\begin{frame}
  \frametitle{Null Values}

%   \begin{goal}{}
  Null values are used to model a variety of  scenarios:
  \smallskip
  \begin{itemize}
  \item \textbf{No value exists.}\\
    \remark{A student might not have an e-mail address.}
  \medskip

  \item \textbf{The attribute is not applicable for this tuple.}\\
    \remark{Some exercises are for training only: no points will be given.}
  \medskip
  
  \item \textbf{A value exists (in the real world), but is not known.}\\
    \remark{In table \sql{Students}, \sql{address} might be unknown for a student.}
  \medskip

  \item \textbf{Any value will do.}
  \smallskip
  \end{itemize}
%   \end{goal}
  \bigskip\pause
  
  \begin{alertblock}{}
    Since the same null value is used for quite different purposes,
    there can be \textbf{no clear semantics.}
  \end{alertblock}
\end{frame}