Transcript Document 7742889

XQL (XML Query Language)
Jonathan Robie (Software AG) <[email protected]>
Eduard Derksen (CSCIO) <[email protected]>
Peter Fankhauser (GMD-IPSI) <[email protected]>
Ed Howland (DEGA) <[email protected]>
Gerald Huck (GMD-IPSI) <[email protected]>
Ingo Macherius (GMD-IPSI) <[email protected]>
Makoto Murata (Fuji Xerox) <[email protected]>
Michael Resnick (Object Design, Incorporated) <[email protected]>
Harald Schöning (Software AG) <[email protected]>
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.
XQL Expressions
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Terms
Namespaces and Names
Comparisons
Hierarchy and Filters
Boolean and Set Operators
Grouping Operator
Sequence
XQL Expressions (cond.)
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Functions
Extensible Functions
References (not incorporated into XQL)
Joins
Renaming Operator
Precedence of Operators
Learn XQL by Examples
• Terms:
– A simple string is interpreted as an element name:
• Query example: author.
– Attribute names begin with "@".
• Query example: address/@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 <novel>.
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[@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’ [@gender='male' and @shoesize>'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
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"
• //b:book
• The first line declares "b" to be a variable equivalent to the
namespace URI "http://www.TwiceSoldTales.com”
• The second line searches for all <book> 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.
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()
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)
– <xql:number> 12345.6789 </xql:number>
• Renaming Operator
– Nodes in a list can be renamed using renaming operator "->".
• Example: //book->a:book
• Rename the book into a new namespace a
Learn XQL by Examples (cond.)
• References:
– XSL provides "id()" function, which returns the element containing a
given id.
• Example: A/id(@HREF)
– Return the node pointed to by an HREF attribute in an <A> 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(@HREF)
• Function ref() returns the node (nodes) to which an XPointer or HTML
HREF points
• Can specify parameters to this function
– Example: A/ref(@HREF, "table")
– Return the referenced node only if it is a 'table" element; otherwise,
will return null.
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.
• /book[$i:=isbn] { isbn | title | author | //review[isbn=$i] { reviewer |
comments }}
– using correlation variables to restrict the reviews to those that
have the same ISBN number as the book.