Relational database management system

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. Most popular commercial and open source databases currently in use are based on the relational model.

A short definition of an 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.

Normalization
A relational database aims to achieve normalization of data. Normalization helps to reduce redundancy and update anomalies. For normalization of data there are some normal forms like:


 * 1) First normal form
 * 2) Second normal form
 * 3) Third normal form
 * 4) BCNF(Boyce-Codd Normal Form)
 * 5) Fourth normal form

Historical usage of the term
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, even if it is not based strictly upon relational theory. By this definition, RDBMS products typically implement some but not all 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 some RDBMSs as Truly-Relational Database Management Systems (TRDBMS), naming others Pseudo-Relational Database Management Systems (PRDBMS).

As of 2009, all commercial relational DBMSes employ SQL as their query language. Alternative query languages have been proposed and implemented, notably the pre-1996 implementation of Berkeley Ingres QUEL. With standardization of the SQL, both commercial and open source DBMSes have adopted some degree of standards compliance.

Market structure
Given below is a list of top RDBMS vendors in 2006 with figures in millions of United States Dollars published in an IDC study.

Low adoption costs associated with open-source RDBMS products such as MySQL, PostgreSQL, Firebird and public domain RDBMS libraries such as SQLite have begun influencing vendor pricing and licensing strategies.

While relational database systems remain the dominant choice for both transactional and analytical applications, newer structures are competing with RDBMSs in data warehouse applications including column-oriented and correlation database systems.