\begin{frame}
  \frametitle{Common Errors}

  \structureAll
  \smallskip
  
  \begin{code}{\textwidth}{Find those students who have neither submitted a homework nor participated in any exam}
    {\small
      \BUseVerbatim{query}
    }
  \end{code}
  \smallskip
  
  \begin{quiz}{\textwidth}{What is the error in this query?}
    What is the output of this query? Fix the query.
  \end{quiz}
  
  %% (1)
  %% Yes, reference to sid in subquery is OK (but it references sid of
  %% Students since there is not sid in Exercises!)
  %% (2)
  %% empty (subquery is correlated)
  %% (3)
  %% Replace Exercises by Results (and optionally introduce tuple
  %% variables to make intent explicit).

\end{frame}