What are databases?

What is SQL?

From guru99:

Structured Query language (SQL) pronounced as "S-Q-L" or sometimes as "See-Quel" is the standard language for dealing with Relational Databases. A relational database defines relationships in the form of tables.

SQL programming can be effectively used to insert, search, update, delete database records.

That doesn't mean SQL cannot do things beyond that. It can do a lot of things including, but not limited to, optimizing and maintenance of databases.

Relational databases like MySQL Database, Oracle, Ms SQL Server, Sybase, etc. use SQL.

What is NoSQL?

NoSQL is used for Big data and real-time web apps. For example companies like Twitter, Facebook, Google that collect terabytes of user data every single day. 

What is firebase Realtime database?