**LaTeX** insert Unicode Characters (relational Database chars)

Question

I recently tried to insert the Characters for the relational Algebra (left/right (semi) join, ...) but failed up to now. I know that it is problematic with Unicode and ASCII encoding, but I thought with [utf8]{inputenc} it should work.

But

\documentclass{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\DeclareUnicodeCharacter{27d7}{⟗}
\begin{document}
⟗
\end{document}

does not work.

! LaTeX Error: Missing \begin{document}.
See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...
l.3 \DeclareUnicodeCharacter{27d7}{⟗}
? q
OK, entering \batchmode

is the font the problem or am I doing something wrong?

Oh and the nicest thing for me would be to know how to create a command \fullOuterJoin which inserts the character for me (easier than always copy the character).

Answer 1

You want to look your symbol up in The Comprehensive LaTeX Symbols List, or one of the methods here. In this case, Ulrike Fischer found it for you and posted it in a comment: \fullouterjoin from stix (or also stix2).

The first argument of \DeclareUnicodeCharacter should have letters capitalized, so 27D4. The second argument should be the command to produce the character. That is, if you already have a command \fullouterjoin, you would use \DeclareUnicodeCharacter to turn the character ⟖ into a macro that runs that command.

In this case, \fullouterjoin is a math-mode command in stix and you want to want to use it in text mode, so \ensuremath{\fullouterjoin} will work.

\documentclass{scrartcl}
\usepackage[utf8]{inputenc} % The default since 2018
\usepackage[T1]{fontenc}
\usepackage{stix2}
\DeclareUnicodeCharacter{27D7}{\ensuremath{\fullouterjoin}}
\begin{document}
( \textnormal{foo} ⟗ \textnormal{bar} )
\end{document}

An alternative, which, unlike \DeclareUnicodeCharacter, works with LuaLaTeX or XeLaTeX, is newunicodechar:

\documentclass{scrartcl}
\usepackage[utf8]{inputenc} % The default since 2018
\usepackage[T1]{fontenc}
\usepackage{stix2}
\usepackage{newunicodechar}
\newunicodechar{⟗}{\ensuremath{\fullouterjoin}}
\begin{document}
( \textnormal{foo} ⟗ \textnormal{bar} )
\end{document}

Either of these had the disadvantage that, to load one symbol, you had to load the entire stix or stix2 package. There is no standard way to load only one symbol from a legacy TeX package, but here’s some code that will work in this case:

\documentclass{scrartcl}
\usepackage[utf8]{inputenc} % The default since 2018
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\DeclareFontEncoding{LS1}{}{}
\DeclareFontSubstitution{LS1}{stix2}{m}{n}
\DeclareFontFamily{LS1}{stix2frak}{\skewchar\font127 }
\DeclareFontShape{LS1}{stix2frak}{m}{n} {<-> stix2-mathfrak}{}
\DeclareFontShape{LS1}{stix2frak}{b}{n} {<-> stix2-mathfrak-bold}{}
\newcommand\textfullouterjoin{%
  {\fontencoding{LS1}\fontfamily{stix2frak}\fontshape{n}\selectfont\symbol{"13}}}
\newcommand\fullouterjoin{%
  \ifmmode\mathbin{\text{\textfullouterjoin}}%
  \else\textfullouterjoin%
  \fi}
\DeclareUnicodeCharacter{27D7}{\fullouterjoin}
\begin{document}
( \textnormal{foo} ⟗ \textnormal{bar} )
\end{document}

Since the manual for stix2-type1 has a font chart that says which slot of which font this symbol is in, I copied the relevant lines from the .sty and .fdd source files for the package, to set up the symbol font. I then wrote a command to either insert the glyph as a text symbol in text mode, or typeset it as a binary operator in math mode. (It therefore no longer needs \ensuremath.) Finally, I copied over the code from my first example to make the Unicode character expand to this command.