\begin{frame}
  \frametitle{Restrictions}
  
  \emph{The following are not allowed:}
  \begin{itemize}
  \medskip
  \item Simple aggregations may not be nested (makes no sense):
    \begin{code}{0.8\textwidth}{\small\bfseries Wrong!}
      \small
      $\quad\cdots$ \sql{sum(avg(A))} $\cdots$
    \end{code}
  \pause\medskip
  \item Aggregations may not be used in the \sql{where} clause:
    \smallskip
    
    \begin{code}{0.8\textwidth}{\small\bfseries Wrong!}
      \small
      $\quad\cdots$ \sql{where sum(A) > 100} $\cdots$
    \end{code}
  \pause\medskip
    
  \item If an aggregation function is used without \sql{group by}, \\
    \emph{no attributes} may appear in the \sql{select} clause:
    \smallskip
    
    \begin{code}{0.8\textwidth}{\small Wrong!}
      \small
      \BUseVerbatim{query}
    \end{code}
  \end{itemize}
\end{frame}