\begin{frame}
  \frametitle{Attribute References}
  
  \structureStudentsResults
  \smallskip
  
  Let $R$ be a tuple variable and $A$ an attribute of $R$.
  \begin{goal}{}
    \emph{Attributes are referenced} in the form
    \begin{tcenter}
      $R\,.\,A$
    \end{tcenter}
    If $R$ is the \emph{only} tuple variable with attribute $A$, then it suffices
    \begin{tcenter}
      $A$
    \end{tcenter}
  \end{goal}
  \smallskip\pause
  
    
  For example,
  \begin{tcenter}
    \begin{code}{0.9\textwidth}{}
      \small
      \BUseVerbatim{query}
    \end{code}
  \end{tcenter}
  \begin{itemize}
    \item \sql{first}, \sql{last} can only refer to \sql{S}
    \item \sql{category}, \sql{number}, \sql{points} can only refer to \sql{R}
    \item \sql{sid} on its would be \emph{ambiguous} (could refer to \sql{S} or \sql{R})
  \end{itemize}
\end{frame}