How to merge row in the first column in LaTeX

You can decipher the following template.

\documentclass[preview,border=12pt,12pt]{standalone}
\usepackage{array,multirow}

\let\mc=\multicolumn
\let\mr=\multirow
\let\cl=\cline

\begin{document}
\begin{tabular}{|*6{c|}l}
\cl{3-6}
\mc{2}{c|}{\mr{2}{*}{empty}} & \mc{4}{c|}{Primes} \\\cl{3-6}
\mc{2}{c|}{}                 & 2 & 3 & 5 & 7 \\\cl{1-6}
\mr{2}{*}{Powers}            & 504 & 3 & 2 & 0 & 1 \\\cl{2-6}
                             & 540 & 2 & 3 & 1 & 0 \\\cl{1-6}
\mr{2}{*}{Powers}            & HCF & 2 & 2 & 0 & 0 & min \\\cl{2-6}
                             & LCM & 3 & 3 & 1 & 1 & max \\\cl{1-6}
\end{tabular}
\end{document}

Warning

\mr can be nested in \mc but the reverse is not possible!

References

My code above was inspired by an example from Wikipedia as follows.

\begin{tabular}{cc|c|c|c|c|l}
\cline{3-6}
& & \multicolumn{4}{ c| }{Primes} \\ \cline{3-6}
& & 2 & 3 & 5 & 7 \\ \cline{1-6}
\multicolumn{1}{ |c  }{\multirow{2}{*}{Powers} } &
\multicolumn{1}{ |c| }{504} & 3 & 2 & 0 & 1 &     \\ \cline{2-6}
\multicolumn{1}{ |c  }{}                        &
\multicolumn{1}{ |c| }{540} & 2 & 3 & 1 & 0 &     \\ \cline{1-6}
\multicolumn{1}{ |c  }{\multirow{2}{*}{Powers} } &
\multicolumn{1}{ |c| }{gcd} & 2 & 2 & 0 & 0 & min \\ \cline{2-6}
\multicolumn{1}{ |c  }{}                        &
\multicolumn{1}{ |c| }{lcm} & 3 & 3 & 1 & 1 & max \\ \cline{1-6}
\end{tabular}

However, as you can see and compare, my code is much much simpler and well refactored yet still readable, right?

What you try to do can be done with this:

\documentclass{standalone}
\usepackage{multirow}
\begin{document}
\begin{tabular}{|c|c|c|c|c} \hline
& & \multicolumn{3}{|c|}{ ABC} \\ \hline
& & A&B&C\\ \hline
\multirow{3}{*}{DEF} & D &1& 4 & 7 \\ \cline{2-5}
&E&  2& 5 & 8 \\ \cline{2-5}
&F&  3& 6 & 9 \\ \cline{2-5}
\end{tabular}
\end{document}

But! LaTeX can make tables that are much nicer that this Excel-style layout. Consider doing something like this, which still nice to look at if you have more than 3 columns.

\documentclass{standalone}
\usepackage{multirow}
\usepackage{booktabs}
\begin{document}
\begin{tabular}{lcccccc} 
\toprule
& \multicolumn{3}{c}{ ABC} & \multicolumn{3}{c}{ ABC} \\\cmidrule(lr){2-4} \cmidrule(lr){5-7}
& A&B&C & A2&B2&C2\\ \midrule
DEF:\\
\;D &1& 4 & 7 &1& 4 & 7 \\ 
\;E&  2& 5 & 8 &1& 4 & 7 \\ 
\;F&  3& 6 & 9 &1& 4 & 7 \\ 
XYZ:\\
\;X &1& 4 & 7 &1& 4 & 7  \\ 
\;Y&  2& 5 & 8 &1& 4 & 7 \\ 
\;Z&  3& 6 & 9 &1& 4 & 7  \\ \bottomrule
\end{tabular}
\end{document}

\multirow command, needs to be act on its own column... So you need to add an extra column on the left side in each column and thus an extra & needed on each left side of every row of your table and an extra columntype (lets say c) at the beginning of your tabular's arguments.

Also, multicolumn in your example will should be 3 rows tall and should start from the row (1,4,7). But then an \hline on one of this or the next rows, would add a line in the "multirowed" cell too, and this is not wanted. So, we have to change some \hlines with \cline{2-4} that is a line from column 2 to column 4 and not from 1st to last.

Finally, you can add empty \multicolumn{1}{c|}{} as first element of your first two rows, in order to change their behavior from |c| to c| and remove this way the unwanted vertical line from their left side:

\documentclass{article}
\usepackage{multirow}
\begin{document}
\begin{table}
\centering
\caption{Table}
\begin{tabular}{|c|c|c|l|} \cline{2-4}
\multicolumn{1}{c|}{}&\multicolumn{3}{|c|}{ ABC} \\ \cline{2-4}
\multicolumn{1}{c|}{}&A&B&C\\ \hline
\multirow{3}{*}{DEF}&1& 4 & 7 \\ \cline{2-4}
&2& 5 & 8 \\ \cline{2-4}
&3& 6 & 9 \\ \hline
\end{tabular}
\end{table}
 \end{document}

PS: Tried to show you what was your mistakes and not to find a way to show your table more beautiful.