Querying XML Documents

7th December 2000

Organisation

• XML query history and QL '98
• XML Query WG history, goals and status
• XML Query Requirements
• XML Query Data Model
• XML Query Algebra
• Questions

XML query history

• Early 1998: ``roll your own query language''
• XSL Working Group - XSLT needed syntax to select nodes
• XML Linking Working Group - XPointer needed syntax to select a location
• February 1999 joint meeting - Rapprochement on 90% of syntax
• XPath - W3C recommendation with XSLT (Used by both XSLT and XPointer candidate recommendations)

XML query history - 2

• Early query facilities for SGML
• Academic research into semi-structured data and its operations
• XQL: See http://metalab.unc.edu/xql (complete (I think) but not very powerful)
• XML-QL, August, 1998 - http://www.w3.org/TR/NOTE-xml-ql/

Query Languages Workshop '98

• W3C sponsored workshop
• Boston (USA), December 2-3, 1998
• 98 participants: W3C members, database vendors, invited experts, etc.
• 66 position papers (Particularly check out: Mayers - "Desiderata for an XML query language" (ten points) Cotton & XXX - "(summary paper)")
• See: http:###

W3C XML Query WG - History

• July 1999 - Working Group proposed as part of XML Activity phase 3 rechartering
• September 1999 - WG chartered
• More than 30 W3C member companies
• Eight F2F meetings and 40+ telecons so far
• Close working relationship with other W3C Working Groups (Schema, XSL, I18N) (This can make life difficult)

W3C XML Query WG - Goals

• `The goal of the XML Query WG is to produce a data model for XML documents, a set of query operators on that data model, and a query language based on those query operators.''

W3C XML Query WG - Status

• Jan 2000 - Requirements Working Draft
• May 2000 - XML Query Data Model WD
• May 2000 - Feedback on Schema Last Call
• August 2000 - Revised Requirements Working Draft wiht Use Cases
• Dec 2000 - XML Query Algebra WD
• Future public WDs every three months
• Proposed recommendation(s) (later in 2001)

XML Query Requirements

• Usage Scenarios ### slide whipped away

General Requirements

• Non-procedural query language (what to find, not how to find it)
• XML syntax for query language but also a readable syntax (Yes, these requirements are contradictory)
• Protocol independent
• Standard error conditions
• Future support for updates

XML Query Data Model

• Built on XML Infoset and PSV (Inforset seems to be something like a parsed document object) (PSV is the Post Schema Validatation info-set: the significant difference here seems to be that the data is typed)
• Namespace aware
• Support for XML Schema data types
• Support for inter- and intra-document references

XML Query Functionality

• Operators on all data types (e.g. "less than" for numeric types)
• Text operators across element boundaries (e.g. proximity matching of text ignoring tags)
• Support for hierarchy and sequence
• Ability to combine data from different locs
• Aggregation and sorting
• Combination of operators including queries as operands (analogous to embedding a query in to FROM clause of an SQL query -- not possible in SQL.)

XML Query Functionality - 2

• Support for NULL values
• Structural preservations
• Identity preservation
• Operations on names (e.g. wildcarding element names -- SELECT *_id FROM ...)
• Operations on ``schemas''
• Extensibility
• Closure

XML Query Data Model WD

• Defines information available to a query processor
• Infoset plus the following
  • - Support for XML Schema data types (PSV)
  • - Support for document collections
  • - Support for references
• Node-labelled tree contructor model with node identity (I think this is a concrete interface to the infoset, which is an abstract concept. I think)
• Mapping from Infoset to Query Data Model defined in Annex A (Most important part of the document!)
• http://www.w3.org/TR/query-datamodel/

XML Query Algebra WD

• Defines operations on Query Data Model
• Simple principles, easy to use
• Firm mathematical foundation
• Many issues still open
  • - References
  • - Unordered data
  • - Algebra subset of syntax?
• http://www.w3.org/TR/query-datamodel/

Introduction to the Query Algebra: Part I: Data model and types

Document                                Algebra
--------                                --------
<bib>                                   bib [
  <book>                                  book [
    <title>Data on the web</title>          title [ "Data on the web" ],
    <year>1999</year>                       year [ 1999 ],
    <author>Fruitbat</author>               author [ "Fruitbat" ]
  </book>                                 ]
</bib>                                  ]

(... and the value of this is? ...)

(This moves too fast to take notes)

Questions

• Today
• Later pcotton@microsoft.com
• Feedback email list: www-xml-query-comments@w3.org
• Public email list: www-ql@w3.org (### not totally sure about this address)