\begin{frame}
  \frametitle{Not Exists}

  \begin{alertblock}{}
    The subquery may use tuple variables from outer query.\\
    The \emph{converse is illegal}!
  \end{alertblock}
  \bigskip
  
  \begin{code}{\textwidth}{Wrong!}
    \small
    $
    \begin{array}{ll}
      \sql{select} & \sql{first, last, {\setlength{\fboxsep}{2pt}\fbox{R.number}}} \\
      \sql{from}   & \sql{Students S}  \\
      \sql{where}  & \sql{not exists (} 
      \begin{array}[t]{@{\,}ll}
        \sql{select} & \sql{*} \\
        \sql{from}   & \sql{Results R} \\
        \sql{where}  & \sql{R.category = \textquotesingle{}homework\textquotesingle{}} \\
        \sql{and}    & \sql{R.sid = S.sid)}
      \end{array}
    \end{array}
    $
  \end{code}
  \vspace{-1ex}
  
  Compare this to \emph{variable scoping} (global/local
  variables) in block-structured programming languages (Java, C).
  \medskip
  
  Subquery tuple variables declarations are ``local.''
\end{frame}