\begin{frame}
  \frametitle{Keys are Contraints}
  
  \begin{goal}{}
    \textbf{Keys are constraints:}
    they refer to all possible DB states, not only the current one.
  \end{goal}

  \begin{center}
    \begin{code}{.6\textwidth}{}
    \small
    \BUseVerbatim{createtable}
    \end{code}
  \end{center}
  \pause
   
  \begin{exampleblock}{}
    \centerline{%
      \ttfamily\footnotesize
      \colorbox{rellight}{%
        \begin{tabular}[t]{|r|r|r|r|}
          \multicolumn{4}{c}{Students}        \\ \hline
          \hd{\underline{sid}} & \hd{first} & \hd{last} & \hd{address} \\ \hline
          101 & George & Orwell & \normalfont\ldots \\ 
          103 & Lisa & Simpson & \normalfont\ldots \\ 
          \hline
        \end{tabular}%
      }      
    }
    \medskip
    
    In this database state \sql{first} is a key for \sql{Students}.
    However, this is too restrictive for all intended database states.
    A future insertion of \sql{George Washington} would be impossible.
  \end{exampleblock}
  \pause
  
  \begin{goal}{}
    When declaring keys, think about all intended database states!
  \end{goal}
\end{frame}