Obtain all different values of a specific column in a datatool database

Question

how can I get all different values of a specific column in a datatool database? Example is the following table read into a datatool database:

Name,Town,Age
Adam,Xcity,20
Berta,Ytown,30
Cesar,Ztington,40
Dora,Ztington,20
Emil,Ytown,30
Franz,Ytown,20

Now I'd like to get a list of all towns (Xcity, Ytown,Ztington) in that database and of all ages (20,30,40) in that database without duplicates, that I can store in macro/whatever to reuse later with datatool or pgffor.

Answer 1

datatool has a macro called \DTLifinlist which can check if an element is in the given comma-separated list, so this can be used when constructing a list of all the towns or ages, like this:

\documentclass{article}

\usepackage{etoolbox}
\usepackage{datatool}

\begin{filecontents}{test.csv}
Name,Town,Age
Adam,Xcity,20
Berta,Ytown,30
Cesar,Ztington,40
Dora,Ztington,20
Emil,Ytown,30
Franz,Ytown,20
\end{filecontents}

\DTLloaddb{data}{test.csv}

\begin{document}

\newcommand*{\uniquetowns}{}
\newcommand*{\uniqueages}{}

\DTLforeach*{data}{\Town=Town,\Age=Age}{%
  \expandafter\DTLifinlist\expandafter{\Town}{\uniquetowns}%
  {}% do nothing, already in list
  {% add to list
    \ifdefempty{\uniquetowns}%
    {\let\uniquetowns\Town}% first element of list
    {% append to list
      \eappto\uniquetowns{,\Town}%
    }%
  }%
  % Similarly for age
  \expandafter\DTLifinlist\expandafter{\Age}{\uniqueages}%
  {}% do nothing, already in list
  {% add to list
    \ifdefempty{\uniqueages}%
    {\let\uniqueages\Age}% first element of list
    {% append to list
      \eappto\uniqueages{,\Age}%
    }%
  }%
}

List of unique towns: \uniquetowns.

List of unique ages: \uniqueages.
\end{document}

Result: