\begin{frame} \frametitle{Self Joins} \begin{code}{\textwidth}{Students that solved at least two homework assignments} \remark{(This may also be solved using aggregations.)} \bigskip \small \BUseVerbatim{query2} \end{code} \medskip \begin{quiz}{\textwidth}{``Unexpected'' result} What is going wrong here? \end{quiz} \pause \smallskip \begin{code}{\textwidth}{} We need to ensure that \sql{R1} and \sql{R2} refer to distinct results:\vspace{-1ex} \begin{talign} &\vdots \\[-1ex] &\sql{and (R1.sid <> R2.sid or }\\[-.5ex] &\sql{\ \ \ \ \ R1.category <> R2.category or} \\[-.5ex] &\sql{\ \ \ \ \ R1.number <> R2.number)} \end{talign} \end{code} \bigskip \end{frame} \theme{Duplicate Elimination}