Boolean conjunctive query
In the theory of relational databases, a Boolean conjunctive query is a conjunctive query without distinguished predicates, i.e., 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 contain the appropriate tuples of values, i.e. the conjunction is valid according to the facts in the database.
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 . 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 an employed child?"