What is a Database?
A database is a collection of information that is organized so that it can be easily accessed, managed and updated.
Data is organized into rows, columns and tables, and it is indexed to make it easier to find relevant information. Data gets updated, expanded and deleted as new information is added.
A relational database is one which employs the relational model, in which the raw data is organized into sets of tuples, and the tuples organized into relations. This relational model imposes structure on its contents, in contrast to unstructured or semi-structured data of the various NoSQL architectures.
NoSQL is an umbrella term, one which encompasses a number of different technologies.
These different technologies aren't necessarily related in any way beyond the single defining characteristic of NoSQL: they are not relational in nature. This lack of relational structure results in unstructured or semi-structured data in storage; there may be structure, but it is loose in nature, lax in enforcement.
Structured Query Language
SQL is a relational database query and manipulation language. Its power and flexibility allows for the creation of databases and query tables, and the manipulation and query of data. More recently, the term has become conflated with relational databses, relational databse management systems, and the relational model, at least as a term used in contrast to the term NoSQL.
Popular DBMSs include MongoDB, Cassandra, Redis, MySQL, Microsoft SQL Server, SQLite, and Oracle, among many, many, many others.
Source: Data Science App