\begin{frame}
  \frametitle{Summary Database States}
  
  \begin{exampleblock}{}
    \begin{center}
    \begin{tikzpicture}[thick,nodes={rectangle,inner sep=1mm}, node distance=15mm, c/.style={rounded corners=1mm,fill=cgreen!30}]
      \node (db) {Database};
      \node (rl) [below of=db, xshift=-22mm] {Relation}; \draw (db) to (rl);
      \node (r)  [below of=db] {Relation}; \draw (db) to (r);
      \node (rr) [below of=db, xshift=22mm] {Relation}; \draw (db) to (rr);
      \node [c,below of=db, xshift=35mm,anchor=west] {$\approx$ Classes};
      \node (tl) [below of=r, xshift=-22mm] {Tuple}; \draw (r) to (tl);
      \node (t)  [below of=r] {Tuple}; \draw (r) to (t);
      \node (tr) [below of=r, xshift=22mm] {Tuple}; \draw (r) to (tr);
      \node [c,below of=r, xshift=35mm,anchor=west] {$\approx$ Objects};
      \node (vl) [below of=t, xshift=-22mm, align=center] {Attribute\\Value}; \draw (t) to (vl);
      \node (v)  [below of=t, align=center] {Attribute\\Value}; \draw (t) to (v);
      \node (vr) [below of=t, xshift=22mm, align=center] {Attribute\\Value}; \draw (t) to (vr);
      \node [c,below of=t, xshift=35mm,anchor=west] {$\approx$ Objects Properties};
    \end{tikzpicture}
    \end{center}
  \end{exampleblock}
\end{frame}