MySQL is one of the most trendy Open Source Relational SQL database management system. It is one of the best RDBMS database used for developing web based software application. It permits the cost effective delivery of reliable, high performance, scalable web based and embedded database applications. Most of the famous and fast growing organizations like Google, Alcatel Lucent, Facebook, Zappos and Adobe depends on MySQL to save money and time to operate their business critical systems, packaged software and high volume websites.
Oracle Database is an Object Relational Database Management System manufactured and promoted by Oracle Corporation. This the first database specially designed for enterprise grid computing which is the most flexible and cost effective way to handle information and application.
PL/SQL stands for Procedural Language/Structured Query Language. It is Oracle Corporations procedural extension for SQL and the Oracle relational database. This language contains procedural language elements like conditions and loops. It enables declaration of constants and variables, procedures and function's, types and variables of those types, and triggers.
4.Structured Query Language (SQL) (http://sql-plsql.blogspot.in/2007/03/sql-introduction.html)
SQL stands for Structured Query Language used for accessing and manipulating databases. This language is ANSI (American National Standards Institute) and ISO standard for relational database management systems. The most standard SQL commands are "Select", "Insert", "Update", "Delete", "Create", and "Drop" that can be used to achieve everything which one need to do with a database.
PostgreSQL is an open source and powerful Object Relational Database System (ORDBMS). Its primary function is to store data securely, supports best practices as well as retrieve data as per the request of other software applications. PostgreSQL simply called as Postgres, which can manage all workloads ranges from single machine application to large internet facing applications with many synchronized users.
It is a cross platform document oriented database system which is classified as a NoSQL database, MongoDB eschews is the traditional table based relational database structure in favor of JSON like documents using dynamic schemas, making the integration of data in some types of applications easily and quickly.
SQLite is a Relational Database Management System (RDBMS) hold in a C programming library. It is not a client server database engine in the comparison of another database management systems. It is ACID compliant as well as implements many SQL standard with dynamically and weakly typed SQL syntax that does not promises the domain integrity.
Neo4j is the most famous and open source graph database which is implemented in Java. It is manufactured by Neo Technology, Inc. based in the San Francisco Bay Area, US and Malmo, Sweden. The developers defines Neo4j as "embedded, disk based, fully transactional Java persistence engine which stores data structured in graphs rather than in tables".