SQL in a Nutshell, 2nd Edition

The Relational Model and ANSI SQL

Relational Database Management Systems (RDBMSs), such as those covered in this book, are the primary engines of information systems worldwide, particularly web applications and distributed client/server computing systems. They enable a multitude of users to quickly and simultaneously access, create, edit, and manipulate data without impeding or impacting other users. They also allow developers to write useful applications to access their resources as well as provide administrators with the capabilities they need to maintain, secure, and optimize organizational data resources.

An RDBMS is defined as a system whose users view data as a collection of tables related to each other through common data values. Data is stored in tables, which are composed of rows and columns. Tables of independent data can be linked (or related) to one another if they each have unique, identifying columns of data (called keys) that represent a data value held in common. E. F. Codd first described relational database theory in his landmark paper “A Relational Model of Data for Large Shared Data Banks,” published in the Communications of the ACM (Association for Computing Machinery) in June, 1970. Under Codd’s new relational data model, data was structured (into tables of rows and columns); manageable using operations like selections, projections, and joins; and consistent as the result of integrity rules like keys and referential integrity. Codd also articulated rules that governed how a relational database should be designed; the process for applying these rules is now known as normalization.