如果未找到条目，datatool 的 \DTLfetch 将返回最后找到的值，而不是“未定义值”

Question 1

我认为最简单的方法是使用\DTLgetvalueforkey（并评估/调用定义的宏\DTLgetvalueforkey）：

\documentclass{article}

\usepackage{datatool}

\begin{filecontents*}{abbreviationss.csv}
    "term", "abbreviation"
    Machine Learning, ML
    Artificial Neural Network, ANN
\end{filecontents*}

\DTLloaddb{abbreviations}{abbreviationss.csv}

\newcommand{\getabbreviation}[1]{% 
    #1 : %
    \DTLgetvalueforkey{\scratchmacro}{abbreviation}{abbreviations}{term}{#1}%
    \scratchmacro
    %\scratchmacro will be \@dtlnovalue -> "Undefined Value" if #1 is not found.
    % Alternatively:
    % \DTLifnull{\scratchmacro}{Hey, this is undefined!}{\scratchmacro}%
}%

\begin{document}

\begin{itemize}
\item\getabbreviation{x}   
\item\getabbreviation{Machine Learning}
\item\getabbreviation{x}
\item\getabbreviation{Artificial Neural Network}
\item\getabbreviation{x}
\end{itemize}
    
\end{document}

与 Simon Dispa 的方法类似，其基于\DTLforeach*：

\documentclass{article}

\usepackage{datatool}

\begin{filecontents*}{abbreviationss.csv}
    "term", "abbreviation"
    Machine Learning, ML
    Artificial Neural Network, ANN
\end{filecontents*}

\DTLloaddb{abbreviations}{abbreviationss.csv}

\newif\ifnotfound
\global\notfoundtrue

\newcommand{\getabbreviation}[1]{% 
    #1 : %
    \global\notfoundtrue
    \DTLforeach*[{\equal{\term}{#1}}]{abbreviations}{\abbreviation=abbreviation, \term=term}%
                {\abbreviation\global\notfoundfalse\dtlbreak}%
    \ifnotfound Undefined Value\fi
}%

\begin{document}

\begin{itemize}
\item\getabbreviation{x}   
\item\getabbreviation{Machine Learning}
\item\getabbreviation{x}
\item\getabbreviation{Artificial Neural Network}
\item\getabbreviation{x}
\end{itemize}
    
\end{document}

如果现实生活中的情况不只是关于某些itemize事情，但你真的需要抓住\DTLfetch找不到的情况⟨column1 value⟩：

! Package datatool Error: No row found in database `abbreviations' for column ` 1' matching `x'.使用 MWE 时，您会在终端和 .log 文件中发现很多错误。

语法\DTLfetch是：

\DTLfetch{⟨db name⟩}%
         {⟨column1 name⟩}%
         {⟨column1 value⟩}%
         {⟨column2 name⟩}%

部分 ”6.11.1 对当前行进行操作“datatool bundle 2.32 版用户手册中写道：

\dtlgetrowindex{⟨row cs⟩}{⟨db name⟩}{⟨col idx⟩}{⟨value⟩}
类似于，\DTLgetrowindex但如果未找到匹配项，则不会产生错误。您可以使用来测试结果。\ifx⟨row cs⟩\dtlnovalue

因此，您可以编写一个\DTLfetch适用的包装器

\dtlgetrowindex{⟨row cs⟩}%
               {⟨db name⟩}%
               {⟨col idx of column1⟩}%
               {⟨column1 value⟩}%

定义⟨row cs⟩然后执行：

\ifx⟨row cs⟩\dtlnovalue\expandafter\@firstoftwo\else\expandafter\@secondoftwo\fi
{%
  ⟨ tokens in case ⟨column1 name⟩ does not have a value ⟨column1 value⟩ ⟩%
}{%
  \DTLfetch{⟨db name⟩}{⟨column1 name⟩}{⟨column1 value⟩}{⟨column2 name⟩}%
}%

您需要 column1/of 的索引⟨col idx of column1⟩。

您可以通过以下方式获得\dtlcolumnindex{⟨db name⟩}{⟨column1 name⟩}

为了在执行之前扩展参数\dtlgetrowindex——尤其⟨col idx of column1⟩是= —— ，可以定义一个临时宏，通过前面添加阻止命令的扩展和待定义命令的扩展，但不阻止其他参数的扩展，然后调用该临时宏：\dtlcolumnindex{⟨db name⟩}{⟨column1 name⟩}\dtlgetrowindex\protected@edef\dtlgetrowindex⟨row cs⟩\noexpand\dtlgetrowindex

\documentclass{article}

\usepackage{datatool}

%\DTLfetchifcolOneValueExists{<db name>}%
%                            {<column1 name>}%
%                            {<column1 value>}%
%                            {<column2 name>}%
%                            {<tokens in case <column1-value> does not exist>}%
%
%   Turn @ into a normal letter so that it can be used as part of the name of a control sequence:
\makeatletter
%   Test if the command \NewDocumentCommand is not available.
%   If so, use traditional \newcommand.  Otherwise use it.
\@ifundefined{NewDocumentCommand}%
{\newcommand\DTLfetchifcolOneValueExists[5]}%
{\NewDocumentCommand\DTLfetchifcolOneValueExists{mmmmm}}%
{%
  % open a local scope:
  \begingroup
  % define the scratch macro
  \protected@edef\@dtl@dogetrowforvalue{%
    \endgroup\noexpand\dtlgetrowindex{\noexpand\dtl@rowidx}{#1}{\dtlcolumnindex{#1}{#2}}{#3}%
  }%
  % Call the scratch-macro:
  \@dtl@dogetrowforvalue
  % Expansion of the scratch-macro yields:
  %   \endgroup %<-after processing \endgroup the local scope is closed and thus the scratch-macro is undefined.
  %   \dtlgetrowindex{\dtl@rowidx}{<expansion of #1>}{<expansion of \dtlcolumnindex{#1}{#2}>}{<expansion of #3>}%
  %   % After processing \dtlgetrowindex the macro \dtl@rowidx is defined.
  \ifx\dtl@rowidx\dtlnovalue\expandafter\@firstoftwo\else\expandafter\@secondoftwo\fi
  {#5}{\DTLfetch{#1}{#2}{#3}{#4}}%
}%
\makeatother


% csv file 1
\begin{filecontents*}{abbreviationss.csv}
    term, abbreviation
    Machine Learning, ML
    Artificial Neural Network, ANN
\end{filecontents*}

\DTLloaddb{abbreviations}{abbreviationss.csv}

\begin{document}

\begin{itemize}
    \item x :
          \DTLfetchifcolOneValueExists{abbreviations}%
                                      {term}%
                                      {x}%
                                      {abbreviation}%
                                      {Column `term' doesn't have a value `x'}
    \item Machine Learning :
          \DTLfetchifcolOneValueExists{abbreviations}%
                                      {term}%
                                      {Machine Learning}%
                                      {abbreviation}%
                                      {Column `term' doesn't have a value `Machine Learning'}
    \item x : 
          \DTLfetchifcolOneValueExists{abbreviations}%
                                      {term}%
                                      {x}%
                                      {abbreviation}%
                                      {\DTLstringnull}
    \item Artificial Neural Network :
          \DTLfetchifcolOneValueExists{abbreviations}%
                                      {term}%
                                      {Artificial Neural Network}%
                                      {abbreviation}%
                                      {Column `term' doesn't have a value `Artificial Neural Network'}
    \item x :
          \DTLfetchifcolOneValueExists{abbreviations}%
                                      {term}%
                                      {x}%
                                      {abbreviation}%
                                      {\dtlnovalue}
\end{itemize}

\end{document}

如果您希望获得一些空值而不是短语“列'term'没有值'x'”，您可以提供其中一个标记/ \DTLstringnull/。（yields。yields。yields \DTLnumbernull。所有三个标记都可以用作等的第一个参数\dtlnovalue。）\DTLstringnullNULL\DTLnumbernull0\dtlnovalueUndefined Value\DTLifnull

Answer