Wikipedia:Category intersection

Many existing categories are logically the intersection of attributes for which "primary" categories exist, for example Category:American actors is logically the intersection of Category:Actors and Category:American people. Although these "primary" categories are today generally subdivided into subcategories, if they were directly (fully) populated the "intersection categories" could be automatically generated. are already organized into fully populated primary categories.

This proposal tries to envision the changes necessary to make category intersection a reality. It is designed to augment the current categorization system, not replace it.

In mid 2005 the category table of contents template, {{}}, was created. With the table of contents it became possible to navigate through very large categories with a few clicks. Due to the combination of the performance change and CategoryTOC, there is no longer any reason that categories need to be small.

Multiple category taxonomies have been part of the categorization scheme from the beginning. It is possible to take a category and subcategorize it in many different ways. Use of these "subset" categories makes it difficult to find all members of a "higher level" category; either articles have to be added to both the "subset" and "higher level" categories or the members of the "subcategories" (and, recursively, their subcategories) have to be enumerated. Precisely defining the circumstances in which articles should be added to both "lower level" and "higher level" categories, and even whether this is ever appropriate, remains a source of continuing discussion among editors (see, for example, and ).

Category intersection has been a desired feature for quite some time. Looking through the , someone even wrote the code . points out its limitation: "I don't see how this can be more than marginally useful unless it also searches all subcategories to infinite depth (with recursion checks?!)."

As of April 2008^[update] it is possible to use the Search tool to find category intersections; however, this facility does not look inside subcategories. To find a category intersection, type incategory:"CategoryName" in the search box for each category of interest. For example, incategory:"German films" incategory:"1998 films" will return the articles that are common to both categories – German films released in 1998. This method will only work for categories which have been directly entered into articles (e.g. Category:1998 films), not where categories are assigned through templates, e.g. from any of the stub message templates or from the template {{}}.

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