Content deleted Content added
m →References: task, replaced: Journal of the ACM (JACM) → Journal of the ACM using AWB |
m Robot: fix links to disambiguation page Validity |
||
Line 1:
In the theory of [[relational databases]], a '''Boolean conjunctive query''' is a [[conjunctive query]] without distinguished predicates, i.e., a query in the form <math>R_1(t_1) \wedge \cdots \wedge R_n(t_n)</math>, where each <math>R_i</math> is a relation symbol and each <math>t_i</math> is a [[tuple]] of variables and constants; the number of elements in <math>t_i</math> is equal to the [[arity]] of <math>R_i</math>. Such a query evaluates to either true or false depending on whether the relations in the database contain the appropriate tuples of values, i.e. the conjunction is [[Validity (logic)|valid]] according to the facts in the database.
As an example, if a database schema contains the relation symbols {{mvar|Father}} (binary, who's the father of whom) and {{mvar|Employed}} (unary, who is employed), a conjunctive query could be <math>Father(\text{Mark}, x) \wedge Employed(x)</math>. This query evaluates to true if there exists an individual {{mvar|x}} who is a child of Mark and employed. In other words, this query expresses the question: "does Mark have an employed child?"
| |||