Introduction to mongodb chapter 03 basic concepts of mongodb

This article will introduce MongoDB Some basic concepts in , For example, documents 、 aggregate 、 Database and namespace .

data format

stay MongoDB in , Data usage JSON perhaps BSON Format for processing and storage .

JSON

JSON Its full name is JavaScript Object Notation, Is a lightweight data exchange format .JSON Grammar is based on JavaScript ECMA-262 3rd edition A subset of .

One JSON A document is a collection of fields and values , Follow a specific format . for example ：

{
    
   "first_name": "John",
   "last_name": "Doe",
   "age": 22,
   "skills": ["Programming","Databases", "API"]
}

BSON

BSON representative Binary JSON, It's a kind of JSON Binary serialization of documents .BSON Pay more attention to the efficiency of storage and processing .

file

MongoDB With BSON Store data records in the form of documents , Short for document （Document）.

Document is a field - A collection of values , The structure is as follows ：

{
    
   field_name1: value1,
   field_name2: value2,
   field_name3: value3,
   ...
}

In the above grammar , The field name is a string , Field values can be numbers 、 character string 、 object 、 Array etc. . for example ：

{
    
    _id: ObjectId("5f339953491024badf1138ec"),
    title: "SQL Programming idea ",
    isbn: "9787121421402",
    publisher: " Electronic industry press ",
    published_date: new Date('2021-10-01'),
    author: {
     first_name: " Xuyang ", last_name: " Dong "}
}

This document contains the following fields - value ：

• _id It's a ObjectId;
• title Is a string ;
• isbn Is a string ;
• publisher Is a string ;
• published_date It's a Date Type data ;
• author Is an embedded document , Contains two fields ： first_name and last_name.

If you know relational data , You can see that a document is like a row of records in a table , But it is more expressive than data rows .

The field names in the document need to follow the following rules ：

• MongoDB Keep field _id Used to uniquely identify a document ;
• Field names cannot contain null character ;
• The outermost field name cannot be entered with $ Symbol start .

aggregate

MongoDB Use set （Colletion） Storing documents . A collection is a set of documents .

MongoDB Collections in are similar to tables in relational databases .

MongoDB	RDBMS
file	That's ok
aggregate	surface

Tables have fixed patterns （ Field definition ）, But the pattern of set is dynamic . Dynamic mode means that the structure of multiple documents in the collection may be completely different . for example , The following documents can be stored in the same collection ：

{
    
    title: "SQL Programming idea ",
    published_date: new Date('2021-10-01')
}

{
    
    title: "MongoDB From entry to business practice ",
    published_date: new Date('2019-09-01'),
    isbn": "9787121372247"
}

The first 2 Documents than 1 Multiple documents with one field . In theory , Each document can have different fields .

A collection has a name , for example books. Collective nouns cannot contain the following ：

• $ character ;
• null（\0） character ;
• An empty string ;
• With system start , because system* yes MongoDB Reserved name of internal set .

database

MongoDB The collection is stored in the database . Single MongoDB Instances can contain multiple databases （Database）.

The database can be referenced by name , for example bookdb. The database name cannot be the following ：

• An empty string （“”）;
• Contains the following characters ：/、\、.、“、*、<、>、:、|、?、$、 Space or \0（ Null character ）;
• The length exceeds 64 byte .

MongoDB It also contains some reserved database names , for example admin、local as well as config, We cannot use these names to create new databases .

Namespace

Namespace （Namespace） It consists of the database name plus the collection name . Namespaces can be used to fully reference a collection .

for example , For the database bookdb The set in books, Namespace is bookdb.books.