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\begin{frame}
  \frametitle{Null Values}
  
  The relational model allows \textbf{missing attribute values}
  \begin{itemize}
  \item table entries may be empty
  \end{itemize}
  \bigskip
  
  \begin{center}\vspace{-2ex}
    \scalebox{1}{{\ttfamily\tablestudent}}
    \vspace{-1ex}
  \end{center}
  \pause

  \begin{block}{}
    Formally, the set of possible values (the domain) for an
    attribute is extended by a new special value \emph{``null''}.    
  \end{block}
  \pause

  \begin{alertblock}{}
    ``Null'' is \emph{not} the number 0 or the empty
        string.  \\A null value is different from all values of any data
        type. \hfill\smash\textdbend 
  \end{alertblock}  
\end{frame}