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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 Edgar F. Codd.
History of the term
Codd introduced the term in his seminal paper A Relational Model of Data for Large Shared Data Banks. 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 in tabular form (as a collection of tables, each table consisting of a set of rows and columns)
- provided operators to manipulate the data in tabular form
The first 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.
Current usage
There is some disagreement about what can or cannot be called a "relational" DBMS.
The most popular definition of a RDBMS is relatively imprecise; some argue that merely presenting a view of data as a collection of rows and columns is sufficient to qualify as a RDBMS. Typically, products that qualify as a RDBMS under this interpretation implement some of Codd's 12 rules, but most popular database systems do not support them all.
Popular commercial DBMSs sold as RDBMSs include Oracle, Microsoft SQL Server, Sybase SQL Server, IBM's DB2, and Microsoft Access. MySQL, PostgreSQL and Firebird are free RDBMSs.
A second school of thought argues that if a database does not implement all of Codd's rules, it is not relational. This view, shared by many theorists and other strict adherents to Codd's principles, would disqualify many database systems from being considered "truly relational". In fact, any database that uses the Structured Query Language (SQL) to access and modify data is not an RDBMS under this definition. Advocates of this philosophy refer to systems that follow some but not all of the rules as Pseudo-Relational Database Management Systems (PRDBMS). For clarification, they often refer to DBMSs that do follow all of the rules Truly-Relational Database Management Systems (TRDBMS).
Recently, the choice of RDBMS has become much less dependent on meeting all of Codd's 12 rules and much more dependent on providing a manageable Total cost of ownership to data management organizations within IT organizations. Today, all RDBMSs with any degree of popularity employ SQL as their query language.
Alphora's Dataphor is a commercially-available RDBMS that follows all of Codd's rules, and is considered a relational DBMS by both groups. However, it is neither popular nor widespread.
External links
- Database Debunkings (http://www.dbdebunk.com)
- 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.
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