Dr. Alexandra I. Cristea CS 253: Topics in Database Systems: XPath, NameSpaces

2 Previously we looked at: –XML Next: –XPath –Namespaces

3 XPath

4 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 )

5 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.

6 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.

7 XPath Terminology Nodes Atomic values Items (atomic values or nodes) Relationships of nodes –Parent –Children –Siblings –Ancestors –Descendants

8 XPath Nodes 7 kinds of nodes: 1.element, 2.attribute, 3.text, 4.namespace, 5.processing-instruction, 6.comment, and 7.document (root) nodes. XML documents are treated as trees of nodes. The root of the tree is called the document node (or root node).

9 Nodes Examples Harry Potter J K. Rowling Document (root) nodeElement node Attribute node

10 Atomic values Examples* Harry Potter J K. Rowling *nodes with no children or parent

11 Selecting nodes ExpressionDescription nodenameSelects all child nodes with this name / Selects from the root node // Selects nodes in the document from the current node down that match the selection no matter where they are. Selects the current node.. Selects the parent of the current Selects attributes

12 Examples of selecting nodes Path ExpressionResult bookstoreSelects all the bookstore elements /bookstoreSelects the root element bookstore Note: If the path starts with a slash ( / ) it always represents an absolute path to an element! bookstore/bookSelects all book elements that are children of bookstore //bookSelects all book elements no matter where they are in the document bookstore//bookSelects all book elements that are descendant of the bookstore element, no matter where they are under the bookstore element all attributes that are named lang

13 Predicates Predicates are used to find a specific node or a node that contains a specific value. Predicates are always embedded in square brackets.

14 Example predicates Path ExpressionResult /bookstore/book[1]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

15 Example predicates – cont. Path ExpressionResult all the title elements that have an attribute named lang all the title elements that have an attribute named lang with a value of 'eng' /bookstore/book[price>35.00] /bookstore/book[price>35.00]/title Selects all the book elements of the bookstore element that have a price element with a value greater than 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

16 Selecting Unknown Nodes WildcardDescription *Matches any element any attribute node node()Matches any node of any kind

17 Example: selecting several paths Path ExpressionResult //book/title | //book/priceSelects all the title as well as price elements of all book elements //title | //price /bookstore/book/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

18 XPath Axes self childparent ancestordescendant ancestor-or-selfdescendant-or- self preceding-siblingfollowing-sibling precedingfollowing attribute namespace

19 axisname::nodetest[predicate] //DDD/parent::*

20 axisname::nodetest[predicate] //BBB/child::* Note: /AAA is equivalent to /child::AAA

21 More examples –Check basics, //, *, predicates, attributes, functions (new ones: count, name, normalize-space, starts- with, contains, string-length, floor, ceiling), axes, operators (mod)predicatesattributescountnamenormalize-spacestarts- withcontainsstring-lengthfloorceilingaxes 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)example

22 XPath Conclusion We have learned: –XPath definition –Path expressions –Standard functions –Terminology –Predicates –Location paths –Axes –Some operators

23 Before we go on, one more thing about XML: XML Namespaces

24 Naming ambiguity

25 The Idea to Solve it Assign a URI (~ URL) to every sub- language: –E.g., for XHTML 1.0: Qnames: Qualify element names with URIs: –{ Web Naming and Addressing Overview (URIs, URLs,...)

26 The actual solution Namespace declarations bind URIs to prefixes: Default namespace (no prefix) declared with: xmlns=… Lexical Scope Attribute names can also be prefixed

27 Applying namespaces

28 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 …