\begin{frame}
  \frametitle{Primary Keys}
  
  \begin{center}
  {\small\ttfamily
    \colorbox{rellight}{%
      \begin{tabular}[t]{|r|r|r|r|r|}
        \multicolumn{5}{c}{Customer}        \\ \hline
        \hd{first-name} & \hd{last-name} & \hd{phone} & \hd{street} & \hd{city} \\ \hline 
        Tom & James & 06-73917384 & Main & London \\
        Joe & Jones & 06-18384405 & Slater & Paris  \\
        \hline
      \end{tabular}%
    }}
  \end{center}
  
  \begin{alertblock}{}
  What would be a good primary key?
  \end{alertblock}
  \medskip
  \pause
    
  Is \ $\{\;\textit{first-name},\;\textit{last-name},\;\textit{phone}\;\}$ a good key?
  \pause
  \begin{itemize}
    \item the phone number can change
    \item is it really unique?
  \end{itemize}
  \medskip
  \pause
  
  \begin{goal}{}
  It is often good to introduce an artificial \emph{internal key}:
  \begin{itemize}
    \item e.g. \textit{customer-id}
    \item \emph{advantage: unique, does not change}
    \item minor disadvantage: no descriptive meaning
  \end{itemize}    
  \end{goal}
  \bigskip\bigskip
  
\end{frame}