\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}