\begin{frame}
  \frametitle{Aggregation}
  
  \begin{exampleblock}{}
    \begin{center}\vspace{-4.7ex}
      \scalebox{.9}{
      \begin{tikzpicture}[every edge/.style={link}]
        \node[entity] (employee) {employee};
        \node[entity,right of=employee, node distance=6cm] (branch) {branch};
        \node[entity,right of=employee, node distance=3cm,yshift=2cm] (job) {job};

        \node[relationship,scale=.9,aspect=1.3] (works-on) at ($(employee)!.5!(branch)$) {works-on} edge (employee) edge (branch) edge (job);

        \node[entity,above of=employee, node distance=2cm,xshift=-2.5cm] (manager) {manager};
        \node[relationship,scale=.9,aspect=1.3] (manages) at ($(manager)+(2.5cm,0cm)$) {manages};
        \draw (manages) to (manager);
        \begin{scope}[thick]
        \draw (manages) to (employee);
        \draw (manages) to[out=45,in=90,looseness=1] (branch);
        \draw (manages) to (job);
        \end{scope}
      \end{tikzpicture}}
    \end{center}
  \end{exampleblock}
  \medskip
  \pause
  
  However, this design is \alert{not good}:
  \pause
  \begin{itemize}
    \item 
      \emph{does not capture:} every \textit{manages} relationship corresponds to a \textit{works-on} relationship;
    \item %relationship set \textit{works-on} and \textit{manages} represent 
      information is represented \emph{redundant/overlapping};
    \item 
      \emph{we cannot discard the \emph{works-on} relationship set:}
      some \textit{works-on} relationships may not correspond to any
      \textit{manages} relationship.
  \end{itemize}
  \pause\vspace{-.5ex}
  \begin{goal}{}
  The solution is to eliminate redundancy using \emph{aggregation}!
  \end{goal}
  \vspace{10cm}
\end{frame}