$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
三、 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})
四、 $project
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 }
}
])
五、 $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}}
}
])
六、 $group
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"}}
}
]
)
七、 $sort
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}
}
])
八、 $limit
db.order.aggregate([
{
$project:{ trade_no:1, all_price:1 }
},
{
$match:{"all_price":{$gte:90}}
},
{
$sort:{"all_price":-1}
},
{
$limit:1
}
])
九、 $skip
db.order.aggregate([
{
$project:{ trade_no:1, all_price:1 }
},
{
$match:{"all_price":{$gte:90}}
},
{
$sort:{"all_price":-1}
},{
$skip:1
}
])
十、 $lookup 表关联
db.order.aggregate([
{
$lookup:
{
from: "order_item",
localField: "order_id",
foreignField: "order_id",
as: "items"
}
}
]){
"_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
}]
}
db.order.aggregate([
{
$lookup:{
from: "order_item",
localField: "order_id",
foreignField: "order_id",
as: "items"
}
},
{
$match:{"all_price":{$gte:90}}
}
])
{
"_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
}]
}
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}},
])
{
"_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}]
}