documentclass{article}

usepackage{listings}
usepackage{upquote}

lstdefinestyle{myStyle}{
  language=SQL,
  basicstyle=smallttfamily,
  moredelim=[is][underbar]{_}{_},
  keepspaces=true
}

begin{document}

begin{lstlisting}[style=myStyle]
SELECT * FROM usr WHERE _id = 7_
end{lstlisting}

end{document}

documentclass{article}

usepackage{listings}
usepackage{upquote}

lstdefinestyle{myStyle}{
  language=SQL,
  basicstyle=smallttfamily,
  moredelim=[is][underbar]{_}{_},
  keepspaces=true
}

begin{document}

begin{lstlisting}[style=myStyle]
SELECT * FROM usr WHERE id ='_7_'
end{lstlisting}

end{document}

documentclass{article}

usepackage{listings}
usepackage{upquote}

lstdefinestyle{myStyle}{
  language=SQL,
  basicstyle=smallttfamily,
  moredelim=[is][underbar]{_}{_},
  keepspaces=true
}

begin{document}

begin{lstlisting}[style=myStyle,escapechar=%]
SELECT * FROM usr WHERE %underline{id} = underline{7}%
end{lstlisting}

end{document}