* Aryeh Gregor Simetrical+wikilist@gmail.com [Sat, 16 Jan 2010 23:06:06 -0500]:
You can do this with database queries fine -- there are already several different toolserver tools that will do category intersections for you, and a couple extensions. In fact, bog-standard search will do it for you, although AFAIK only for categories added literally (not by templates):
http://en.wikipedia.org/w/index.php?title=Special:Search&redirs=1&se...
Intersections probably are inefficient when someone needs a range search between, let's say 1944 and 1965. SMW has probably right approach that something sequental and numerical like date, mass, speed should not be a Category but a Property.. Also, it's a bit sad that so many toolserver tools are standalone and are not a part of MediaWiki distribution. That tool should be a part of Special:Search.
It wouldn't be that hard to allow template-added categories too. I assume you have categories like "books published in America", "books published in 1872", and "biographies" -- if not, you can easily add them via your templates (although that wouldn't work right now with standard search AFAIK, it would work with things like CatScan).
When comes to subcategories, I always wondered why they have to include the name of parent category: http://en.wikipedia.org/wiki/Category:Books The word "Books" is repeated many times through the nested categories, although we already know these are the "Books". However, this brings the problem with "de-parenting" of categories, which is hard to resolve, because the Categories are the part of source text. Perhaps, a full category name and a shorted subcategory alias, defined at NS_CATEGORY pages. Dmitriy