NoSQL databases have become increasingly popular in recent years as alternatives to traditional relational databases like SQL. The term “NoSQL” stands for “non-SQL” or “not only SQL”, referring to the fact that these databases do not rely on the traditional relational database model and SQL querying language.
A key question that often comes up is whether NoSQL databases are considered relational or not. The short answer is no, NoSQL databases are generally considered non-relational. However, the longer answer depends on the specific type of NoSQL database and architecture being used.
In this article, we’ll take a deeper look at whether different categories of NoSQL databases can be considered relational or not.
Relational Database Model
First, let’s do a quick recap of what constitutes a relational database. The relational model for databases was introduced by Edgar Codd in 1970 and has been the dominant model used in commercial databases since then.
These are the characteristics of relational databases:
- Data is organized into tables with rows and columns
- Tables have defined relationships between them
- Data adheres to a predefined schema
- SQL is used to manipulate and query data
- ACID (Atomicity, Consistency, Isolation, Durability) principles are applied for reliability
The rigid structure of relational databases makes them great for applications that require complex transactions and analytics. However, the predefined schema can also be restrictive for other applications like web apps where data requirements evolve quickly.
Are NoSQL Databases Relational?
With the background on relational databases, let’s examine different categories of NoSQL databases and whether they can be considered relational or not.
Key-value stores are one of the simplest NoSQL database models. As the name suggests, data is stored as key-value pairs. Some examples are Redis, DynamoDB, Aerospike.
Key-value stores are non-relational as they do not enforce any schema on the data. The values can contain any type of data like strings, JSON, binary data etc. There are no structural relationships enforced between the data either.
Document databases like MongoDB and CouchDB store data in document formats like JSON or XML. The documents contain key-value pairs but can be nested in complex hierarchies.
Document databases are considered non-relational as the data storage does not follow the relational model. The documents do not enforce strict schemas and the relationships between documents are managed programmatically by the application code.
However, some document databases like MongoDB do allow modeling one-to-many relationships between documents using features like document references. But overall, document databases are still considered non-relational.
Wide-column stores like Cassandra and HBase store data in columns instead of rows. Each column can contain huge amounts of data without limiting the number of columns.
Wide-column stores lack structural relationships between columns and do not adhere to relational models. They can be considered non-relational databases.
Graph databases like Neo4J structure data as nodes and edges/relationships. They allow flexible schema modeling and are optimized for storing interconnected data.
These databases allow modeling entity relationships, they do not follow rigid relational structure. Their flexible schemas and non-tabular storage makes them non-relational.
When NoSQL Databases Can Be Relational
Most NoSQL databases are considered non-relational, there are some exceptions where they provide features to work in a relational way:
- Some document databases allow enforcing schemas and foreign key constraints in documents to model relationships. With these additional constraints, they can support relational-style data modeling.
- Certain wide-column stores like Apache Phoenix offer a relational interface on top of a NoSQL database to support SQL querying and joins across columns.
- Some graph databases allow representing entities from a relational database as nodes and relations as edges within the graph. This provides a graph view of the relational data.
However, in general practice, most NoSQL databases are used in a non-relational fashion for their flexible schemas and superior performance at scale. The relational features are usually optional add-ons and not the primary design goal.
- NoSQL databases are considered non-relational as they do not rigidly adhere to the formal relational model like SQL databases.
- Most NoSQL databases lack relational concepts like predefined schemas, foreign keys, and table joins. But some provide optional features to work in a relational way if needed.
- The non-relational nature of NoSQL databases provides them flexibility in data modeling and performance at scale for modern applications.
- For most use cases, NoSQL databases are preferred for their non-relational architecture. But traditional relational databases still excel at complex transactions and analytics.
- NoSQL databases are generally non-relational, they can sometimes be used in a relational style depending on the specific database and its optional features. The key is choosing the right database for your application needs.