\begin{frame}
  \frametitle{Subqueries under From}
  
  \begin{alertblock}{}
    Inside the subquery, tuple variables introduced in the same
    \sql{from} clause \emph{may not be referenced.} \hfill\smash\textdbend
  \end{alertblock}
  %   This is for historic reasons and could be considered an SQL ``bug.''
  \medskip
  
  \begin{code}{\textwidth}{Not allowed in SQL!}
    \small
    \BUseVerbatim{query}
  \end{code}
\end{frame}