\begin{frame}
  \frametitle{Not Exists}
  
  \begin{block}{}
    The construct \sql{not exists} enables the main (or outer) query
    to check whether the \emph{subquery result is  empty}.
  \end{block}
  \pause\bigskip
  
  \query
  
  In the subquery, tuple variables declared in the \sql{from}
  clause of the outer query may be referenced.\\
  \begin{itemize}
    \item Then the outer query and subquery are \emph{correlated}.
    \item The subquery is said to be ``\emph{parameterized}''.
  \end{itemize}
  \remark{%
    You may also do so for \sql{in} subqueries.
  }
\end{frame}