\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{6}{lccccc}}
\toprule
 &  & \multicolumn{4}{c}{\textbf{y}} \\
\cmidrule{3-6}
 &  & A & \multicolumn{2}{c}{B} & A \\
 &  & \textbf{z} & \multicolumn{2}{c}{\textbf{z}} & \textbf{z} \\
\cmidrule{3-3}\cmidrule{4-5}\cmidrule{6-6}
 & \textbf{Total} & C & C & D & D \\
\midrule
\textbf{x} &  &  &  &  &  \\
\hspace{12.0pt}Mean (SD) & 3.5 (1.87) & 1.5 (0.707) & 3 (NaN) & 4.5 (0.707) & 6 (NaN) \\
\hspace{12.0pt}Median [Min, Max] & 3.5 [1, 6] & 1.5 [1, 2] & 3 [3, 3] & 4.5 [4, 5] & 6 [6, 6] \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}