\begin{frame}
  \frametitle{Foreign Keys}
  
  The relational model does \textbf{not provide explicit relationships, links, or pointers}.
  \pause\smallskip
  
  \begin{goal}{}
  \emph{Idea: use the key attributes to reference a tuple.}\\
  \remark{%
    The values for the key attributes uniquely identify a tuple.
  }
  \end{goal}
  \pause\smallskip

  \begin{block}{Foreign keys}
    To refer from a relation $R$ to tuples of $S$:\\
    add the \textbf{primary key attributes} of $S$ to the attributes of $R$
  \end{block}
  \pause
  
  \begin{alertblock}{}
    Such a reference is only ``\emph{stable}'' if the ``address'' of
    a tuple does not change, that is, \emph{if the key attributes are not updated}.
  \end{alertblock}
  \end{frame}