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A relational database management system (RDBMS) is a database management system (DBMS) that is based on the relational model as introduced by E. F. Codd. Relational databases are the most common kind of database in use today (assuming one does not count a file system as a database).^{[citation needed]}

A short definition of a RDBMS may be a DBMS in which data is stored in the form of tables and the relationship among the data is also stored in the form of tables.

E. F. Codd introduced the term in his seminal paper "A Relational Model of Data for Large Shared Data Banks", published in 1970. In this paper and later papers he defined what he meant by relational. One well-known definition of what constitutes a relational database system is Codd's 12 rules. However, many of the early implementations of the relational model did not conform to all of Codd's rules, so the term gradually came to describe a broader class of database systems. At a minimum, these systems::

• presented the data to the user as relations (a presentation in tabular form, i.e. as a collection of tables with each table consisting of a set of rows and columns, can satisfy this property)
• provided relational operators to manipulate the data in tabular form

The first systems that were relatively faithful implementations of the relational model were from the University of Michigan; Micro DBMS (1969) and from IBM UK Scientific Centre at Peterlee; IS1 (1970–72) and its followon PRTV (1973–79). The first system sold as an RDBMS was Multics Relational Data Store, first sold in 1978. Others have been Berkeley Ingres QUEL and IBM BS12.

The most popular definition of an RDBMS is a product that presents a view of data as a collection of rows and columns not based strictly upon relational theory. The majority of real world popular RDBMS products implement some of Codd's 12 rules.

A second, theory-based school of thought argues that if a database does not implement all of Codd's rules (or the current understanding on the relational model, as expressed by Christopher J Date, Hugh Darwen and others), it is not relational. This view, shared by many theorists and other strict adherents to Codd's principles, would disqualify most DBMSs as not relational. For clarification, they often refer to RDBMSs as Truly-Relational Database Management Systems (TRDBMS), naming others Pseudo-Relational Database Management Systems (PRDBMS).

Almost all DBMSes employ SQL as their query language. Alternative query languages have been proposed and implemented, but very few have become commercial products.

Given below is a list of top RDBMS vendors in 2005 with figures in millions of United States Dollars published in a IDC study.

Low adoption costs associated with open-source RDBMS products such as MySQL and PostgreSQL have begun influencing vendor pricing and licensing strategies.

• Comparison of relational database management systems
• Comparison of object-relational database management systems
• Structured Query Language (SQL)
• Life cycle of a relational database
• Navigational Database

• Database Debunkings - by Fabian Pascal. Critical point of view that argues that it is important that the predicate relational should be reserved for those database systems that are fully faithful to the relational model.
• Database Vendors Directory - A list of RDBMS and related products.
• A Brief History of IT Management and the RDBMS - A brief history of modern RDBMS technology from the operational perspective of information technology management practice.
• Analysis of modern DBMS design By an industry analyst who has become a true-relational skeptic.
• The Relational Database Dictionary
• Relational Database Management System FAQs