\frametitle{Primary Keys}
        \multicolumn{5}{c}{customer}        \\ \hline
        \hd{first-name} & \hd{last-name} & \hd{phone} & \hd{street} & \hd{city} \\ \hline 
        Tom & James & 06-73917384 & Main & London \\
        Jow & Jones & 06-18384405 & Slater & Paris  \\
  What would be a good primary key?
  Is \ $\{\;\textit{first-name},\;\textit{last-name},\;\textit{phone}\;\}$ a good key?
    \item is it really unique?
    \item the phone number can change
  It is often good to introduce an artificial \emph{internal key}:
    \item e.g. \textit{customer-id}
    \item advantage: unique, does not change
    \item disadvantage: no descriptive meaning