You’ll find that most databases used in businesses these days are relational databases, as opposed to a flat file or hierarchical database. Relational databases have the clout to handle multitudes of data and complex queries, whereas a flat file takes up more space and memory, and is less efficient.

PL/SQL
Communication that database understands. Whether its for recreational or manipulation purpose.
Mongo DB
MongoDB is an open-source document database and leading NoSQL database.