A relational database management system is a database management system that is based
on the relational model as introduced by E. F. Codd, of IBM's San Jose Research Laboratory.
Many popular databases currently in use are based on the relational database model.
RDBMSs have become a predominant choice for the storage of information in new databases
used for financial records, manufacturing and logistical information, personnel data,
and much more since the 1980s. Relational databases have often replaced legacy hierarchical
databases and network databases because they are easier to understand and use. However,
relational databases have been challenged by object databases, which were introduced
in an attempt to address the object-relational impedance mismatch in relational database,
and XML databases.
Market share According to research company Gartner, the
five leading commercial relational database vendors by revenue in 2011 were Oracle, IBM,
Microsoft, SAP including Sybase, and Teradata. The three leading open source implementations
are MySQL, PostgreSQL, and SQLite. MariaDB is a prominent fork of MySQL prompted by Oracle's
acquisition of MySQL AB. According to Gartner, in 2008, the percentage
of database sites using any given technology were:
Oracle Database - 70% Microsoft SQL Server - 68%
MySQL - 50% IBM DB2 - 39%
IBM Informix - 18% SAP Sybase Adaptive Server Enterprise - 15%
SAP Sybase IQ - 14% Teradata - 11%
According to DB-Engines, the most popular systems are Oracle, MySQL, Microsoft SQL Server,
PostgreSQL and IBM DB2. History
In 1974, IBM began developing System R, a research project to develop a prototype RDBMS.
Its first commercial product was SQL/DS, released in 1981. However, the first commercially available
RDBMS was Oracle, released in 1979 by Relational Software, now Oracle Corporation. Other examples
of an RDBMS include DB2, SAP Sybase ASE, and Informix.
Historical usage of the term The term "relational database" was invented
by E. F. Codd at IBM in 1970. 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 composed of 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, which at a minimum:
Present the data to the user as relations; Provide 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,
the Massachusetts Institute of Technology;, and from IBM UK Scientific Centre at Peterlee;
IS1 and its followon PRTV. 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 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 most DBMSs as not relational. For clarification,
they often refer to some RDBMSs as Truly-Relational Database Management Systems, naming others
Pseudo-Relational Database Management Systems. As of 2009, most 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.
References