Category talk:Book:Structured Query Language

Two categories?
Why do we have the two different categories SQL and Structured Query Language? I know, we have a book Structured Query Language and a lot of pages about SQL (which are not part of the book). But I think that it is part of the nature of categories that they appertain to different objects.--Kelti (discuss • contribs) 14:00, 27 February 2012 (UTC)
 * Because in the main categories are used as the container for all pages in a book. Therefore |Category:Structured Query Language contains all pages of the book Structured Query Language rather than on, say, Wikipedia, where it would contain all pages about structured query language. In comparison |Category:SQL is a "meta category" which contains all books about SQL or that have SQL as a major aspect. QU TalkQu 14:36, 27 February 2012 (UTC)