\begin{frame}
  \frametitle{Group By}

  Grouping by \sql{E.number} \emph{and} \sql{E.topic} yields the desired result!
  \medskip
  
  \begin{code}{\textwidth}{}
    {\small
    \BUseVerbatim{query}}
    \medskip
    
    \begin{tcenter}
      \footnotesize\ttfamily
      \colorbox{rellight}{%
        \begin{tabular}{|r|r|r|}
          \hline
          \hd{E.number} & \hd{E.topic} & \hd{avg(points)} 
          \\
          \hline
          1 & Rel.Alg. & 8 \\
          2 & SQL      & 8.5 \\
          \hline
        \end{tabular}%
      }
    \end{tcenter}
  \end{code}
  \vspace{-1ex}
  
  Now the DBMS has a \emph{syntactic clue} that \sql{E.topic} is unique.
  \pause\bigskip

  \begin{goal}{}
    The order of the \sql{group by} attributes is not important.
  \end{goal}  
\end{frame}