# 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 ${\displaystyle R_{1}(t_{1})\wedge \cdots \wedge R_{n}(t_{n})}$, where each ${\displaystyle R_{i}}$ is a relation symbol and each ${\displaystyle t_{i}}$ is a tuple of variables and constants; the number of elements in ${\displaystyle t_{i}}$ is equal to the arity of ${\displaystyle R_{i}}$. 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 ${\displaystyle Father}$ (binary, who's the father of whom) and ${\displaystyle Employed}$ (unary, who is employed), a conjunctive query could be ${\displaystyle Father(Mark,x)\wedge Employed(x)}$. This query evaluates to true if there exists an individual ${\displaystyle x}$ who is a child of Mark and employed. In other words, this query expresses the question: "does Mark have employed children?"