\begin{frame} \frametitle{Null Values} % \begin{goal}{} Null values are used to model a variety of scenarios: \smallskip \begin{itemize} \item \textbf{No value exists.}\\ \remark{A student might not have an e-mail address.} \medskip \item \textbf{The attribute is not applicable for this tuple.}\\ \remark{Some exercises are for training only: no points will be given.} \medskip \item \textbf{A value exists (in the real world), but is not known.}\\ \remark{In table \sql{Students}, \sql{address} might be unknown for a student.} \medskip \item \textbf{Any value will do.} \smallskip \end{itemize} % \end{goal} \bigskip\pause \begin{alertblock}{} Since the same null value is used for quite different purposes, there can be \textbf{no clear semantics.} \end{alertblock} \end{frame}