NoSQL databases represent a category of non-relational database systems designed for high availability, scalability, and flexibility with various data models. The four primary types of NoSQL databases are key-value stores, document databases, column-family stores, and graph databases. A key-value store is one of the most fundamental NoSQL models, storing data as a simple collection of key-value pairs. The key is a unique identifier used to retrieve an associated value, making this model highly efficient for session management, caching, and user profiles where data is accessed via a known key.

A. Hierarchical: This is an early, pre-relational database model with a tree-like structure, distinct from the modern NoSQL database categories.

C. Cloud: This refers to a deployment model or environment (e.g., public, private, hybrid cloud) and not a type of database structure.

D. Object-oriented: This is a separate database model that stores data as objects, as defined in object-oriented programming, and is not one of the primary NoSQL types.

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1. Carnegie Mellon University

Database Group. (2022). 15-445/645 Intro to Database Systems

Lecture #21: NoSQL Databases. Slide 17

"NoSQL System Types". The lecture explicitly categorizes NoSQL systems into four types: Key-Value

Document

Column-Family

and Graph. Retrieved from https://15445.courses.cs.cmu.edu/fall2022/slides/21-nosql.pdf

2. Amazon Web Services (AWS). (n.d.). What is a Key-Value Database? Official AWS Documentation. In the introductory paragraph

it states

"A key-value database is a type of nonrelational database... Key-value databases are one of four common types of NoSQL databases..." Retrieved from https://aws.amazon.com/nosql/key-value/

3. Han

J.

Kamber

M.

& Pei

J. (2012). Data Mining: Concepts and Techniques (3rd ed.). Morgan Kaufmann. In Chapter 1

Section 1.4.3

"Other Kinds of Data

" the text distinguishes between traditional database models (relational

hierarchical

network) and newer systems

including object-oriented databases and transactional databases

while implicitly placing NoSQL types like key-value stores in a modern context for handling unstructured data.