column separation lines now showing up in table

Question

I have the following code that produces the table that is shown. What I am trying to do is to get the fourth and fifth column titles vertically aligned at the top. Is this possible?

\documentclass{article}
\usepackage{multirow}
\usepackage{array}

\newcolumntype{x}[1]{%
>{\raggedright\hspace{0pt}}p{#1}}%

\begin{document}
\centering
\renewcommand{\arraystretch}{1.2}
\begin{tabular}{| m{2cm} | p{2cm} | p{2cm} | p{2cm} | p{2cm} | p{2cm} |} \hline
    \multirow{3}{1.7cm}{Data Set} &%
    \multicolumn{3}{c |}{Raw Data} &%
    Offline Processing Time &
    Index Size (KB)\\ \cline{2-4}
%
    & %
    RDF N3 File SizeKB (sec)    &%
    Number of Triples &%
    Number of Entities\\    \hline
\end{tabular}
\end{document}

Answer 1

Better don't use them (vertical lines) at all!

I think your problem is missing vertical lines. You have to use the ampersands even if those cells are empty, like

Number of Entities & & \\    \hline

You are done as those titles are already aligned at the top.

Answer 2

Not sure if this is the desired layout:

\documentclass{article}
\usepackage{multirow}
\usepackage{array}

\newcolumntype{x}[1]{%
>{\raggedright\arraybackslash\hspace{0pt}}p{#1}}%

\begin{document}
\centering
\renewcommand{\arraystretch}{1.2}
\begin{tabular}{| m{2cm} | x{2cm} | x{2cm} | x{2cm} | x{2cm} | x{2cm} |} \hline
    \multirow{4}{*}{Data Set} &%
    \multicolumn{3}{c |}{Raw Data} &%
    \multirow{3}{2cm}{Offline Processing Time} &
    \multirow{3}{2cm}{Index Size (KB)}\\ \cline{2-4}
%
    & %
    RDF N3 File SizeKB (sec)    &%
    Number of Triples &%
    Number of Entities & &\\    \hline
\end{tabular}
\end{document}

Some remarks: the lines were missing because you didn't complete the columns in the last row (adding the necessary & characters). Also, since you have narrow columns, it is better to have the contents \ragedright, so I changed your p{} columns to x{} columns (adding \arraybackslash).

You should consider not using vertical rules in your tables; the booktabs package discusses this and helps you to create better looking tables.