## relational algebra

<database, theory>

A family of algebra with a well-founded semantics used for modelling the data stored in relational databases, and defining queries on it. The main operations of the relational algebra are the set operations (such as union, intersection, and cartesian product), selection (keeping only some lines of a table) and the projection (keeping only some columns).

The relational data model describes how the data is structured.

Codd's reduction algorithm can convert from relational calculus to relational algebra.

Last updated: 1997-02-17

## relational calculus

<database>

An operational methodolgy, founded on predicate calculus, dealing with descripitive expressions that are equivalent to the operations of relational algebra. Codd's reduction algorithm can convert from relational calculus to relational algebra.

Two forms of the relational calculus exist: the tuple calculus and the domain calculus.

["An Introduction To Database Systems" (6th ed), C. J. Date, Addison Wesley].

Last updated: 1998-10-05

## relational database

<database>

(RDBMS - relational database management system) A database based on the relational model developed by E.F. Codd. A relational database allows the definition of data structures, storage and retrieval operations and integrity constraints. In such a database the data and relations between them are organised in tables. A table is a collection of rows or records and each row in a table contains the same fields. Certain fields may be designated as keys, which means that searches for specific values of that field will use indexing to speed them up.

Where fields in two different tables take values from the same set, a join operation can be performed to select related records in the two tables by matching values in those fields. Often, but not always, the fields will have the same name in both tables. For example, an "orders" table might contain (customer_id, product_code) pairs and a "products" table might contain (product_code, price) pairs so to calculate a given customer's bill you would sum the prices of all products ordered by that customer by joining on the product-code fields of the two tables. This can be extended to joining multiple tables on multiple fields. Because these relationships are only specified at retreival time, relational databases are classed as dynamic database management system.

The first commercial RDBMS was the Multics Relational Data Store, first sold in 1978.

INGRES, Oracle, Sybase, Inc., Microsoft Access, and Microsoft SQL Server are well-known database products and companies. Others include PostgreSQL, SQL/DS, and RDB.

["Managing Data Bases, Four Critical Factors" Michael M. Gorman, QED Information Sciences, Inc.].

["An Introduction To Database Systems" (6th ed) C. J. Date, Addison Wesley (an excellent source of detailed info)].

["An End-User's Guide to Data Base" James Martin, Prentice Hall (excellent place to begin learning about DBMS)].

Last updated: 2002-06-10

## relational database management system

relational database

## relational data model

<database>

(Or "relational model") A data model introduced by E.F. Codd in 1970, particularly well suited for business data management. In this model, data are organised in tables. The set of names of the columns is called the "schema" of the table.

Here is an example table with the schema (account number, amount) and 3 lines.

``` account number     amount
--------------  ---------
12343243546456  +30000.00
23149875245824   +2345.33
18479827492874    -123.25

```
The data can be manipulated using a relational algebra. SQL is a standard language for talking to a database built on the relational model (a "relational database").

["A relational model for large shared data banks" Communications of ACM 13:6, pp 377-387].

Last updated: 1998-10-05

## relational DBMS

relational database

## relational language

<language>

Any kind of programming language that specifies output in terms of some property and some arguments. For example, if Tom has two brothers, Dick and Harry, a relational language will respond to the query "Who is Tom's brother?" with either Dick or Harry. Notice that unlike functional languages, relational languages do not require a unique output for each predicate/argument pair. Prolog is the best known relational language.

Last updated: 2004-05-17

## relational model

relational data model

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