Customizing biblatex "alphabetic" style

Question

I'm trying to customize my citation a little, but don't know how to achieve my goals.

MWE:

\documentclass[
fleqn,                                          
fontsize=11pt,
paper=a4,
bibliography=totoc,listof=totoc,BCOR=25mm,
english,
DIV = 12
]{scrreprt}
\usepackage[english]{babel}
\usepackage[style=alphabetic, autocite=footnote]{biblatex}
\usepackage[babel]{csquotes}

\usepackage{filecontents}
\begin{filecontents}{mybib.bib}
@online{test,
url = {http://test.test},
urldate = {2018-12-15},
title = {Test Bib},
label = {TB}
}
\end{filecontents} 

\bibliography{mybib}
\begin{document}
This is the first example\autocite{test}.\\
This is the second example\footfullcite{test}.
\printbibliography
\end{document}

My Goal is to combine the "footnotes" of the first example in brackets and the second example, such that it prints (the 1 should be subscripted):

1 [TB] Test Bib. URL: http://test.test (visited on 12/15/2018)

For compiling I'm using LuaLaTex,

Best, cortesis

Answer 1

You can define a new \cite... command based on \footfullcite that also prints the alphabetic label

\documentclass[
  fontsize=11pt,
  paper=a4,
  bibliography=totoc,listof=totoc,BCOR=25mm,
  english,
  DIV = 12
]{scrreprt}
\usepackage{babel}
\usepackage[autostyle]{csquotes}
\usepackage[style=alphabetic, autocite=footnote]{biblatex}

\DeclareCiteCommand{\footfullcitewithalphalabel}[\mkbibfootnote]
  {\usebibmacro{prenote}}
  {\printtext[labelalphawidth]{%
     \printfield{labelprefix}%
     \printfield{labelalpha}%
     \printfield{extraalpha}}%
   \printunit{\addspace}%
   \usedriver
     {\DeclareNameAlias{sortname}{default}}
     {\thefield{entrytype}}}
  {\multicitedelim}
  {\usebibmacro{postnote}}

\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@online{test,
url = {http://test.test},
urldate = {2018-12-15},
title = {Test Bib},
label = {TB}
}
\end{filecontents} 
\addbibresource{\jobname.bib}

\begin{document}
This is the first example\autocite{test}.

This is the second example\footfullcitewithalphalabel{test}.

\printbibliography
\end{document}

I personally think that an alphabetic bibstyle does not really lend itself to footnote citations. Like numeric labels alphabetic labels are intended for use in the text directly, that's why they are reasonably short and compact. Indeed, since each footnote starts in a new line, you may end up wasting more space when you use alphabetic in footnotes rather than inline.