How can I write specific table code?

Question

I want to write table code for the table in attached image. I tried but unable to get this form.

    \begin{tabular}{|llllllll|}
\hline

A & \multicolumn{7}{|l}{B} \\

\hline

&\multicolumn{3}{|l|}{C} & \multicolumn{2}{l|}{D} & \multicolumn{2}{l}{E}\\

\hline

&\multicolumn{1}{|l|}{F}& \multicolumn{1}{l|}{G} & \multicolumn{1}{l|}{H} & \multicolumn{1}{l|}{I} & \multicolumn{1}{l|}{J} & \multicolumn{1}{l|}{K}& \multicolumn{1}{l}{L}\\

\hline
M &&&&&&&\\

\hline

\end{tabular}

Answer 1

Here are some working assumptions I had to make:

• The eight columns should all be equally wide
• The table should span the full width of the text block
• Material in the left-hand column should be left-aligned
• Material in the header cells above columns 2 thru 8 should be centered
• Material in the data cells in columns 2 thru 8 should be left-aligned
• Automatic line-breaking should be enabled in all cells, including all header cells.

If these assumptions are correct, the following code should be of interest to you. Both solutions shown here are based on the tabularx package, and they both satisfy the criteria laid out above. The first solution uses a "traditional" layout, with lots of horizontal and vertical bars. The second solution, with goes for a more open "look", uses no vertical bars at all and uses fewer but well-spaced horizontal lines.

\documentclass{article}
\usepackage{tabularx,ragged2e,booktabs}
\newcolumntype{C}{>{\Centering\arraybackslash}X}
\newcolumntype{L}{>{\RaggedRight\arraybackslash}X}

\newcommand\mC[1]{\multicolumn{1}{C|}{#1}} % handy shortcut macro
\begin{document}

\setlength\extrarowheight{2pt}
\noindent
\begin{tabularx}{\textwidth}{|*{8}{L|}}
\hline
& \multicolumn{7}{c|}{B}\\
\cline{2-8}
A & \multicolumn{3}{c|}{C} & \multicolumn{2}{c|}{D} & \multicolumn{2}{c|}{E} \\
\cline{2-8}
& \mC{F} & \mC{G} & \mC{H} & \mC{I} & \mC{J} & \mC{K} & \mC{L} \\
\hline
M & & & & & & & \\
\hline
N & & & & & & & \\
\hline
\end{tabularx}

\renewcommand\mC[1]{\multicolumn{1}{C}{#1}}
\setlength\extrarowheight{0pt}
\bigskip\noindent
\begin{tabularx}{\textwidth}{*{8}{L}}
\toprule
& \multicolumn{7}{c}{B}\\
\cmidrule(l){2-8}
A & \multicolumn{3}{c}{C} & \multicolumn{2}{c}{D} & \multicolumn{2}{c}{E} \\
\cmidrule(lr){2-4} \cmidrule(lr){5-6} \cmidrule(l){7-8}
& \mC{F} & \mC{G} & \mC{H} & \mC{I} & \mC{J} & \mC{K} & \mC{L} \\
\midrule
M & & & & & & & \\
N & & & & & & & \\
\bottomrule
\end{tabularx}
\end{document}

Answer 2

In the following MWE, I have removed superfluous \multicolumn{1} commands, replaced two \hlines by \cline, added | where needed and replaced l column types by c column types (horizontally centered).

\documentclass{article}
\begin{document}

\begin{tabular}{|c|c|c|c|c|c|c|c|}
\hline
 & \multicolumn{7}{|c|}{B} \\
\cline{2-8}
A & \multicolumn{3}{|c|}{C} & \multicolumn{2}{c|}{D} & \multicolumn{2}{c|}{E}\\
\cline{2-8}
& F & G & H & I & J & K & L\\
\hline
M &&&&&&&\\
\hline
\end{tabular}

\end{document} 

Depending on the real contents of your table, you might consider removing the vertical lines completely and replace \hline by the rules from the booktabs package.