table nesting multicolumn into multirow

Question

I am trying to nest multicolumn in a multirow but my code doesn't properly close the table in the first row. How to fix this?

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Answer 1

First off, it's not clear why you choose an very small column width for all five columns. Second, since line-breaking within cells doesn't seem to be needed (or even permitted), I wouldn't use a column type that permits line breaking and use the w column type instead. Third, the missing vertical line arises because you use \multicolumn{2}{c}{...} instead of \multicolumn{2}{c|}{...}.

\documentclass{article} 
\usepackage{array} % for 'm' and 'w' column types
\usepackage{multirow}
\usepackage{newtxtext,newtxmath}      % optional
\renewcommand\thetable{\Roman{table}} % optional
\begin{document}
\setcounter{table}{4} % just for this example

\begin{table}[h]
\centering
\caption{Dummy table}
\begin{tabular}{| *{5}{wc{3mm}} |} % choose suitable column types/widths
\hline
\multirow{2}{*}{a} 
   & \multicolumn{2}{c }{b-c} 
   & \multicolumn{2}{c|}{b-c} \\ % <- note the "|" symbol
   & b2 & c2 & d2 & e2 \\ \hline
r  & 1  & 2  & 3  & 4  \\ \hline
\end{tabular}
\end{table}
\end{document}

Answer 2

tabularray is much simpler.

\documentclass{article}
\usepackage{tabularray}
\begin{document}
\begin{table}
\caption{title}
\centering
\begin{tblr}
{
colspec      = {Q[c,m]Q[c,m]Q[c,m]Q[c,m]Q[c,m]},
hline{1,3,Z} = {},
vline{1,Z}   = {},
cell{1}{2,4} = {c=2}{},
cell{1}{1}   = {r=2}{},
}
a & b-c &    & b-c &    \\
  & b2  & c2 & d2  & e2 \\
r & 1   & 2  & 3   & 4  \\
\end{tblr}
\end{table}
\end{document}