\begin{frame}
  \frametitle{Cycles in the Connection Graph}
  
  \begin{goal}{}
    The connection graph may contain \emph{cycles}, 
    which makes the selection of the ``right path'' more difficult (and error-prone).
  \end{goal}
  \medskip

  \begin{exampleblock}{}
    A database of course enrolments, could have the cycle:
    \smallskip
    \begin{center}
      \begin{tikzpicture}[n/.style={rectangle,rounded corners=1mm,draw=black,fill=cblue!30,inner sep=1mm}]
        \node (S) [n] {\sql{Students}};
        \node (T) [n,right of=S,node distance=35mm,yshift=7mm] {\sql{Teaching Assistants}};
        \node (E) [n,right of=S,node distance=35mm,yshift=-7mm] {\sql{Enrolments}};
        \node (C) [n,right of=T,node distance=35mm,yshift=-7mm] {\sql{Courses}};
        \draw (S) to (T);
        \draw (S) to (E);
        \draw (C) to (T);
        \draw (C) to (E);
      \end{tikzpicture}\vspace{-1ex}
    \end{center}
    \smallskip
  \end{exampleblock}
\end{frame}