What is the database? 
 In computer science, a database management system usually refers to the software that stores and organizes reliably, and provides efficient and simple methods to modify and access data for end-users and applications.

The database systems are classified according to the database model they support. The most popular database models include: relational, object, key-value and document model. After the 1980s, the relational database dominated the industry and other models were used in niche environments.

The relation database is widely accepted because it provides a highly abstract and simple interface, which organizes data into rows and columns of a set of tables, that reduces the complexity for end-users and applications. In relational databases, the SQL language is used to modify and access data.

Different database systems have different core functionalities based on their targeting working environments. However, most of them share following characteristics:
-- Store, modify and access data efficiently,
-- Provide access to the metadata or the catalog of user data,
-- Handle concurrent modification with some degree of consistency,
-- Support transaction basic isolation level,
-- Control the access control to user data for security enforcement.