Oracle
An Oracle database is a collection of data treated as a unit. The purpose of a database is to store and retrieve related information. A database server is the key to solving the problems of information management.
SQL
SQL is a standard language for accessing and manipulating databases. Using SQL, you can insert records, update records, and delete records. You can also create new database objects such as databases and tables. And you can drop (delete) them.
MongoDB
The motivation of the MongoDB language is to implement a data store that provides high performance, high availability, and automatic scaling. MongoDB is extremely simple to install and Implement. MongoDB uses JSON or BSON documents to store data.
DB2
"DB2" was used to indicate a shift from hierarchical databases;like the Information Management System (IMS) popular at the time; to the new relational databases." There was a SQL/DS product on MVS mainframe. When they released new relational db  so it become database 2.
