There are two primary types of solutions for databases: SQL and NoSQL – or relational databases and non-relational databases.
Relational databases are structured and have predefined schemas, like phone books that store phone numbers and addresses. Non-relational databases are unstructured, distributed, and have a dynamic schema, like file folders that hold everything from a person’s address and phone number to their Facebook ‘likes’ and online shopping preferences.
Relational databases store data in rows and columns. Each row contains all the information about one entity, and columns are all the separate data points.
Following are the most common forms of NoSQL:
Data is stored in an array of key-value pairs. The “key” is an attribute name, which is linked to a “value”.
Example: Redis, and Memcached.
In these databases, data is stored in documents instead of rows and columns in a table, and these documents are grouped in collections. Each document can have an entirely different structure.
Instead of ‘tables,’ wide-column databases have column families, which are containers for rows. Unlike relational databases, we don’t need to know all the columns upfront, and each row doesn’t have to have the same number of columns. Thus, columnar databases are best suited for analyzing large datasets.
Example: Cassandra and HBase.
These databases are used to store data whose relations are best represented in a graph. Data is saved in graph structures with nodes (entities), properties (information about the entities), and lines (connections between the entities).
Search Engine Databases
They are used to search and analyze unstructured data.
Example: ElasticSearch, Splunk, and Solr
A multi-model database is a database management system designed to support multiple data models against a single, integrated backend.
Example: DynamoDb and CosmosDb
Time Series Databases
They are optimized for fast, high-availability storage and retrieval of time series data in fields such as operations monitoring, application metrics, Internet of Things sensor data, and real-time analytics.
Differences between SQL and NoSQL
SQL stores data in tables, where each row represents an entity, and each column represents a data point about that entity; for example, if we are storing a car entity in a table, different columns could be ‘Color,’ ‘Make,’ ‘Model,’ and so on.
NoSQL databases have different data, storage models. The main ones are key-value, document, graph, and columnar.
In SQL, each record conforms to a fixed schema, meaning the columns must be decided and chosen before data entry, and each row must have data for each column. The schema can be altered later, but it involves modifying the whole database and going offline.
Whereas in NoSQL, schemas are dynamic. Columns can be added on the fly, and each ‘row’ (or equivalent) doesn’t have to contain data for each ‘column.’
SQL databases use SQL (structured query language) to define and manipulate the data, which is very powerful.
In a NoSQL database, queries are focused on a collection of documents. Sometimes it is also called UnQL (Unstructured Query Language). Different databases have different syntax for using UnQL.
SQL databases are vertically scalable in most common situations, i.e., by increasing the horsepower (higher Memory, CPU, etc.) of the hardware, which can get very expensive. It is possible to scale a relational database across multiple servers, but this is a challenging and time-consuming process.
On the other hand, NoSQL databases are horizontally scalable, meaning we can add more servers easily in our NoSQL database infrastructure to handle large traffic. Any cheap commodity hardware or cloud instances can host NoSQL databases, thus making it a lot more cost-effective than vertical scaling. A lot of NoSQL technologies also distribute data across servers automatically.
ACID Compliance (Atomicity, Consistency, Isolation, Durability)
The vast majority of relational databases are ACID compliant. So, when it comes to data reliability and a safe guarantee of performing transactions, SQL databases are still the better bet.
Most of the NoSQL solutions sacrifice ACID compliance for performance and scalability.
Which one to use?
When it comes to database technology, there’s no one-size-fits-all solution. That’s why many businesses rely on both relational and non-relational databases for different needs. Even as NoSQL databases are gaining popularity for their speed and scalability, there are still situations where a highly structured SQL database may perform better; choosing the right technology depends on the use case.
Reasons to use SQL database
- We need to ensure ACID compliance. ACID compliance reduces anomalies and protects the integrity of your database by prescribing exactly how transactions interact with the database. Generally, NoSQL databases sacrifice ACID compliance for scalability and processing speed, but for many e-commerce and financial applications, an ACID-compliant database remains the preferred option.
- Your data is structured and consistent. If your business is not experiencing massive growth that would require more servers, and you are working with data that is consistent, then there may be no reason to use a system designed to support various data types and high traffic volume.
Reasons to use NoSQL database
- When Storing large volumes of data that often have little to no structure. A NoSQL database sets no limits on the types of data we can keep together and allows us to add different new types as the need changes. With document-based databases (like MongoDB and ElasticSearch), you can store data in one place without defining their “types” in advance.
- Making the most of cloud computing and storage. Cloud-based storage is an excellent cost-saving solution but requires data to be easily spread across multiple servers to scale up. Using commodity (affordable, smaller) hardware on-site or in the cloud saves you the hassle of additional software, and NoSQL databases like Cassandra are designed to be scaled across multiple data centers out of the box without a lot of headaches.
- For Rapid development: NoSQL is extremely useful for rapid development as it doesn’t need to be prepped ahead of time. If you’re working on quick iterations of your system that require frequent updates to the data structure without a lot of downtime between versions, a relational database will slow you down.
As of June 2021, 18 out of the top 30 most used databases are relational databases.
Oracle, MySQL, and SQL Server are the most used SQL/Relational databases while MongoDB, Redis, and Elasticsearch are the most used NoSQL databases.