The scenario is, some posts have one category, ok. Some others have more than one category. I must create post with content and also slideshow thumbnails for use with Nivo plugin gallery. Ok.
I want to filter all the posts in the "slideshow" category when in index pages, so I use a simply
query_posts($query_string . '&cat=-1,-36');
Category number 1 is "Undefined", and number 36 is "Slideshow". Ok. But... I have some posts that have another category, lets say, number 50, "Applications" and I wanna show these posts, despite they are too in "Slideshow" category.
I cannot find the way to say WP "show me posts, except the ones in categories 1 and 36, but show me them if they have more categories".
Googling and reading a ton of docs and pages I haven't found any way to know the categories count by post (not how many posts by category), maybe to build a query like "show me that, filter that EXCEPT if count(categories are my post present) > 2 ).