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\begin{frame}
  \frametitle{Query Formulation}
  
  It is not always that trivial\ldots
  \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
  \pause

  \begin{exampleblock}{}
    Consider a course registration database that also contains teaching assistants (TA) assignments:
    \begin{talign}
    \xymatrix@C=1.5cm@R=2mm{%
      & *+[F]{\sql{~~~TA~~~}} \\
      *+[F]{\sql{STUDENTS}} \ar@{-}[ur] \ar@{-}[dr] & & 
      *+[F]{\sql{COURSES}}  \ar@{-}[ul] \ar@{-}[dl]\\
      & *+[F]{\sql{ENROLLMENTS}} 
      }
    \end{talign}
  \end{exampleblock}
\end{frame}