MySQL, Oracle, SQL Server, PostgreSQL, and so on, are some Relational Database Management Systems (RDBMSs) that dominated the database community for decades, until they faced the limitation of scaling to very large-scale datasets. Some important database properties that relational databases follow are Atomicity, Consistency, Isolation, and Durability (ACID). Relational stores can be classified into two major families:

Figure 2.2 illustrates how information in a department can be stored and the relationship between them. The diagram clearly illustrates the relationship that is established between each of the tables. For example, the employees table stores the employee number as the identifier, the birth date, the first name, the last name, the gender, and the hired date. The salaries of the corresponding employees are stored in the salaries table.