\documentclass{beamer}

\usepackage{lmodern}
\renewcommand{\thesection}{\alph{section}}
\renewcommand{\thesubsection}{\alph{subsection}}

\makeatletter
\AtBeginSubsection{%
  \immediate\write\@auxout%
    {\string\def\csname Section\Alph{section}subsection\Alph{subsection}\endcsname{%
        \string\tableofcontents[currentsubsection,hideothersubsections,sectionstyle=hide/hide,subsectionstyle=show/hide]}}
}
\makeatother

\begin{document}

\begin{frame}
  \frametitle{contents of all subsections}

  % The commands below shall be defined
  \csname SectionAsubsectionA\endcsname
  \csname SectionAsubsectionB\endcsname
  \csname SectionBsubsectionA\endcsname
\end{frame}

\section{section 1}

\subsection{subsection 1.1}

\subsection{subsection 1.2}

\section{section 2}

\subsection{subsection 2.1}

\end{document}