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 formula_1, where each formula_2 is a relation symbol and each formula_3 is a tuple of variables and constants; the number of elements in formula_3 is equal to the arity of formula_2. 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 (binary, who's the father of whom) and (unary, who is employed), a conjunctive query could be formula_6. This query evaluates to true if there exists an individual who is a child of Mark and employed. In other words, this query expresses the question: "does Mark have an employed child?"