Symbols for Outer Joins

With unicode-math and XeLaTeX or LuaLaTeX, you can use these symbols directly:

% compile with xelatex or lualatex
\documentclass{article}
\usepackage{fontspec}
\usepackage{unicode-math}
\setmathfont{XITS Math}

\begin{document}
$A ⟖ B$
\end{document}

They also have aliases: \leftouterjoin, \rightouterjoin, \fullouterjoin

\documentclass[a4paper]{article}
\usepackage{amssymb}

\def\ojoin{\setbox0=\hbox{$\bowtie$}%
  \rule[-.02ex]{.25em}{.4pt}\llap{\rule[\ht0]{.25em}{.4pt}}}
\def\leftouterjoin{\mathbin{\ojoin\mkern-5.8mu\bowtie}}
\def\rightouterjoin{\mathbin{\bowtie\mkern-5.8mu\ojoin}}
\def\fullouterjoin{\mathbin{\ojoin\mkern-5.8mu\bowtie\mkern-5.8mu\ojoin}}

\begin{document}
\[A\leftouterjoin B\]
\[A\rightouterjoin B\]
\[A\fullouterjoin B\]
\end{document}