First we must clear up a common misunderstanding. SQL Server, Oracle, MySQL, DB2 and other similar pieces of software are not relational databases. They are Relational Database Management Systems (RDBMS). RDBMSs support the relational data model but can be used for storage and retrieval as well.
Consider a web application that uses MySQL to store information used to construct web pages. MySQL is an RDBMS but the information in it does not constitute a relational database. It is the structure and nature of the information that makes it a relational database not the software used to store it.
Information in a relational database is inherently categorical data. That is, the information is stored according to categories. In a simple academic relational database we might have categories like Student, Course and Professor. Once the database is populated with information we use SQL to ask questions about the categories. How many students do we have? How many classes are being offered? We can ask much more complicated question such as: How many Students have taken a class with a Professor with the same last name?
The purpose of a Relational Database Management System is to provide storage and retrieval as well as data management for databases some of which are relational. The purpose of a relational database is to store information in properly defined categories so that we can answer questions about the data and the categories.
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