\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{8}{lccccccc}}
\toprule
 &  & \multicolumn{6}{c}{\textbf{group1}} \\
\cmidrule{3-8}
 &  & \multicolumn{3}{c}{a} & \multicolumn{3}{c}{b} \\
 &  & \multicolumn{3}{c}{\textbf{group2}} & \multicolumn{3}{c}{\textbf{group2}} \\
\cmidrule{3-5}\cmidrule{6-8}
 & \textbf{Total} & e & f & Total & e & f & Total \\
\midrule
\textbf{value1} &  &  &  &  &  &  &  \\
\hspace{12.0pt}Mean (SD) & 4.5 (2.45) & 2 (1.41) & 3 (1.41) & 2.5 (1.29) & 6 (1.41) & 7 (1.41) & 6.5 (1.29) \\
\hspace{12.0pt}Median [Min, Max] & 4.5 [1, 8] & 2 [1, 3] & 3 [2, 4] & 2.5 [1, 4] & 6 [5, 7] & 7 [6, 8] & 6.5 [5, 8] \\
\textbf{value2} &  &  &  &  &  &  &  \\
\hspace{12.0pt}a & 3 (37.5\%) & 1 (50\%) & 1 (50\%) & 2 (50\%) & 1 (50\%) & 0 (0\%) & 1 (25\%) \\
\hspace{12.0pt}b & 3 (37.5\%) & 0 (0\%) & 1 (50\%) & 1 (25\%) & 1 (50\%) & 1 (50\%) & 2 (50\%) \\
\hspace{12.0pt}c & 2 (25\%) & 1 (50\%) & 0 (0\%) & 1 (25\%) & 0 (0\%) & 1 (50\%) & 1 (25\%) \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}