XQL

Welcome to the XQL FAQ - a Frequently Asked Questions list for the XML Query Language. This FAQ and the associated XQL Mailing List are intended to point you to resources that will help you learn, use, or implement XQL. XQL has already been implemented in web browsers, document repositories, XML middleware, Perl libraries, and command-line utilities. If you are looking for an XML query language that is simple, powerful, easily implemented, and available now, XQL is probably your best bet. The best way to learn about XQL depends on your learning style. The following table lists different ways to begin learning about XQL: If you are a hands-on learner, you will probably want to get one of the implementations listed in the next section and play around with the language. Here are implementations that I am aware of. If you want your XQL implementation listed here, please send email to me at [email protected] . I have not tested most of these implementations; the descriptions are lightly edited versions of texts found on the web sites that describe them. Implementations are listed in alphabetical order. In addition, the proposal contains more than I think should really be required, so I am defining a minimum implementation set will be drawn from the "XQL Proposal" listed above. I also plan to post test suites. If people have test suites they would like to share, please let me know about them. The XQL Proposal submitted with webMethods and Microsoft includes a few features that do not fit comfortably into the XQL evaluation model, and which must be optional. Granted, XQL is a three letter acronym that begins with X and ends with L, and the proposal was submitted to a W3C Query Language Workshop, but that does not make it a W3C standard. It is very likely that the W3C will be hosting a Query Language Activity, and we would be very interested in participating in this activity, but there is no way to guarantee how much influence XQL or any other language will have on that activity. Given the track record of W3C standards, there probably won't be a W3C recommendation for a query language for at least a year, and one of the big reasons I have frozen the current version of XQL is to give people something that they can implement while waiting for a W3C query language. The people who claim XQL is too simple complain about the lack of joins and transformations, which David Maier considered important parts of an XML Query Language in his paper, Database Desiderata for an XML Query Language . Joins : XQL allows semi-joins, but not full joins. For instance, it is possible to ask for all books whose author is the same as the author of "Moby Dick": XQL has been criticized for being too simple, and it has also been criticized for being too complex. Marcelo of Cantos, who is associated with the Structured Information Manager , has said that XQL "offers a good compromise between expressivity and simplicity" (XML Dev List, Fri, 26 May 1999). The minimal implementation set of XQL is intended to be simple, easy to implement, and easily optimizable. Nevertheless, some minimalists have argued that even the minimal implementation set of XQL is too complex. Tim Bray has written a useful paper that indicates what may be the smallest possible useful query language; the paper is called "Element Sets: A Minimal Basis for an XML Query Engine" . The best implementation depends a great deal on what you are doing, eg. whether you are dealing with single documents in RAM or large document repositories. Most implementations that work on single documents seem to be basically navigational, implementing the basic operations of XQL in a very straightforward manner. [Burkowski1992] F.J. Burkowski. An algebra for hierarchically organized text-dominated databases. Information Processing & Management, 28(3):333--348, 1992. [Dao1997] T.Dao, R.Sacks-Davis, and J.A. Thom. An indexing scheme for structured documents and its implementation. In DASFAA '97, 199