Recently I saw a topic in the forum XnView Classic with someone posting exactly the same question I was asking myself: http://newsgroup.xnview.com/viewtopic.php?f=35&t=24822. Unfortunately there was no answer, so I post it here in MP quoting the question and hoping there will be an answer

"It would really be a great feature if there was a possibility to exclude some categories.
Ideally it would be just magical if user could construct his own searches with categories. Like you would like to find all pictures with keyword vacation that also include keywords for your wife or daughter but not with the beach keyword. That would make you cooler than ACDSee and Adobe Bridge.
Is it hard to implement? Shouldn't that be one of basic SQL features since XnView MP uses databases to store data?"