Boolean conjunctive query

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In the theory of relational databases, a Boolean conjunctive query is a conjunctive query without distinguished predicates, i.e., a query in the form R_1(t_1) \wedge \cdots \wedge R_n(t_n), where each R_i is a relation symbol and each t_i is a tuple of variables and constants; the number of elements in t_i is equal to the arity of R_i. 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 Father (binary, who's the father of whom) and Employed (unary, who is employed), a conjunctive query could be Father(Mark, x) \wedge Employed(x). This query evaluates to true if there exists an individual x who is a child of Mark and employed. In other words, this query expresses the question: "does Mark have employed children?"

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  • 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.