\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}