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Relational database management system

A relational database management system is a database management system that is based on the relational model as introduced by Edgar F. Codd. Strictly speaking it should also satisfy Codd's 12 rules but in practice there is no DBMS that satisfies all these rules. In fact, most successful DBMSs (and the query language SQL) that are considered to be relational violate the relational model in several important ways. However, most database practitioners and researchers use the term in a loose way such that most databases that support SQL are also included.

The first released RDBMS that was a relatively faithful implementation of the relational model was the Multics Relational Data Store, first sold in 1978. Others have been Berkeley Ingres QUEL and IBM BS12.

Today, popular commercial RDBMS's for large databases include Oracle, Microsoft SQL Server, Sybase SQL Server, and IBM's DB2. The most commonly used free RDBMS's are MySQL and PostgreSQL.