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\begin{frame}
  \frametitle{Summary Relational Database Schemas}
  
  \begin{exampleblock}{}
  $$
  \xygraph{ !{ 0;<10mm,0mm>:<0mm,10mm>:: }
    [] *{\txt{Database (Schema)}} (
      -[dll] *{\txt{Relation}},
      -[d]   *{\txt{Relation}} (
        -[dll] *{\txt{Tuple}},
        -[d]   *{\txt{Tuple}} (
           -[dll] *{\txt{Attribute \\ Value}},
           -[d] *{\txt{Attribute \\ Value}},
           -[drr] *{\txt{Attribute \\ Value}} [rrr]  *{\txt{\small Data}}
        ),
        -[drr] *{\txt{Tuple}} [rrr]  *{\txt{\small$\approx$ Objects}}
      ),
      -[drr] *{\txt{Relation}} [rrr] *{\txt{\small$\approx$ Classes}}
    )
  }
  $$
  \end{exampleblock}
\end{frame}