\begin{frame}
  \frametitle{Differences: ER Models vs. UML Class Diagrams}

  \begin{itemize}
    \item visual differences --- no big deal
  \medskip
    \item \emph{keys}:
      \begin{itemize}
        \item ER supports keys (underlining)
        \item UML has no standard for indicating keys\\
          \remark{Some people underline, others write \textit{PK} after the attribute.}
      \end{itemize}
%   \pause
%   \begin{exampleblock}{}
%     \begin{minipage}{.4\textwidth}
%     In case of composition in UML, one may use \emph{association qualifiers} to indicate the key. 
%     \end{minipage}~\quad~%
%     \begin{minipage}{.49\textwidth}
%     \begin{center}
%     \scalebox{.8}{
%     \begin{tikzpicture}
%     \umlclass{book}{ 
%       +ISBN \\
%       +title \\
%       +author
%       }{ 
%       } 
%     \umlclass[x=5]{section}{ 
%       +nr \\
%       +title
%       }{ 
%       } 
%     \node [rectangle,draw,fill=yellow!20,anchor=west] at (book.east) (isbn) {ISBN};
%     \umlcompo[arg1=$1..1$,mult2=$\,$,arg2=$0..*$,mult2=$\,$,pos2=0.8]{isbn}{section} 
%     \end{tikzpicture} \vspace{-2ex}
%     }
%     \end{center}
%     \end{minipage}
%   \end{exampleblock}
  \medskip
    \item \emph{aggregation}: means something very different
      \begin{itemize}
        \item in ER: treating a relationship set as an entity
        \item in UML: a part-whole relation\\(non-exclusive form of composition)
      \end{itemize}
  \medskip
    \item \emph{weak entities}:
      \begin{itemize}
        \item in ER: weak entities are entities without own key
        \item in UML: composition is similar, but says nothing about keys
      \end{itemize}
  \end{itemize}
\end{frame}