Non-relational (NoSQL)

A non-relational database does not use tables, rows, columns, primary keys, relationships, or schemas. Instead, a NoSQL database stores data using various storage models, depending on the type of data stored.
Due to the lack of a defined structure for the database, NoSQL databases are very scalable and flexible. When dealing with datasets that are not very well defined and structured, a NoSQL database would be the best choice for storing our data.
There are 4 common storage models for NoSQL databases:
- Key-Value
- Document-Based
- Wide-Column
- Graph
Each of the above models has a different way of storing data. For example, the Key-Value model usually stores data in JSON or XML, and has a key for each pair, storing all of its data as its value:


graph LR
    subgraph Posts 
        box1[id
date
content]
        box2[id
date
content]
        box3[id
date
content]
    end

    box1 --> Key1[Key]
    box1 --> Value1[Value]

    box2 --> Key2[Key]
    box2 --> Value2[Value]

    box3 --> Key3[Key]
    box3 --> Value3[Value]

The above example can be represented using JSON as follows:

{
  "100001": {
    "date": "01-01-2021",
    "content": "Welcome to this web application."
  },
  "100002": {
    "date": "02-01-2021",
    "content": "This is the first post on this web app."
  },
  "100003": {
    "date": "02-01-2021",
    "content": "Reminder: Tomorrow is the ..."
  }
}

It looks similar to a dictionary/map/key-value pair in languages like Python or PHP ‘i.e. {'key':'value'}’, where the key is usually a string, the value can be a string, dictionary, or any class object.
The Document-Based model stores data in complex JSON objects and each object has certain meta-data while storing the rest of the data similarly to the Key-Value model.
Some of the most common NoSQL databases include:

Type	Description
MongoDB	The most common `NoSQL` database. It is free and open-source, uses the `Document-Based` model, and stores data in `JSON` objects
ElasticSearch	Another free and open-source `NoSQL` database. It is optimized for storing and analyzing huge datasets. As its name suggests, searching for data within this database is very fast and efficient
Apache Cassandra	Also free and open-source. It is very scalable and is optimized for gracefully handling faulty values

Other common NoSQL databases include: Redis, Neo4j, CouchDB, and Amazon DynamoDB.

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