Relational databases have been at the top of the database programming world for over 50 years. They were invented by E.F. Codd while working at IBM's San Jose Research Laboratory. NoSQL databases also known as no-relational databases are the new wave of formless data stores that don't use any schema i.e (formless). They work with all kinds of data including data collected from smart city IoT sensors. The four main ones that have a solid user interface and an online free trial are.
www.RavenDB.com - Raven is a document datastore NoSQL database, that is entirely schema-less meaning no primary and foreign key relationships are required. Raven has an easy-to-use user interface and many functions for new database developers. RQL Raven querying languages is the database programming language used to write, read, update and delete records.
www.Aerospike.com - If transactions are your main goal then Aerospike is the right choice. Aerospike delivers speed when you need to access important read or write functions on credit card reports. Aerospike DB can handle acid transactions more efficiently than most relational databases since it was designed to do this.
www.Neo4j.com - Neo4j is a NoSQL graph database that is truly a winner in the non-relational database world. Ne04j is used in many of the most popular web apps since the data can be visualized in simple nodes.
www.datastax.com - Datastax astra is a serverless NoSQL multi module Column-Oriented database owned by parent company Cassandra. CQL with an almost SQL a-like syntax is the database programming language used to write, read, update and, delete records. API access is also made available through API services like GraphQL and Swagger.
A few others worth a mention are MongoDB and Firebase-DB both document based databases. For new beginners in the world of non-relational database technology, I would recommend. This technology is fairly new so don't be scared off and choose the best one that meets your development needs. My final point is that relational databases stack vertically (scale-up) while non-relational SQL databases stack horizontally (scale-out). Less resources are used when scaling-out across the network. Services are shared between different servers, this saves on costs and reduces the power consumed.
