\begin{frame}
  \frametitle{Single Value Subqueries}
  
  \begin{goal}{}
    Use constraints to ensure that the query returns only one row!\\
    \remark{%
      The DBMS will yield a runtime error if the subquery returns two or more rows.
    }
  \end{goal}
  \pause\bigskip

  \begin{block}{}
    If the subquery has an \emph{empty result}, the \emph{null value} is returned.
  \end{block}
  \pause\smallskip

  \begin{code}{\textwidth}{Bad style!}
    \small
    \BUseVerbatim{query}
  \end{code}
\end{frame}

\theme{Views \& Subqueries under From}