\begin{frame}
  \frametitle{Joins}

  \begin{alertblock}{}
    It is almost always an \emph{error} if there are
    two tuples variables which are \emph{not linked} (directly or
    indirectly) via join conditions.
  \end{alertblock}  
  \medskip\pause
  
  In this query, all three tuple variables are connected:
  \begin{tcenter}
    \begin{code}{\textwidth}{}
      \small
      \BUseVerbatim{query}
    \end{code}
  \end{tcenter}
  \medskip\pause
  
  The tuple variable are connected as follows:
  \begin{center}
    \begin{tikzpicture}[n/.style={circle,draw=black,fill=cblue!30,inner sep=.5mm}]
      \node (S) [n] {S};
      \node (R) [n,right of=S,node distance=35mm] {R};
      \node (E) [n,right of=R,node distance=55mm] {E};
      \draw (S) to node [below,align=center] {\sql{S.sid = R.sid}} (R);
      \draw (R) to node [below,align=center] {\sql{R.category = E.category}\\\sql{and R.number = E.number}} (E);
    \end{tikzpicture}\vspace{-1ex}
  \end{center}
  \begin{goal}{}
    Typically (just like in this example), the conditions correspond to the \emph{foreign key relationships} between the tables.
  \end{goal}
  \bigskip
\end{frame}