\begin{frame}
  \frametitle{Nested Subqueries}
  
  \begin{goal}{}
    \emph{Subqueries may be nested!}
  \end{goal}
  \pause\smallskip
  
  \begin{code}{\textwidth}{List the students who solved all homework assignments}
    \small
    \mpause[2]{\BUseVerbatim{query}}
  \end{code}
  
  \pause
  \emph{Inner} query: all results for student \sql{S} and homework \sql{E}.\\
  \emph{Middle} query: homework of student \sql{S} for which no result exists.\\
  \emph{Outer} query: students that have no homework without results.\hspace*{-2cm}\ \\
  \bigskip
  
  %% ``List those Students S, where there is no homework E for which there
  %% exists no entry for (S,E) in the Results.''
  %%
  %% { (S.first, S.last) | S:Students /\ 
  %%                       not (exists E (E:Exercises /\ E.cat = 'H' /\
  %%                                      not (exists R (R:Results /\
  %%                                                     R.sid = S.sid /\
  %%                                                     R.eno = E.eno /\
  %%                                                     R.cat = 'H')))) }  
\end{frame}