Relational Databases

RDBMS(Relational Database Management System):数据库模式中的每个表都有唯一的名称，并定义了其所存储数据的结构,包括:

数据类型:Data types of the data columns
关系:Relationships between tables
Constraints:Primary keys/Foreign keys

RDBMS的局限性:

Schema固定,没法快速修改结构，也不能按需设置动态字段
以来vertical scaling数据量增长是需要扩展单一服务器，centralized而非distributed

NoSQL Databases

代表”Not Only SQL”,而不是“No SQL”,是一类无schema的数据库系统,具有horizontal scalability,代表其可以将数据distribute到多台服务器上,其支持多种不同的数据类型(如图片等) 如:MongoDB 是一个开源、面向文档的数据库，设计时兼顾了可扩展性和开发者的灵活性。与关系数据库中将数据存储在表和行中不同，MongoDB 中你存储的是带有动态模式（无模式、无模式）的 JSON 文档。

CRUD Operations in MongoDB

create

//Create database
db.createCollection("posts") //显式创建：直接创建一个空集合
db.posts.insertOne({"title":"post 1"}) //隐式创建：向不存在的集合中插入item时,会自动创建一个集合
show collections //查看所有collections
 
//Create document
//插入单个文档
db.posts.insertOne(
  {
    "title": "Post 1 Title",
    "author": "Author 1",
    "content": "Content of post 1",
    "tags": ["tag1", "tag2"]
  }
)
//插入多个文档
db.posts.insertMany([
  {
    "title": "Post 1 Title",
    "author": "Author 1",
    "content": "Content of post 1",
    "tags": ["tag1", "tag2"]
  },
  {
    "title": "Post 2 Title", 
    "author": "Author 2",
    "content": "Content of post 2",
    "tags": ["tag3", "tag4"]
  },
  {
    "title": "Post 3 Title",
    "author": "Author 3", 
    "content": "Content of post 3",
    "tags": ["tag1", "tag5"]
  }
])

read

//查找所有文档
db.posts.find() 
db.posts.find({"author": "Author 1"}) 
db.posts.find({                       
  "author": "Author 1",
  "tags": "tag1"
})
 
//查找单个文档
db.posts.findOne({"title": "Post 1 Title"})
 
//查找并返回指定字段/定义Projection参数
db.posts.find(
  {"author": "Author 1"},
  {"title": 1, "author": 1, "_id": 0} //只返回title和author字段，不返回_id
)

Update

//更新单个文档
db.posts.updateOne(
  {"title": "Post 1 Title"},           
  {$set: {"author": "New Author"}}      
)
 
//更新多个文档
db.posts.updateMany(
  {"author": "Author 1"},              
  {$set: {"status": "updated"}}        
)
 
//替换整个文档
db.posts.replaceOne(
  {"title": "Post 1 Title"},            
  {                                      
    "title": "New Post Title",
    "author": "New Author",
    "content": "Updated content of the post",
    "tags": ["updated", "new", "post"],
    "date": new Date(),
    "views": 150
  }
)

Delete

//删除单个文档
db.posts.deleteOne({"title": "Post 1 Title"})
 
//删除多个文档
db.posts.deleteMany({"author": "Author 1"})
 
//删除所有文档
db.posts.deleteMany({})
 
//删除整个集合
db.posts.drop()                         //删除整个集合及其索引
 
//查找并删除
db.posts.findOneAndDelete({"title": "Post to Delete"})

Advanced Query Operators (MQL)

笔记

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