\begin{frame}
  \frametitle{Ambiguous Attribute References}
  
  \structureExercisesResults
  \medskip

  Consider the following query:
  \begin{tcenter}
    \begin{code}{\textwidth}{}
      \small
      \BUseVerbatim{query}
    \end{code}
  \end{tcenter}
  \pause
  In the \sql{select} clause \sql{number} is ambiguous.
  \pause

  \begin{alertblock}{}
    Although forced to be equal by the join condition, SQL
    requires the user to specify whether \sql{number} refers to \sql{R} or \sql{E}.
  \end{alertblock}

  The unambiguity check is purely \emph{syntactic} and does
  not depend on the query semantics.
  \bigskip
\end{frame}