通过实例讲清楚MongoDB九种聚合操作
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1 文章概述
1.1 基本概念
MongoDB是一种非关系型数据库，数据最终存储为BSON（Binary JSON）类型。MongoDB包含三个重要逻辑概念：数据库、集合和文档，与关系型数据库相关概念映射如下图：
1.2 数据准备
1.2.1 逻辑设计
bookdb数据库包含library与book两个集合，相关字段说明如下图：
1.2.2 新增数据
// 创建数据库
use bookdb
// 新增图书馆
db.library.insertMany([
{
"_id": 1,
"library_name": "图书馆_1"
},
{
"_id": 2,
"library_name": "图书馆_2"
},
{
"_id": 3,
"library_name": "图书馆_3"
}
])
// 新增图书
db.book.insertMany([
{
"_id": 1,
"book_type": 1,
"book_name": "图书_1",
"book_price": 10,
"book_count": 100,
"borrowers": [{
"name": "张三",
"id": 100
}, {
"name": "李四",
"id": 200
}],
"library_id": 1
},
{
"_id": 2,
"book_type": 2,
"book_name": "图书_2",
"book_price": 20,
"book_count": 100,
"borrowers": [{
"name": "张三",
"id": 100
}],
"library_id": 2
},
{
"_id": 3,
"book_type": 1,
"book_name": "图书_3",
"book_price": 30,
"book_count": 100,
"borrowers": [{
"name": "张三",
"id": 100
}, {
"name": "王五",
"id": 300
}],
"library_id": 2
}
])2 聚合概念
2.1 基本语法
MongoDB聚合语法第一个参数是管道：文档在上个管道处理完后传递给下个管道，第二个参数是选项：设置聚合操作特性
db.collection.aggregate(pipeline, options)2.2 聚合管道
常用聚合管道有以下九种类型：
$project：对文档进行投影
$limit：输出管道内前N个文档
$skip：跳过管道内前N个文档
$sort：对文档进行排序
$out：输出管道中文档
$match：对文档进行筛选
$unwind：铺平文档中的数组字段
$lookup：对文档进行查询
$group：对文档进行分组
3 聚合实例
3.1 project
1表示展示某字段
0表示不展示某字段
借阅人编号和姓名拆分成两个数组
db.book.aggregate({
$project: {
"_id": 0,
"book_name": 1,
"borrowerIds": "$borrowers.id",
"borrowerNames": "$borrowers.name"
}
})
---------------------------------------------------------
// 1
{
"book_name": "图书_1",
"borrowerIds": [
100,
200
],
"borrowerNames": [
"张三",
"李四"
]
}
// 2
{
"book_name": "图书_2",
"borrowerIds": [
100
],
"borrowerNames": [
"张三"
]
}
// 3
{
"book_name": "图书_3",
"borrowerIds": [
100,
300
],
"borrowerNames": [
"张三",
"王五"
]
}3.2 limit
只展示一个投影结果
db.book.aggregate([
{
$project: {
"_id": 0,
"book_name": 1,
"borrowerIds": "$borrowers.id",
"borrowerNames": "$borrowers.name"
}
},
{
$limit: 1
}
])
---------------------------------------------------------
// 1
{
"book_name": "图书_1",
"borrowerIds": [
100,
200
],
"borrowerNames": [
"张三",
"李四"
]
}3.3 skip
跳过一个且只展示一个投影结果
db.book.aggregate([
{
$project: {
"_id": 0,
"book_name": 1,
"borrowerIds": "$borrowers.id",
"borrowerNames": "$borrowers.name"
}
},
{
$skip: 1
},
{
$limit: 1
}
])
---------------------------------------------------------
// 1
{
"book_name": "图书_2",
"borrowerIds": [
100
],
"borrowerNames": [
"张三"
]
}3.4 sort
db.book.aggregate([
{
$project: {
"_id": 1,
"book_name": 1,
"library_id": 1
}
},
{
$sort: {
"library_id": - 1, // 降序
"_id": 1 // 升序
}
}
])
---------------------------------------------------------
// 1
{
"_id": 2,
"book_name": "图书_2",
"library_id": 2
}
// 2
{
"_id": 3,
"book_name": "图书_3",
"library_id": 2
}
// 3
{
"_id": 1,
"book_name": "图书_1",
"library_id": 1
}MongoDB内存排序有100M限制，如果排序数据过多需要设置选项allowDiskUse=true，表示数据可以写入临时文件进行排序
db.book.aggregate([
{
$project: {
"_id": 1,
"book_name": 1,
"library_id": 1
}
},
{
$sort: {
"library_id": - 1,
"_id": 1
}
}
], {
allowDiskUse: true
})3.5 out
投影结果输出到新集合
db.book.aggregate([
{
$project: {
"_id": 0,
"book_name": 1,
"borrowerIds": "$borrowers.id",
"borrowerNames": "$borrowers.name"
}
},
{
$out: "newCollection"
}
])
db.newCollection.find()
---------------------------------------------------------
// 1
{
"_id": ObjectId("62bec0636f9c37787b9590b9"),
"book_name": "图书_1",
"borrowerIds": [
100,
200
],
"borrowerNames": [
"张三",
"李四"
]
}
// 2
{
"_id": ObjectId("62bec0636f9c37787b9590ba"),
"book_name": "图书_2",
"borrowerIds": [
100
],
"borrowerNames": [
"张三"
]
}
// 3
{
"_id": ObjectId("62bec0636f9c37787b9590bb"),
"book_name": "图书_3",
"borrowerIds": [
100,
300
],
"borrowerNames": [
"张三",
"王五"
]
}3.6 match
where book_name = xxx
db.book.aggregate([
{
$match: {
"book_name": "图书_2"
}
},
{
$project: {
"_id": 1,
"book_name": 1,
"library_id": 1
}
}
])
---------------------------------------------------------
// 1
{
"_id": 2,
"book_name": "图书_2",
"library_id": 2
}where library_id = 2 and price > 15
db.book.aggregate([
{
$match: {
$and: [
{
"library_id": 2
},
{
"book_price": {
$gt: 25
}
}
]
}
},
{
$project: {
"_id": 1,
"book_name": 1,
"library_id": 1,
"book_price": 1
}
}
])
---------------------------------------------------------
// 1
{
"_id": 3,
"book_name": "图书_3",
"book_price": 30,
"library_id": 2
}3.7 unwind
文档按照借阅人数组铺平
includeArrayIndex表示索引
db.book.aggregate([
{
$unwind: {
path: "$borrowers",
includeArrayIndex: "idx"
}
}
])
---------------------------------------------------------
// 1
{
"_id": 1,
"book_type": 1,
"book_name": "图书_1",
"book_price": 10,
"book_count": 100,
"borrowers": {
"name": "张三",
"id": 100
},
"library_id": 1,
"idx": NumberLong("0")
}
// 2
{
"_id": 1,
"book_type": 1,
"book_name": "图书_1",
"book_price": 10,
"book_count": 100,
"borrowers": {
"name": "李四",
"id": 200
},
"library_id": 1,
"idx": NumberLong("1")
}
// 3
{
"_id": 2,
"book_type": 2,
"book_name": "图书_2",
"book_price": 20,
"book_count": 100,
"borrowers": {
"name": "张三",
"id": 100
},
"library_id": 2,
"idx": NumberLong("0")
}
// 4
{
"_id": 3,
"book_type": 1,
"book_name": "图书_3",
"book_price": 30,
"book_count": 100,
"borrowers": {
"name": "张三",
"id": 100
},
"library_id": 2,
"idx": NumberLong("0")
}
// 5
{
"_id": 3,
"book_type": 1,
"book_name": "图书_3",
"book_price": 30,
"book_count": 100,
"borrowers": {
"name": "王五",
"id": 300
},
"library_id": 2,
"idx": NumberLong("1")
}3.8 lookup
查询图书馆有哪些图书
lookup可以实现连表查询
MongoDB 3.4之前聚合语法：
from：待关联集合【book】
localField: 本集合关联键【library】
foreignField：待关联键【book】
as：待关联集合数据【book】
db.library.aggregate([
{
$lookup:
{
from: "book",
localField: "_id",
foreignField: "library_id",
as: "books_info"
}
}
])
---------------------------------------------------------
// 1
{
"_id": 1,
"library_name": "图书馆_1",
"books_info": [
{
"_id": 1,
"book_type": 1,
"book_name": "图书_1",
"book_price": 10,
"book_count": 100,
"borrowers": [
{
"name": "张三",
"id": 100
},
{
"name": "李四",
"id": 200
}
],
"library_id": 1
}
]
}
// 2
{
"_id": 2,
"library_name": "图书馆_2",
"books_info": [
{
"_id": 2,
"book_type": 2,
"book_name": "图书_2",
"book_price": 20,
"book_count": 100,
"borrowers": [
{
"name": "张三",
"id": 100
}
],
"library_id": 2
},
{
"_id": 3,
"book_type": 1,
"book_name": "图书_3",
"book_price": 30,
"book_count": 100,
"borrowers": [
{
"name": "张三",
"id": 100
},
{
"name": "王五",
"id": 300
}
],
"library_id": 2
}
]
}
// 3
{
"_id": 3,
"library_name": "图书馆_3",
"books_info": [ ]
}MongoDB 3.4之后聚合语法：
from：待关联集合【book】
let：声明本集合字段在管道使用
pipeline：操作管道
db.library.aggregate([
{
$lookup:
{
from: "book",
let: {
"lid": "$_id"
},
pipeline: [
{
$match: {
$expr: {
$and: [
{
$eq: ["$$lid", "$library_id"]
}
]
}
}
}
],
as: "books_info"
}
}
])
---------------------------------------------------------
// 1
{
"_id": 1,
"library_name": "图书馆_1",
"books_info": [
{
"_id": 1,
"book_type": 1,
"book_name": "图书_1",
"book_price": 10,
"book_count": 100,
"borrowers": [
{
"name": "张三",
"id": 100
},
{
"name": "李四",
"id": 200
}
],
"library_id": 1
}
]
}
// 2
{
"_id": 2,
"library_name": "图书馆_2",
"books_info": [
{
"_id": 2,
"book_type": 2,
"book_name": "图书_2",
"book_price": 20,
"book_count": 100,
"borrowers": [
{
"name": "张三",
"id": 100
}
],
"library_id": 2
},
{
"_id": 3,
"book_type": 1,
"book_name": "图书_3",
"book_price": 30,
"book_count": 100,
"borrowers": [
{
"name": "张三",
"id": 100
},
{
"name": "王五",
"id": 300
}
],
"library_id": 2
}
]
}
// 3
{
"_id": 3,
"library_name": "图书馆_3",
"books_info": [ ]
}新增价格大于20查询条件
db.library.aggregate([
{
$lookup:
{
from: "book",
let: {
"lid": "$_id"
},
pipeline: [
{
$match: {
$expr: {
$and: [
{
$eq: ["$$lid", "$library_id"]
},
{
$gt: ["$book_price", 20]
}
]
}
}
}
],
as: "books_info"
}
}
])
---------------------------------------------------------
// 1
{
"_id": 1,
"library_name": "图书馆_1",
"books_info": [ ]
}
// 2
{
"_id": 2,
"library_name": "图书馆_2",
"books_info": [
{
"_id": 3,
"book_type": 1,
"book_name": "图书_3",
"book_price": 30,
"book_count": 100,
"borrowers": [
{
"name": "张三",
"id": 100
},
{
"name": "王五",
"id": 300
}
],
"library_id": 2
}
]
}
// 3
{
"_id": 3,
"library_name": "图书馆_3",
"books_info": [ ]
}3.9 group
3.9.1 简单统计
_id：图书类型作为分组键
count：每个类型有多少种书
db.book.aggregate([
{
$group: {
_id: "$book_type",
count: {
$sum: 1
}
}
}
])
---------------------------------------------------------
// 1
{
"_id": 2,
"count": 1
}
// 2
{
"_id": 1,
"count": 2
}3.9.2 复杂统计
_id：图书类型作为分组键
type_count：每个类型有多少种书
type_book_count：每个类型有多少本书
minTotalPrice：每个类型总价最小值
maxTotalPrice：每个类型总价最大值
totalPrice：每个类型总价
avgPrice：每个类型平均价
db.book.aggregate([
{
$group: {
_id: "$book_type",
type_count: {
$sum: 1
},
type_book_count: {
$sum: "$book_count"
},
minTotalPrice: {
$min: {
$multiply: ["$book_price", "$book_count"]
}
},
maxTotalPrice: {
$max: {
$multiply: ["$book_price", "$book_count"]
}
},
totalPrice: {
$sum: {
$multiply: ["$book_price", "$book_count"]
}
},
avgPrice: {
$avg: "$book_price"
}
}
}
])
---------------------------------------------------------
// 1
{
"_id": 2,
"type_count": 1,
"type_book_count": 100,
"minTotalPrice": 2000,
"maxTotalPrice": 2000,
"totalPrice": 2000,
"avgPrice": 20
}
// 2
{
"_id": 1,
"type_count": 2,
"type_book_count": 200,
"minTotalPrice": 1000,
"maxTotalPrice": 3000,
"totalPrice": 4000,
"avgPrice": 20
}3.9.3 空分组键
_id：空分组键表示统计全量数据
db.book.aggregate([
{
$group: {
_id: null,
type_count: {
$sum: 1
},
type_book_count: {
$sum: "$book_count"
},
minTotalPrice: {
$min: {
$multiply: ["$book_price", "$book_count"]
}
},
maxTotalPrice: {
$max: {
$multiply: ["$book_price", "$book_count"]
}
},
totalPrice: {
$sum: {
$multiply: ["$book_price", "$book_count"]
}
},
avgPrice: {
$avg: "$book_price"
}
}
}
])
---------------------------------------------------------
// 1
{
"_id": null,
"type_count": 3,
"type_book_count": 300,
"minTotalPrice": 1000,
"maxTotalPrice": 3000,
"totalPrice": 6000,
"avgPrice": 20
}4 文章总结
第一介绍了MongoDB与关系型数据库关系，并且准备本文需要的测试数据，第二介绍了聚合语法和聚合管道相关概念，第三通过实例介绍了如何使用聚合操作，希望本文对大家有所帮助。
JAVA前线
欢迎大家关注公众号「JAVA前线」查看更多精彩分享，主要内容包括源码分析、实际应用、架构思维、职场分享、产品思考等等，同时也非常欢迎大家加我微信「java_front」一起交流学习
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