The Relational Model of Data was introduced in 1970 by Ted Codd of IBM in his paper "A Relational Model of Data for Large Shared Data Banks" Communications of the ACM, Vol 13, No 6, June 1970.

The first relational database management system (RDBMS) was System R, developed in the IBM San Jose laboratory where Codd was based. IBM's DB2, Microsoft's SQL Server and Oracle all bear traces of System R's DNA.

Codd was motivated primarily by the notion of 'data independence': that programs should not need to take account of how data is organized in the machine.

 

An RDBMS uses one or more uniquely-structured tables for each distinct type of thing about which it records data: customers, orders, products etc.

Each table (or "relation") comprises a number of columns. Each row (or "tuple") contains data about an instance of the table's type.

Each row is uniquely identified by the values in one or more of its columns which together are called its "primary key". When a corresponding set of column values in a row (a "foreign key") match the primary key of another row, this records an association between the two rows.

Here's a simple example:

Employees
Employee number
(Primary key)
First name
Family name
Date joined
Department code
(Foreign key)
Salary
123
David
Jones
15-Feb-2005
DEV
£60,000
234
Alice
Hoffman
4-Mar-2008
SLS
£44,000
345
Degar
Bonesh
31-Aug-2007
MKT
£34,000

Departments
Department code
(Primary key)
Department name
Manager's Employee number
(Foreign key)
DEV
Development
123
SLS
Sales
456
SPT
Support
567
MKT
Marketing
789
 

Many people who hear the term "relational" for the first time assume that it refers to relationships between the things in the database.

This is not the case: Codd was a mathematician, and "relation" is a mathematical concept whose definition is:

"Given sets S1, S2, . . ., Sn, R is a relation on these n sets if it is a set of n-tuples, the first component of which is drawn from S1, the second component from S2, and so on."

 
   
     

Applications access relational databases via a data sub-language called SQL, which is used for both data definition and data manipulation.

Originally called Structured English Query Language or SEQUEL, it was invented in 1974 by a research group at IBM in Yorktown Heights, NY.

 

 

Other data sub-languages for the relational model have been proposed, including Codd's "ALPHA", "Query Language" (QUEL) and "Query by Example" (QBE), but SQL is now an ANSI and ISO standard, and is universally accepted as the means of accessing relational databases.

 

     
 
 
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