\begin{frame}
  \frametitle{Group By}

  \begin{quiz}{\textwidth}{Is there any semantical difference between these queries?}
    {\footnotesize
    \BUseVerbatim{query}\\[-.5ex]
    \sql{group by topic}}
    \bigskip\medskip
    
    {\footnotesize
    \BUseVerbatim{query}\\[-.5ex]
    \sql{group by topic,\setlength{\fboxsep}{2pt}\fbox{E.number}}}
  \end{quiz}
  
  %% Yes: if table Exercises contains multiple Exercises with the same
  %% topic (these will then be grouped together which may not be what
  %% was desired).  
  %%
  %% In the current database state: no observable difference (topic
  %% unique)
\end{frame}