1 XPath Path Expressions Conditions. 2 Paths in XML Documents uXPath is a language for describing paths in XML documents. uReally think of the semistructured.

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Presentation transcript:

1 XPath Path Expressions Conditions

2 Paths in XML Documents uXPath is a language for describing paths in XML documents. uReally think of the semistructured data graph and its paths.

3 Example DTD <!DOCTYPE BARS [ ]>

4 Example Document … <BEER name = “Bud” soldBy = “JoesBar SuesBar … ”/> …

5 Path Descriptors uSimple path descriptors are sequences of tags separated by slashes (/). uIf the descriptor begins with /, then the path starts at the root and has those tags, in order. uIf the descriptor begins with //, then the path can start anywhere.

6 Value of a Path Descriptor uEach path descriptor, applied to a document, has a value that is a sequence of elements. uAn element is an atomic value or a node. uA node is matching tags and everything in between. wI.e., a node of the semistructured graph.

7 Example: /BARS/BAR/PRICE … <BEER name = “Bud” soldBy = “JoesBar SuesBar …”/> … /BARS/BAR/PRICE describes the set with these two PRICE elements as well as the PRICE elements for any other bars.

8 Example: //PRICE … <BEER name = “Bud” soldBy = “JoesBar SuesBar …”/>… //PRICE describes the same PRICE elements, but only because the DTD forces every PRICE to appear within a BARS and a BAR.

9 Wild-Card * uA star (*) in place of a tag represents any one tag. uExample: /*/*/PRICE represents all price objects at the third level of nesting.

10 Example: /BARS/* … <BEER name = “Bud” soldBy = “JoesBar SuesBar …”/> … /BARS/* captures all BAR and BEER elements, such as these.

11 Attributes uIn XPath, we refer to attributes by to their name. uAttributes of a tag may appear in paths as if they were nested within that tag.

12 Example: … <BEER name = “Bud” soldBy = “JoesBar SuesBar …”/> … selects all name attributes of immediate subelements of the BARS element.

13 Selection Conditions uA condition inside […] may follow a tag. uIf so, then only paths that have that tag and also satisfy the condition are included in the result of a path expression.

14 Example: Selection Condition u/BARS/BAR[PRICE < 2.75]/PRICE … The condition that the PRICE be < $2.75 makes this price but not the Miller price satisfy the path descriptor.

15 Example: Attribute in Selection = “Miller”] … Now, this PRICE element is selected, along with any other prices for Miller.

16 Axes uIn general, path expressions allow us to start at the root and execute steps to find a sequence of nodes at each step. uAt each step, we may follow any one of several axes. uThe default axis is child:: --- go to all the children of the current set of nodes.

17 Example: Axes u/BARS/BEER is really shorthand for /BARS/child::BEER. is really shorthand for the attribute:: axis. wThus, = “Bud” ] is shorthand for /BARS/BEER[attribute::name = “Bud”]

18 More Axes uSome other useful axes are: 1.parent:: = parent(s) of the current node(s). 2.descendant-or-self:: = the current node(s) and all descendants. wNote: // is really shorthand for this axis. 3.ancestor::, ancestor-or-self, etc.