\begin{frame}
  \frametitle{Aggregation}
  
  \begin{block}{}
    \emph{Aggregation}:
    \begin{itemize}
      \item treat relationship set as an abstract entry\\
            \remark{abstraction of a relationship into a new entry}
      \item allows relations between relations
    \end{itemize}
  \end{block}

  \begin{exampleblock}{}
    \begin{center}
      \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=1.6cm] (job) {job};

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

        \draw [very thick,blue!50!black!100] (-1.2cm,2.2cm) rectangle (7.1cm,-1cm);

        \node[entity,above of=employee] at ($(employee)!.5!(branch) + (0,-4.5cm)$) (manager) {manager};
        \node[relationship,scale=.9,aspect=1.3] (manages) at ($(manager)!.4!(works-on)$) {manages} edge[thick] (manager) edge[thick] (works-on);
      \end{tikzpicture}}
    \end{center}
  \end{exampleblock}
\end{frame}