Transcript Title Slide - Computer Science Department @ University of

CS 253: Topics in Database
Systems: XPath, NameSpaces
Dr. Alexandra I. Cristea
http://www.dcs.warwick.ac.uk/~acristea/
• Previously we looked at:
– XML
• Next:
– XPath
– Namespaces
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XPath
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XPath
• XPath is a syntax for defining parts of an XML
document
• XPath uses path expressions to navigate in
XML documents
• XPath contains a library of standard functions
• XPath is a major element in XSLT
• XPath is a W3C recommendation, thus a
Standard (16. November 1999 )
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XPath Path Expressions
• Uses path expressions to select nodes
or node-sets in an XML document.
– These path expressions look very much
like the expressions you see when you
work with a traditional computer file
system.
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XPath Standard Functions
• over 100 built-in functions.
– string values,
– numeric values,
– date and time comparison,
– node and QName manipulation,
– sequence manipulation,
– Boolean values,
– and more.
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XPath Terminology
•
•
•
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Nodes
Atomic values
Items (atomic values or nodes)
Relationships of nodes
– Parent
– Children
– Siblings
– Ancestors
– Descendants
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XPath Nodes
•
7 kinds of nodes:
1.
2.
3.
4.
5.
6.
7.
element,
attribute,
text,
namespace,
processing-instruction,
comment, and
document (root) nodes.
•
XML documents are treated as trees of nodes. The root
of the tree is called the document node (or root node).
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Nodes Examples
<?xml version="1.0" encoding="ISO-8859-1"?>
<bookstore>
Document (root) node
<book>
<title lang="en">Harry Potter</title>
Element node
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
Attribute node
</bookstore>
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Atomic values Examples*
<?xml version="1.0" encoding="ISO-8859-1"?>
<bookstore>
<book>
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
</bookstore>
*nodes with no children or parent
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Selecting nodes
Expression
Description
nodename
Selects all child nodes with this name
/
//
Selects from the root node
.
..
@
Selects the current node
Selects nodes in the document from the current node
down that match the selection no matter where they are
Selects the parent of the current node
Selects attributes
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Examples of selecting nodes
Path Expression
bookstore
Result
Selects all the bookstore elements
/bookstore
Selects the root element bookstore
Note: If the path starts with a slash ( / ) it always
represents an absolute path to an element!
Selects all book elements that are children of
bookstore
bookstore/book
//book
bookstore//book
//@lang
Selects all book elements no matter where they are
in the document
Selects all book elements that are descendant of the
bookstore element, no matter where they are under
the bookstore element
Selects all attributes that are named lang
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Predicates
• Predicates are used to find a specific
node or a node that contains a specific
value.
• Predicates are always embedded in
square brackets.
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Example predicates
Path Expression
/bookstore/book[1]
Result
Selects the first book element that is the
child of the bookstore element
/bookstore/book[last()]
Selects the last book element that is the
child of the bookstore element
/bookstore/book[last()-1]
Selects the last but one book element that
is the child of the bookstore element
/bookstore/book[position()<3] Selects the first two book elements that
are children of the bookstore element
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Example predicates – cont.
Path Expression
Result
//title[@lang]
Selects all the title elements that
have an attribute named lang
//title[@lang='eng']
Selects all the title elements that
have an attribute named lang with a
value of 'eng'
Selects all the book elements of the
/bookstore/book[price>35.00]
bookstore element that have a price
element with a value greater than 35.00
/bookstore/book[price>35.00]/title Selects all the title elements of the
book elements of the bookstore
element that have a price element
with a value greater than 35.00
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Selecting Unknown Nodes
Wildcard
*
@*
node()
Description
Matches any element node
Matches any attribute node
Matches any node of any kind
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Example: selecting several paths
Path Expression
//book/title | //book/price
Result
Selects all the title as well as price
elements of all book elements
//title | //price
Selects all the title as well as price
elements in the document
Selects all the title elements of the
book element of the bookstore
element as well as all the price
elements in the document
/bookstore/book/title | //price
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XPath Axes
self
child
parent
ancestor
descendant
ancestor-or-self
descendant-orself
preceding-sibling following-sibling
preceding
following
attribute
namespace
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axisname::nodetest[predicate]
• //DDD/parent::*
<AAA>
<BBB>
<DDD>
</DDD>
</BBB>
</AAA>
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axisname::nodetest[predicate]
• //BBB/child::*
<AAA>
<BBB>
<DDD>
</DDD>
</BBB>
</AAA>
Note: /AAA is equivalent to /child::AAA
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More examples
• http://www.zvon.org/xxl/XPathTutorial/General/examples.html
– Check basics, //, *, predicates, attributes, functions
(new ones: count, name, normalize-space, startswith, contains, string-length, floor, ceiling), axes,
operators (mod)
– Note: The ancestor, descendant, following,
preceding and self axes partition a document
(ignoring attribute and namespace nodes): they do
not overlap and together they contain all the
nodes in the document. (see example)
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XPath Conclusion
• We have learned:
–
–
–
–
–
–
–
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XPath definition
Path expressions
Standard functions
Terminology
Predicates
Location paths
Axes
Some operators
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• Before we go on, one more thing about
XML:
• XML Namespaces
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Naming ambiguity
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The Idea to Solve it
• Assign a URI (~ URL) to every sublanguage:
– E.g., for XHTML 1.0:
http://www.w3.org/1999/xhtml
• Qnames: Qualify element names with
URIs:
– {http://www.w3.org/1999/xhtml}head
Web Naming and Addressing Overview (URIs, URLs, ...)
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The actual solution
• Namespace declarations bind URIs to
prefixes:
• Default namespace (no prefix) declared
with: xmlns=“…”
• Lexical Scope
• Attribute names can also be prefixed
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Applying namespaces
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• Next we look at how to query XML
• This can be done, to some extent, as
we have seen, within XSLT,
• but the main language developed for
this purpose is …
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