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\begin{frame}
  \frametitle{Queries in Databases: Relational Model}
  
  \begin{exampleblock}{}
  \begin{center}
    \tablepersons
  \end{center}
  \end{exampleblock}
  \smallskip
  \pause
  
  \begin{goal}{Relational Model}
    The table is modelled by a ternary predicate \textit{Person}:
    \begin{talign}
      \text{\textit{Person}(101, Ann, Amsterdam)}
    \end{talign}
    Likewise, the predicate holds for every row of the table.
  \end{goal}
  \pause

  We can formulate \emph{queries using free variables}.
  \begin{exampleblock}{}
    The names of people living in Amsterdam:
    \pause
    \begin{talign}
      \{\; y \;\mid\; \myex{i}{\text{\textit{Person}}(i,y,\text{Amsterdam})} \; \}
      \mpause[1]{ \;\;=\;\; \{\;\text{Ann},\; \text{Mia}\;\} }
    \end{talign}
  \end{exampleblock}
\end{frame}