\begin{frame}
  \frametitle{Entity Relationship (ER) Model}

  \begin{block}{Entity relationship (ER) model}
    \begin{itemize}
      \item entities = objects
        \begin{itemize}
          \item e.g. customers, accounts, bank branches
        \end{itemize}
      \item relationship between entities
        \begin{itemize}
          \item e.g. account 217 is held by customer Elvis
          \item \emph{relationship set descriptor} links customers with accounts
        \end{itemize}
    \end{itemize}
  \end{block}
  \smallskip
  
  \begin{exampleblock}{}
    \begin{center}
      \scalebox{.85}{
      \begin{tikzpicture}[every edge/.style={link}]
        \node[entity] (customer) {Customers};
        \node[attribute] (id) [above left of=customer,node distance=1.7cm,shift={(-2mm,-3mm)}] {id} edge (customer);
        \node[attribute] (name) [above of=customer,node distance=1.3cm] {name} edge (customer);
        \node[attribute] (street) [left of=customer,node distance=2.1cm] {street} edge (customer);
        \node[attribute] (city) [below left of=customer,node distance=1.7cm] {city} edge (customer);

        \node[entity,right of=customer, node distance=6cm] (account) {Accounts};
        \node[attribute] (account-number) [above of=account,node distance=1.2cm] {account-number} edge (account);
        \node[attribute] (balance) [below right of=account,node distance=1.7cm,xshift=-5mm] {balance} edge (account);

        \node[relationship] (does) at ($(customer)!.5!(account)$) {depositor} edge (customer) edge (account);
      \end{tikzpicture}}
    \end{center}
  \end{exampleblock}
\end{frame}