SQL preferences: where does the _DOMAIN_ in the query come from

[Guido Goluke, Majorlabel]

I'm in the process of setting up my preferences through SQL. Now spamc 
is invoked through a Postfix milter, but that's besides the point, since 
whatever way spamc is called, it can only specify one -u param as the 
username. However, the WIKI and Docs version of the proposed query use 
both a _USERNAME_ and _DOMAIN_ variable. In my setup, the domain 
variable is empty. How do I get the _DOMAIN_ variable to be filled so 
that I can make a flexible SQL setup?

Guido Goluke
Majorlabel