Category query cleanups

[skeltoac]

Category queries concatenate category_id conditions.

How about doing it this way:

?cat=1,2,3,-4,-5,6,-8,-9

 AND category_id IN (1, 2, 3, 6) AND category_id NOT IN (4, 5, 8, 9)

We also need to put UNIQUE back in due to the way JOIN returns one row for every matching post2cat record.

Um, DISTINCT.

Due to the JOIN, NOT IN can fail in this circumstance: a post is in two categories, one of which is not in the NOT IN list. The usual case here is that a post is in categores "Parent" and "Child", and the excluded category is "Child." In this case, the post will appear because the JOIN returns a row with the the category_id of Parent, which is not an excluded category.

I think this is acceptable. Any argument?

Patch attached. I also changed the field list to

apply_filters('posts_fields', "$wpdb->posts.*");

which returns fewer columns (none from JOINed tables) and makes DISTINCT work. If anybody knows of a need for JOINed columns, speak now.

(In [3767]) Category query cleanups from skeltoac. fixes #2709