The Relational Database
The technical definition of a relational database is a database that stores data as a collection of related entities. These entities are composed of attributes that describe the entity, and each entity has a collection of rows. Another way to think about a relational database is that it stores information on real-world objects (the entities). The information about the objects is contained in the attributes for the object.
Since real world objects have some type of relation to each other, we must have a facility for expressing relations between the objects in the database. The relationships between the database objects is described using a query language, the most popular of which is the Structured Query Language (SQL). (Chapter 2 will describe relational databases and SQL in more detail.)
The relational database is the predominant form of database in use today. Other database types include hierarchical, network, flat-file databases, and object databases. Though the hierarchical database is still common on many mainframe systems, it is not commonly used on other platforms.