XML-QL A Query Language for XML

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XML-QL A Query Language for XML

• Charuta Nakhe
• charuta_at_cse.iitb.ernet.in

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Querying XML document

• What is a query language?
• Why not adapt SQL or OQL to query XML data?
• What is an XML query?
• What is the database? -- XML documents
• What is input to the query? XML document
• What is the output of the query? XML document

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Requirements of XML query language

• Query operations
• Selection eg. Find books with S. Sudarshan as
  author
• Extraction eg. Extract the publisher field of
  above books
• Restructuring Restructuring of elements
• Combination Queries over more than one
  documents
• Must be able to transform create XML structures
• Capability for querying even in absence of schema

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The XML-QL language

• The XML-QL language is designed with the
  following features
• it is declarative, like SQL.
• it is relational complete, e.g. it can express
  joins.
• it can be implemented with known database
  techniques.
• it can extract data from existing XML documents
  and construct new XML documents.
• XML-QL is implemented as a prototype and is
  freely available in a Java version.

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Example XML document

• ltbibgt  
• ltbook year1997gt     
• lttitlegtInside COMlt/titlegt
•      ltauthorgtDale Rogersonlt/authorgt
•     ltpublishergtltnamegtMicrosoftlt/namelt/publish
  ergt  
• lt/bookgt  
• ltbook year1998gt     
• lttitlegtDatabase system conceptslt/titlegt
•      ltauthorgtS. Sudarshanlt/authorgt
• ltauthorgtH. Korthlt/authorgt
• ltpublishergt ltnamegtMcGrawHilllt/namelt/publi
  shergt lt/bookgt
• lt/bibgt

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Matching data using patterns

• Find those authors who have published books
  for McGraw Hill
• WHERE
• ltbibgtltbookgt
• ltpublishergtltnamegtMcGraw Hilllt/gtlt/gt
• lttitlegttlt/gt
• ltauthorgtalt/gt
• lt/bookgtlt/bibgt IN bib.xml
• CONSTRUCT ltresultgtlttitlegttlt/gtltauthorgtalt/gtlt/gt
  
• the t and a are variables that pick out
  contents.
• the output is a collection of author names.

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Result XML document

• ltresultgt
•      lttitlegtDatabase system
  conceptslt/titlegt
•      ltauthorgtS. Sudarshanlt/authorgt
• lt/resultgt
• ltresultgt
•      lttitlegtDatabase system
  conceptslt/titlegt
•      ltauthorgtH. Korthlt/authorgt
• lt/resultgt

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Grouping with Nested Queries

• Group results by book title
• WHERE
• ltbib.bookgtplt/gt IN bib.xml,
• lttitlegttlt/gt
• ltpublishergtltnamegtMcGraw Hilllt/gtlt/gt IN p
• CONSTRUCT ltresultgt lttitlegttlt/gt
• WHERE ltauthorgtalt/gt IN p CONSTRUCT a
• lt/gt
• Produces one result for each title and contains a
  list of all its authors

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Result XML document

• ltresultgt
•      lttitlegtDatabase system
  conceptslt/titlegt
•      ltauthorgtS. Sudarshanlt/authorgt
•      ltauthorgtH. Korthlt/authorgt
• lt/resultgt
• .
• .

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Constructing XML data

• Results of a query can be wrapped in XML   
• WHERE ltbib.bookgt
•           ltpublishergtltnamegtMcGrawHilllt/gtlt/gt
•           lttitlegttlt/gt
• ltauthorgtalt/gt
•         lt/gt IN bib.xml   
• CONSTRUCT ltresultgtltauthorgtalt/gtlttitlegttlt/gtlt/gt
• Results are grouped in elements.
• The pattern matches once for each author, which
  may give duplicates of books.

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Joining elements by value

• Find all articles that have at least one
  author who has also written a book since 1995
• WHERE ltbib.articlegt
• ltauthorgtnlt/gt
• I lt/gt CONTENT_AS a IN bib.xml,
• ltbook yearygt
• ltauthorgtnlt/gt
• lt/gt IN bib.xml,
• y gt 1995
• CONSTRUCT ltarticlegtalt/gt
• CONTENT_AS a following a pattern binds the
  content of the matching element to the variable
  a

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Tag variables

• Find all publications in 1995 where Smith is
  either an author or editor
• WHERE ltbib.pgt
• lttitlegttlt/gt ltyeargt1995lt/gt ltegtSmithlt/gt
• lt/gt IN bib.xml,
• e IN author, editor
• CONSTRUCT ltpgtlttitlegttlt/gtltegtSmithlt/gtlt/gt
• p matches book and article.
• e matches author and editor.

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Regular-path expressions

• Find the name of every part element that
  contains a brand element equal to Ford,
  regardless of the nesting level at which r
  occurs.
• WHERE ltpartgt
• lttitlegtrlt/gt
• ltnamegtFordlt/gt
• lt/gt IN bib.xml
• CONSTRUCT ltresultgtrlt/gt
• Regular path expressions can specify element
  paths of arbitrary depth

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Other interesting features

• Constructing explicit root element
• Grouping of data
• Transforming XML data
• Integrating data from different XML sources

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Links for more information

• www.w3.org/TR/NOTE-xml-ql The XML-QL W3C Note
• www.research.att.com/mff/xmlql/doc The XML-QL
  home page
• www.w3.org/XML/Activity.htmlquery-wg The XML
  Query Working Group
• www.w3.org/TR/xmlquery-req XML Query
  Requirements (W3C Working Draft)
• www.oasis-open.org/cover/xmlQuery.html Robin
  Cover's page on XML query languages

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Example DTD

• lt!ELEMENT book (author, title, publisher)gt
• lt!ATTLIST book year CDATAgt
• lt!ELEMENT article (author, title, year?)gt
• lt!ATTLIST article type CDATAgt
• lt!ELEMENT publisher (name, address)gt
• lt!ELEMENT author (firstname?, lastname)gt

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Creating an explicit root element

• Every XML document must have a single root.
  XML-QL supplies an ltXMLgt element as default, but
  others may be specified   
• CONSTRUCT ltresultsgt    
• WHERE ltbib.bookgt
•        ltpublishergtltnamegtMcGrawHilllt
  /gtlt/gt            lttitlegt  t lt/gt
  
•       ltauthorgt a lt/gt         
• lt/gt IN bib.xml
• CONSTRUCT ltresultgtltauthorgtalt/gtlttitlegttlt/gtlt/gt
•   lt/resultsgt