Multiple alignments within tables

Question

How can I create tables with multiple alignments as shown? The first row (i.e. the headings) is centre aligned and the subsequent rows are left aligned?

\documentclass[a4paper]{book}
\usepackage{multicol} 
\usepackage{float}


\begin{document}

\begin{table}[H]
\centering

\begin{tabular}{|l|l|}
 \hline
 \multicolumn{1}{|c}{Compressible} & \multicolumn{1}{|c|}{Incompressible}  \\  \hline

aaaaaaaaaaaaaaaaaaaaaaaa & bbbbbbb bbbbbbbbbbbbb bbbbbbbbbbb bbbbbbbbbbbbbbb bbbbbbbbbb bbbbbbbbbb bbbbbb \\
cccccccccccccccccccccccccccc & ddddddddddddddddddddddddddddd  \\  \hline
\end{tabular}


\end{table}


\end{document}

Answer 1

I think the easiest way is to use tabularx.

Please note that multicol package is for having a document in two (or more) columns, it is not for \multicolumn in tables.

Please also observe that because I've set the width of the tabularx environment to equal \linewidth, the \centering instruction is no longer needed.

As egreg pointed out in his comment, if the table has no caption, a center environment suffices; if it has a caption, then table should not have the [H] option.

For more professional tables, use booktabs.

\documentclass[a4paper]{book}
\usepackage{tabularx} 
\usepackage{ragged2e}
\usepackage{float}
\usepackage{caption}
\usepackage{booktabs}
\begin{document}
If your table is at a fixed position, you can use 
\texttt{center} environment.
%\begin{table}[H]
\begin{center}
%%\centering %% not needed since width of tabularx env. = \linewidth
\begin{tabularx}{\linewidth}{|l|>{\RaggedRight}X|}
 \hline
 \multicolumn{1}{|c}{Compressible} & \multicolumn{1}{|c|}{Incompressible}  \\  \hline
aaaaaaaaaaaaaaaaaaaaaaaa & bbbbbbb bbbbbbbbbbbbb bbbbbbbbbbb bbbbbbbbbbbbbbb bbbbbbbbbb bbbbbbbbbb bbbbbb \\
cccccccccccccccccccccccccccc & ddddddddddddddddddddddddddddd  \\  \hline
\end{tabularx}
%\end{table}
\end{center}

If your table has a caption, the \texttt{[H]} option should not be used, because it can float.
Table \ref{tab:mytab} is an example
with \texttt{booktabs}.
\begin{table}[htb]
\caption{\label{tab:mytab}A professional table}
\begin{tabularx}{\linewidth}{l>{\RaggedRight}X}
\toprule
\multicolumn{1}{c}{Compressible} & \multicolumn{1}{c}{Incompressible} \\  
\midrule
aaaaaaaaaaaaaaaaaaaaaaaa & bbbbbbb bbbbbbbbbbbbb bbbbbbbbbbb bbbbbbbbbbbbbbb bbbbbbbbbb bbbbbbbbbb bbbbbb \\
cccccccccccccccccccccccccccc & ddddddddddddddddddddddddddddd \\  
\bottomrule
\end{tabularx}
\end{table}
\end{document}