\begin{frame}
  \frametitle{Exists}

  We can also use \sql{exists} without negation:
  \smallskip
  
  \begin{code}{\textwidth}{Who has submitted at least one homework?}
    \small
    \BUseVerbatim{query}
  \end{code}
  \begin{center}
    \tableSmall
    \colorbox{rellight}{%
      \begin{tabular}[t]{|r|r|r|}
        \multicolumn{3}{c}{Query Result} \\ \hline
        \hd{sid} & \hd{first} & \hd{last} \\ \hline
        101 & George & Orwell \\ 
        102 & Elvis & Presley \\ 
        103 & Lisa & Simpson \\
        \hline
      \end{tabular}%
    }%
  \end{center}
  \pause
  
  \begin{quiz}{\textwidth}{Can we reformulate the above without using \sql{exists}?}
    \vskip.75cm\ 
  \end{quiz}
  
  %% Yes (semi-join):
  %% select distinct S.sid, first, last
  %% from   Students S, Results R
  %% where  S.sid = R.sid and R.cat = 'H'
\end{frame}