Codd's theorem states that relational algebra and the domain-independent relational calculus queries, two well-known foundational query languages for the relational model, are precisely equivalent in expressive power. That is, a database query can be formulated in one language if and only if it can be expressed in the other. The theorem is named after Edgar F. Codd, the father of the relational model for database management. Relational calculus is essentially equivalent to first-order logic, and indeed, Codd's Theorem had been known to logicians since the late 1940s.
| Attributes | Values |
|---|
| type
| |
| sameAs
| |
| wasDerivedFrom
| |
| dbpedia-owl:abstract
| - El teorema de Codd afirma que l' àlgebra relacional i les consultes de càlcul relacional independents del domini, dos llenguatges de consulta fundacionals coneguts per al model relacional, són exactament equivalents en potència expressiva. És a dir, una consulta de base de dades es pot formular en un idioma si i només si es pot expressar en l’altre. El teorema porta el nom d’ Edgar F. Codd, el pare del model relacional per a la gestió de bases de dades. The domain independent relational calculus queries are precisely those relational calculus queries that are invariant under choosing domains of values beyond those appearing in the database itself. That is, queries that may return different results for different domains are excluded. An example of such a forbidden query is the query "select all tuples other than those occurring in relation R", where R is a relation in the database. Assuming different domains, i.e., sets of atomic data items from which tuples can be constructed, this query returns different results and thus is clearly not domain independent. El teorema de Codd és notable ja que estableix l'equivalència de dos llenguatges sintàcticament força diferents: l’ àlgebra relacional és un llenguatge lliure de variables, mentre que el càlcul relacional és un llenguatge lògic amb variables i quantificació . El càlcul relacional és essencialment equivalent a la lògica de primer ordre, i, de fet, el teorema de Codd era conegut pels lògics des de finals dels anys quaranta. Codd va anomenar relacionalment complets als llenguatges de consulta equivalents en potència expressiva a l’àlgebra relacional . Pel teorema de Codd, això inclou el càlcul relacional. La integritat relacional clarament no implica que cap consulta de base de dades interessant es pugui expressar en idiomes relacionalment complets. Exemples coneguts de consultes inexpressables inclouen agregacions simples (comptar tuples o sumar valors que es produeixen en tuples, que són operacions expressables en SQL però no en àlgebra relacional) i calcular el tancament transitiu d’un gràfic donat per la seva relació d’aresta binària (vegeu també poder expressiu ). El teorema de Codd tampoc no considera nuls SQL i la lògica de tres valors que comporten; el tractament lògic dels nuls continua sumit en la controvèrsia. A més, SQL té una semàntica multiconjunt i permet duplicar files. Tot i això, la integritat relacional constitueix un punt de referència important pel qual es pot comparar el poder expressiu dels llenguatges de consulta.
- Codd's theorem states that relational algebra and the domain-independent relational calculus queries, two well-known foundational query languages for the relational model, are precisely equivalent in expressive power. That is, a database query can be formulated in one language if and only if it can be expressed in the other. The theorem is named after Edgar F. Codd, the father of the relational model for database management. The domain independent relational calculus queries are precisely those relational calculus queries that are invariant under choosing domains of values beyond those appearing in the database itself. That is, queries that may return different results for different domains are excluded. An example of such a forbidden query is the query "select all tuples other than those occurring in relation R", where R is a relation in the database. Assuming different domains, i.e., sets of atomic data items from which tuples can be constructed, this query returns different results and thus is clearly not domain independent. Codd's Theorem is notable since it establishes the equivalence of two syntactically quite dissimilar languages: relational algebra is a variable-free language, while relational calculus is a logical language with variables and quantification. Relational calculus is essentially equivalent to first-order logic, and indeed, Codd's Theorem had been known to logicians since the late 1940s. Query languages that are equivalent in expressive power to relational algebra were called relationally complete by Codd. By Codd's Theorem, this includes relational calculus. Relational completeness clearly does not imply that any interesting database query can be expressed in relationally complete languages. Well-known examples of inexpressible queries include simple aggregations (counting tuples, or summing up values occurring in tuples, which are operations expressible in SQL but not in relational algebra) and computing the transitive closure of a graph given by its binary edge relation (see also expressive power). Codd's theorem also doesn't consider SQL nulls and the three-valued logic they entail; the logical treatment of nulls remains mired in controversy. Additionally, SQL has multiset semantics and allows duplicate rows. Nevertheless, relational completeness constitutes an important yardstick by which the expressive power of query languages can be compared.
- En théorie des bases de données, le théorème de Codd affirme l'équivalence entre l'algèbre relationnelle et le calcul relationnel (restreint aux requêtes indépendantes du domaine). Ce théorème est important pour les bases de données relationnelle, car il assure que toute requête « naturelle » (i.e. du calcul relationnel) peut se traduire en algèbre relationnelle, et donc en un langage de requêtes intelligible par un ordinateur (en particulier le SQL). Ce théorème a été démontré par Edgar Frank Codd en 1971.
|
| dbpedia-owl:wikiPageExternalLink
| |
| dbpedia-owl:wikiPageID
| |
| dbpedia-owl:wikiPageRevisionID
| |
| comment
| - El teorema de Codd afirma que l' àlgebra relacional i les consultes de càlcul relacional independents del domini, dos llenguatges de consulta fundacionals coneguts per al model relacional, són exactament equivalents en potència expressiva. És a dir, una consulta de base de dades es pot formular en un idioma si i només si es pot expressar en l’altre. El teorema porta el nom d’ Edgar F. Codd, el pare del model relacional per a la gestió de bases de dades.
- Codd's theorem states that relational algebra and the domain-independent relational calculus queries, two well-known foundational query languages for the relational model, are precisely equivalent in expressive power. That is, a database query can be formulated in one language if and only if it can be expressed in the other. The theorem is named after Edgar F. Codd, the father of the relational model for database management. Relational calculus is essentially equivalent to first-order logic, and indeed, Codd's Theorem had been known to logicians since the late 1940s.
- En théorie des bases de données, le théorème de Codd affirme l'équivalence entre l'algèbre relationnelle et le calcul relationnel (restreint aux requêtes indépendantes du domaine). Ce théorème est important pour les bases de données relationnelle, car il assure que toute requête « naturelle » (i.e. du calcul relationnel) peut se traduire en algèbre relationnelle, et donc en un langage de requêtes intelligible par un ordinateur (en particulier le SQL). Ce théorème a été démontré par Edgar Frank Codd en 1971.
|
| label
| - Codd's theorem
- Teorema de Codd
- Teorema di Codd
- Théorème de Codd
|
| dbpprop:wikiPageUsesTemplate
| |
| described by
| |
| topic
| |
| Subject
| |
| is primary topic of
| |
| dbpedia-owl:wikiPageLength
| |
| dbpedia-owl:wikiPageWikiLink
| |
| is sameAs
of | |
| is topic
of | |
| is dbpprop:knownFor
of | |
| is dbpedia-owl:wikiPageRedirects
of | |
| is primary topic
of | |
| is dbpedia-owl:wikiPageWikiLink
of | |
| is inDataset
of | |
| is dbpedia-owl:knownFor
of | |
![[RDF Data]](/fct/images/sw-rdf-blue.png)
