\begin{frame}
  \frametitle{How to Determine Keys: Examples}

  \begin{exampleblock}{}
    Find \emph{all} minimal keys the relation $\sql{R}$
    \begin{tcenter}
      {\ttfamily\centering
        \colorbox{rellight}{%
          \begin{tabular}[t]{|r|r|r|r|r|}
            \multicolumn{5}{c}{R} \\ \hline
            \hd{A} & \hd{B} & \hd{C} & \hd{D} & \hd{E} \\
            \hline
          \end{tabular}%
        }}
    \end{tcenter}
    with the functional dependencies
    \begin{talign}
      A &\to D &
      B &\to C &
      B &\to D &
      D &\to E
    \end{talign}
    We get
    \begin{enumerate}
    \pause
      \item $\textit{Candidates} = \{\; \{\, A,B \,\} \;\}$\\
         since $A,B$ do not occur in any right-hand side
    \pause
      \item $\{\, A,B \,\}^+ = \pause \{\, A,B,C,D,E \,\}$\\
         So $\{\, A,B \,\}$ is a key. 
    \pause
      \item $\textit{Candidates} = \{\; \;\}$ \\
        No more candidate keys to check, we terminate. 
    \end{enumerate}
  \end{exampleblock}
\end{frame}