\begin{frame}
  \frametitle{Nested Aggregations}
  
  \begin{goal}{}
    \emph{Nested aggregations} require a subquery in the \sql{from} clause.
  \end{goal}
  \medskip
  
  \begin{code}{\textwidth}{%
      What is the average number of homework points (excluding those
      Students who did not submit anything)?}
    \small
    \BUseVerbatim{query}

    \medskip
    \begin{center}
    \footnotesize\ttfamily
    \colorbox{rellight}{%
      \begin{tabular}{|r|r|}
        \multicolumn{2}{c}{X} \\
        \hline
        \hd{sid} & \hd{homeworkPoints} 
        \\
        \hline
        101 & 18 \\
        102 & 18 \\
        103 & 5  \\
        \hline
      \end{tabular}%
    }
    \qquad
    \colorbox{rellight}{%
      \begin{tabular}{|r|}
        \hline
        \hd{avg(X.homeworkPoints) }
        \\
        \hline
        13.67 \\
        \hline
      \end{tabular}%
    }
    \end{center}
  \end{code}
\end{frame}