\begin{frame}
  \frametitle{Union}
  
  \begin{goal}{}
    ``Union'' allows to combine the results of two queries.
  \end{goal}
  
  This is needed since there is no other method to
  construct one result column that draws from different
  tables/columns.
  \pause\medskip

  \begin{goal}{}
    ``Union'' is necessary, for example, if specialisations of a concept
    (``subclasses'') are stored in separate tables.  
  \end{goal}  

  \begin{exampleblock}{}
      For instance, if we have tables
      \begin{itemize}
        \item \sql{graduate\_courses} and 
        \item \sql{undergraduate\_courses} 
      \end{itemize}
      both of which are specialisations of the concept \sql{course}.
  \end{exampleblock}
  \pause\medskip
  
  \begin{goal}{}
    ``Union'' is also commonly used for \emph{case analysis}
      (cf., the $\sql{if}\ldots\sql{then}\ldots$ cascades in
      programming languages).
  \end{goal}
\end{frame}