\begin{frame}
  \frametitle{Composite Keys}
  
  \begin{goal}{}
    If $\{\,A,B\,\}$ is a key, 
    rows may agree in $A$ \emph{or} $B$, but not \emph{both}.
  \end{goal}
  
  \begin{exampleblock}{}
    \centerline{%
      {\tableSmall
      \colorbox{rellight}{%
        \begin{tabular}[t]{|r|r|r|c|}
          \multicolumn{4}{c}{Students} \\ \hline
          \hd{\underline{sid}} & \hd{first} & \hd{last} & \hd{address} \\ \hline
          103 & Lisa & Simpson & \normalfont\ldots \\ 
          104 & Bart & Simpson & \normalfont\ldots \\ 
          106 & Bart & Smit & \normalfont\ldots \\ 
          \hline
        \end{tabular}%
      }%
      }
    }

    This relation 
    \begin{itemize}
    \item \textbf{violates} the key constraint \sql{first},
    \item \textbf{violates} the key constraint \sql{last},
    \item but \textbf{satisfies} the key constraint $\{\, \sql{first}, \sql{last} \,\}$.
    \end{itemize}
  \end{exampleblock}
  \pause\bigskip
  
  \begin{quiz}{\textwidth}{Quiz}
    Do all relations have a key?
  \end{quiz}
\end{frame}