\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}