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 contains the appropriate tuples of values.
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 . 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 employed children?"