\begin{frame}
  \frametitle{Inner and Outer Joins}

  \structureAll
  
  \begin{code}{\textwidth}{Number of submission per homework (0 if no submission)}
    \small
    \BUseVerbatim{query}
  \end{code}
  \pause\smallskip
  
  All Exercises are present in the result of the left (outer) join.
  \begin{itemize}
  \item 
    for exercises without solutions,
    \sql{sid} and \sql{points} will be \sql{null}
  \item \sql{count(sid)} ignores rows where \sql{sid is null}.
  \end{itemize}
  \pause\medskip
  
  Could also be solved using \emph{union}, but less elegant (longer).
\end{frame}