\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}