Boolean conjunctive query
Information about Boolean conjunctive query
In the theory of relational databases, a Boolean conjunctive query is a query in the form 
, where each 
is a relation symbol and each 
is a tuple of variables and constants; the number of elements in is equal to the arity of . Such a query evaluates to either true or false depending on whether the relations in the database contains the appropriate tuples of values.
As an example, if a database schema contains the relation symbols
(binary, who's the father of who) and 
(unary, who is employeed), a conjunctive query could be 
. This query evaluates to true if there exists an individual 
who is a child of Mark and employed. In other worlds, this query expresses the question: "do Mark have employed children?"
, where each is a relation symbol and each is a tuple of variables and constants; the number of elements in is equal to the arity of . Such a query evaluates to either true or false depending on whether the relations in the database contains the appropriate tuples of values.
As an example, if a database schema contains the relation symbols
(binary, who's the father of who) and (unary, who is employeed), a conjunctive query could be . This query evaluates to true if there exists an individual who is a child of Mark and employed. In other worlds, this query expresses the question: "do Mark have employed children?"
See also
References
- G. Gottlob, N. Leone, F. Scarcello (2001). "The complexity of acyclic conjunctive queries". Journal of the ACM (JACM) 48 (3): 431-498. DOI:10.1145/382780.382783.
A relational database is a database that conforms to the relational model, and refers to a database's data and schema (the database's structure of how that data is arranged).
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In logic, mathematics, and computer science, the arity (synonyms include type, adicity, and rank) of a function or operation is the number of arguments or operands that the function takes.
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In logic and/or mathematics, logical conjunction or and is a two-place logical operation that results in a value of true if both of its operands are true, otherwise a value of false!
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