$\ast$

\ast

The "\ast" is derived from the "asterisk."

$\star$

\star

$\ltimes$

\ltimes

It is a combination of the left line and the times symbol.

$\rtimes$

\rtimes

It is a combination of the right line and the times symbol.

$\Join$

\Join

It is a symbol used to represent natural join. It can also be represented using the "\bowtie", named after its shape.