Introduction to the Relational Model and ANSI SQL

Relational database management systems (RDBMS) such as those covered on this website are the principal engines of structured data systems worldwide. They enable a multitude of users to quickly and simultaneously access, create, edit, and manipulate data without impacting other users. And they enable developers to write useful applications to access their data while providing administrators with the consistent and reliable capabilities they need to maintain, secure, and optimize organizational data resources.
An RDBMS may be 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 data values held in common between the tables.
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 such as selections, projections, and joins; and consistent as the result of integrity rules such as 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.

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