XQL (XML Query Language)

1
XQL (XML Query Language)

• Jonathan Robie (Software AG) ltjonathan.robie_at_sagus
  .comgt
• Eduard Derksen (CSCIO) ltenno_at_att.comgt
• Peter Fankhauser (GMD-IPSI) ltfankhaus_at_darmstadt.gm
  d.degt
• Ed Howland (DEGA) ltEd_at_dega.comgt
• Gerald Huck (GMD-IPSI) lthuck_at_darmstadt.gmd.degt
• Ingo Macherius (GMD-IPSI) ltmacherius_at_darmstadt.gmd
  .degt
• Makoto Murata (Fuji Xerox) ltmurata_at_apsdc.ksp.fujix
  erox.co.jpgt
• Michael Resnick (Object Design, Incorporated)
  ltresnick_at_odi.comgt
• Harald Schöning (Software AG) ltharald.schoening_at_so
  ftwareag.degt

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XQL vs. SQL

• What is the database?
• SQL A set of tables
• XQL A set of one or more XML documents.
• What is the query language?
• SQL A query language that uses the structure of
  tables as a basic model.
• XQL A query language that uses the structure of
  XML documents as a basic model.
• What is the input to a query?
• SQL The FROM clause determines the tables which
  are examined by the query.
• XQL A query is given a list of input nodes from
  one or more documents.
• What is the result of a query?
• SQL The result of a query is a table containing
  a set of rows this table may serve as the basis
  for further queries.
• XQL The result of a query is a list of XML
  document nodes, which may serve as the basis for
  further queries.

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XQL Expressions

• Terms
• Namespaces and Names
• Comparisons
• Hierarchy and Filters
• Boolean and Set Operators
• Grouping Operator
• Sequence

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XQL Expressions (cond.)

• Functions
• Extensible Functions
• References (not incorporated into XQL)
• Joins
• Renaming Operator
• Precedence of Operators

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Learn XQL by Examples

• Terms
• A simple string is interpreted as an element
  name
• Query example author.
• Attribute names begin with "_at_".
• Query example address/_at_type'email.
• Hierarchy
• Child operator /
• Query example /novel/author
• Paths are described from the top down and usually
  the right-most element on the path is returned
• Descendant operator ("//") indicates any number
  of intervening levels
• Example novel//address
• Shows addresses anywhere within ltnovelgt.

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Learn XQL by Examples (cond.)

• Filters
• Filter operator (" ") filters the set of nodes
  to its left based on the conditions inside the
  brackets.
• Example novel/author/address_at_type'email'
• Returns addresses each address must have an
  attribute called "type" with the value "email.
• Boolean Operations and Comparisons
• Boolean operators can be used to combine
  conditions.
• Example
• novel/author Tom _at_gender'male' and
  _at_shoesizegt'9EEEE'
• Set Operations
• Can be used to combine Terms or other XQL
  expressions.
• Example q1 union q2 (also can be written as q1
  q2)
• Example q1 intersect q2

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Learn XQL by Examples (cond.)

• Namespaces and Names
• Names may be associated with namespace prefixes
• A namespace prefix can be declared using a
  variable.
• Example
• b "http//www.TwiceSoldTales.com"
• //bbook
• The first line declares "b" to be a variable
  equivalent to the namespace URI
  "http//www.TwiceSoldTales.com
• The second line searches for all ltbookgt elements
  in this namespace.
• Grouping Operator
• Useful to group results.
• Example //book .//author
• The element to the left of the is used to
  group the results of the query within the braces.

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Learn XQL by Examples (cond.)

• Sequence Operators
• a before b (returns a list of all "a"s that
  precede a "b)
• a after b (returns a list of all
  "a"s that occur after a "b)
• a, b (returns a list containing all "a"s,
  followed by all "b"s)
• XQL operators generally maintain document order.
• Useful for specifying order in return lists.
• Functions
• XQL provide some functions.
• Example
• element(), element(name)
• returns elements in the context. If a name
  argument is supplied, returns the elements with
  the given name.
• count()

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Learn XQL by Examples (cond.)

• Extensible Functions
• Allow users to write their own functions
• Functions are passed the list of nodes in the
  current context. If the function has parameters,
  these are passed as strings.
• The result of a function call is also a nodelist.
  
• Example
• User can add a function that computes the average
  for a list of values and call it in an XQL query
• average(property//price)
• ltxqlnumbergt 12345.6789 lt/xqlnumbergt
• Renaming Operator
• Nodes in a list can be renamed using renaming
  operator "-gt".
• Example //book-gtabook
• Rename the book into a new namespace a

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Learn XQL by Examples (cond.)

• References
• XSL provides "id()" function, which returns the
  element containing a given id.
• Example A/id(_at_HREF)
• Return the node pointed to by an HREF attribute
  in an ltAgt element
• Actually it is a join. But this syntax less
  complex than join.
• XQL extends to incorporate any kind of link, not
  just ID/IDREF.
• Example A/ref(_at_HREF)
• Function ref() returns the node (nodes) to which
  an XPointer or HTML HREF points
• Can specify parameters to this function
• Example A/ref(_at_HREF, "table")
• Return the referenced node only if it is a
  'table" element otherwise, will return null.

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Learn XQL by Examples (cond.)

• Joins
• Combine information from multiple sources to
  create one unified view.
• Example
• /book isbn title author //review
  reviewer comments
• Only correct when the database consists only of
  this one book and this one review.
• /bookiisbn isbn title author
  //reviewisbni reviewer comments
• using correlation variables to restrict the
  reviews to those that have the same ISBN number
  as the book.