\begin{frame}
  \frametitle{Not Exists}

  \begin{alertblock}{}
    \emph{Non-correlated subqueries} with \sql{not exists} are
    almost always an indication of an error!
  \end{alertblock}
  \bigskip
    
  \begin{code}{\textwidth}{Wrong!}
    \small
    \BUseVerbatim{query}
  \end{code}
  
  If there is at least one homework result, then the overall result will be empty.
  \pause
  \medskip
  
  \begin{goal}{}
    Non-correlated subqueries evaluate to a set/relation
    \emph{constant} and may make perfect sense 
    (e.g., when used with (\sql{not}) \sql{in}).
  \end{goal}
\end{frame}