relational algebra - definition. What is relational algebra
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%ما هو (من)٪ 1 - تعريف

FAMILY OF ALGEBRAS USED FOR MODELLING THE DATA STORED IN RELATIONAL DATABASES, AND DEFINING QUERIES ON IT
Natural join; Antijoin; Relational Algebra; Semijoin; ▷; Join (relational algebra); ⋈; Relational logic; Theta join; Extended projection; ⟕; ⟖; ⟗; ⨝

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. (1997-02-17)
Relational algebra         
In database theory, relational algebra is a theory that uses algebraic structures with a well-founded semantics for modeling data, and defining queries on it. The theory was introduced by Edgar F.
Relational art         
TENDENCY IN FINE ART
Relational Aesthetics; Relational Esthetics; L'esthétique relationnelle; Relational Art; Esthétique relationnelle; Relation Aesthetics; Relation Art
Relational art or relational aesthetics is a mode or tendency in fine art practice originally observed and highlighted by French art critic Nicolas Bourriaud. Bourriaud defined the approach as "a set of artistic practices which take as their theoretical and practical point of departure the whole of human relations and their social context, rather than an independent and private space.

ويكيبيديا

Relational algebra

In database theory, relational algebra is a theory that uses algebraic structures for modeling data, and defining queries on it with a well founded semantics. The theory was introduced by Edgar F. Codd.

The main application of relational algebra is to provide a theoretical foundation for relational databases, particularly query languages for such databases, chief among which is SQL. Relational databases store tabular data represented as relations. Queries over relational databases often likewise return tabular data represented as relations.

The main purpose of the relational algebra is to define operators that transform one or more input relations to an output relation. Given that these operators accept relations as input and produce relations as output, they can be combined and used to express potentially complex queries that transform potentially many input relations (whose data are stored in the database) into a single output relation (the query results).

Unary operators accept as input a single relation; examples include operators to filter certain attributes (columns) or tuples (rows) from an input relation.

Binary operators accept as input two relations; such operators combine the two input relations into a single output relation by, for example, taking all tuples found in either relation, removing tuples from the first relation found in the second relation, extending the tuples of the first relation with tuples in the second relation matching certain conditions, and so forth.

Other more advanced operators can also be included, where the inclusion or exclusion of certain operators gives rise to a family of algebras.