EMERGING TECHNOLOGIES

Transcription

1 EMERGING TECHNOLOGIES XML (Part 2): Data Model for XML documents and XPath

2 Outline 1. Introduction 2. Structure of XML data 3. XML Document Schema 3.1. Document Type Definition (DTD) 3.2. XMLSchema 4. Data Model for XML documents. 5. XML Query Languages 5.1. XPath 5.2. XQuery

3 4. Data Model for XML (1) W3C query languages view XML documents as trees whose nodes are elements, attributes, comments, and text. There is a special root node of the tree which should not be confused with the root element of the XML document. The root node serves as a gathering point for all the document components, including the comments that are allowed to occur outside the scope of the root element of the document.

4 4. Data Model for XML (2) Example:We use the following XML document as a running example for XPath. <?xml version= 1.0?> <!- some comment -> <Students> <Student StudentID= s > <Name> <First>John</First> <Last>Doe</Last ></Name> <Status>U4</Status> <CrsTaken CrsCode= CS308 Semester= F1997 /> <CrsTaken CrsCode= MA123 Semester= F1997 /> </Student> <Student StudentID= s > <Name> <First>Bart</First ><Last>Simpson</Last> </Name> <Status>U4</Status> <CrsTaken CrsCode= CS308 Semester= F1994 /> </Student> </Students>

5 4. Data Model for XML (3) Example A tree representation for part of the XML document of the previous example. Root Comment Students Comment Student Student Name Status StudID First Last U4 CrsTaken CrsTaken John Doe CrsCode Semester CrsCode Semester Comment Legend : text Element Attribute Attribute Root

6 5. XML Query Languages If data is represented as XML documents, one needs a query language to query them. We will see two query languages: XPath and XQuery.

7 5.1 XPath (1) Introduction XPath is a query language for XML. XPath is intended to be simple and efficient. It is based on the idea of path expressions of object-oriented and object-relational databases. XPath extends path expressions with query facilities by allowing the programmer to replace parts of the route to data elements with search conditions.

8 5.1 XPath (2) Path Expressions XPath provides operators for navigating the document and accessing its various components. The basic syntax is that of the UNIX file naming schema. In the XML document tree: / represents the root.. represents the current node... represents the parent node. An XPath expression takes an XML document tree and returns a set of nodes.

9 5.1 XPath (3) Path Expressions Many uses of XPath identify a node in the XML tree as the current node. Path expressions can be relative or absolute. An absolute path expression returns nodes that are reachable from the root. Example The path expression: /Students/Student/CrsTaken returns the set of nodes that correspond to courses taken.

10 5.1 XPath (4) Path Expressions A relative path expression considers as its departure point for navigation the current node rather than the root. Example If the current node corresponds to the element Name, then First and./first both refer to its child element First.

11 5.1 XPath (5) Path Expressions To access an attribute, the is used. Example The list of attribute nodes that correspond to CrsCode is obtained using the path expression /Students/Student/CrsTaken/@CrsCode.

12 5.1 XPath (6) Path Expressions Text nodes are accessed using the text() function. Example The collection of nodes that represent the text content of an element First can be obtained using the path expression /Students/Student/Name/First/text(). The two comments can be selected using the expression /comment().

13 5.1 XPath (7) Advanced Navigation The more advanced features of XML navigation include facilities to select specific nodes of an XML document as well as facilities to jump through an intermediate number of children. Example /Students/Student[1]/CrsTaken[2] selects the second course taken by John Doe. /Students/Student/CrsTaken[last()] selects the last course taken by each student. It returns: <CrsTaken CrsCode= MAT123 Semester= F1997 /> <CrsTaken CrsCode= CS308 Semester= F1994 />

14 5.1 XPath (8) Advanced Navigation XPath provides several wildcard facilities: Descendent-or-self operation (//): //CrsTaken is an absolute path expression that starts at the root and selects all CrsTaken nodes in the entire tree. /Students//CrsTaken selects all CrsTaken elements that are descendants of student nodes regardless of the level of nesting.

15 5.1 XPath (9) Advanced Navigation The descendant operation can be used in relative expressions as well..//crstaken searches through all descendants (or self) of the current node to find the CrsTaken elements../crstaken and CrsTaken are the same..//crstaken, CrsTaken and //CrsTaken are all different.

16 5.1 XPath (10) Advanced Navigation Another wildcard, *, let us collect all element children of a node irrespective of type. Example Student/* selects all element children of the Student children of the current node. /*//* selects the element descendents of all child elements of the root (recall that // is descendent-or-self). The * wildcard can also be applied to attributes. Example CrsTaken/@* selects all attribute values of the CrsTaken nodes that sit below the current node.

17 5.1 XPath (11) Advanced Navigation Note that * in Student/* does not include the text nodes that could possibly exist among the element children of the Student element. To select those, the expression Student/text() is used.

18 5.1 XPath (12) Syntax The general form of an XPath query is locationstep 1 /locationstep 2 / or /locationstep 1 /locationstep 2 / where each location step is of the form axis::nodeselector[selectioncondition] The term axis refers to the navigation axis, which indicates the direction along which navigation is taking place in the corresponding location step. The available axis are child (go to a child node), parent, descendant (i.e. child, grandchild, etc), ancestor, descendant-or-self, ancestor-or-self, etc.

19 The node selector is either: 5.1 XPath (13) Syntax - the name of the node (e.g. the name of an element or attribute); - a selector for an unnamed node, such as text() or comment(); or - a wildcard, such as * The optional selectioncondition in a location step selects a subset of nodes reachable by the location step.

20 5.1 XPath (14) Syntax Example child::student is a simple location step that directs navigation from the current node down to a child element named Student. descendant-or-self::@semester is a simple location step that directs navigation from the current node to a Semester attribute in either the current node or in a descendant of the current node.

21 5.1 XPath (15) Syntax Because the syntax of XPath is so verbose, most axis have convenient abbreviations (this is what we have been using up until now and will continue using). Example child::student (relative expression) abbreviates as Student. (relative expression) abbreviates (absolute expression) abbreviates as

22 5.1 XPath (16) Semantics The value of a locationstep axis::nodeselector[selectioncondition] on an instance document is the set of all nodes of the form nodeselector that are reachable from the current node by the navigation axis and that satisfy selectioncondition. Example.//@Semester specifies the set of attribute nodes called Semester, which are reachable from the current node by the axis descendantor-self.

23 5.1 XPath (17) Semantics The value of an XPath expression locationstep1/locationstep2/ on an instance document is the set of all nodes computed as follows: - From the current node find all nodes reachable by locationstep 1. - For each such node, N, find all nodes that are reachable from N via locationstep 2. Take the union of all node steps reachable by location Step2. - Apply locationstep3 to each node in the union etc., until the last location step is reached. - The value of the XPath expression is the set of all nodes obtained at this last step.

24 5.1 XPath (18) A Bigger Example XML document Example The first part of an XML document

25 5.1 Xpath (19) A Bigger Example XML document Example (cont.) The second part of the XML document

26 5.1 XPath (20) Selection Conditions XPath queries can include selection conditions at any step in the navigation process. Example Select all student nodes where the student has taken a course in fall //Student[CrsTaken/@Semester = "F1994"] The expression //Student selects all Student nodes under the root node. The expression between [ and ] is a selection condition that retains a Student node only if it satisfies the condition that the path expression CrsTaken/@Semester can be applied to this node and the set of values returned includes F1994.

27 5.1 XPath (21) Selection Conditions Elements can also be selected based on the content of an element than of an attribute. Selection conditions can be combined using and, or and not. Example //Student[Status = "U3" and.//last = "Public" and not(.//last =.//First)] Select all students who have the status U3, whose last name is Public, and whose last and first names are different. Note that in order to evaluate comparisons, XPath converts every node returned by a path expression into its string value. For element nodes, the string value is simply the text inside the element.

28 5.1 XPath (22) Selection Conditions Selection conditions can be applied at different levels and multiple times in a path expression. Example //Student[Status = "U4"]/CrsTaken[@CrsCode = "CS305"] Select all the CrsTaken elements in the document whose CrsCode attribute has the value CS305 and occur in Students elements with status U4.

29 5.1 XPath (23) Selection Conditions Multiple selection conditions can also be applied at the same level in a path expression. Example //Student/CrsTaken[@CrsCode = "MAT123"] [@Semester = "F1994"] Select all the CrsTaken elements that occur in a Student element and their value for attribute CrsCode is MAT123 and for attribute Semester is F1994. The same expression can be written as: //Student/CrsTaken[@CrsCode= "MAT123" The previous two expressions are different than: //Student[CrsTaken/@CrsCode = "MAT123" and CrsTaken/@Semester = "F1994"]

30 5.1 XPath (24) Selection Conditions A path expression can also be used as a selection predicate rather than as an argument to a predicate. Example Suppose that Grade is an optional attribute of element CrsTaken. //Student[CrsTaken/@Grade] selects all Student elements that have a CrsTaken child element with an explicitly specified Grade attribute (regardless of its value). The query: //Student[Name/First or CrsTaken] selects all Student elements that have either the element First as a grandchild or the element CrsTaken as a child.

31 5.1 XPath (25) Union Operator XPath allows the union operator (denoted by the symbol ). Example //Class[Semester="F1994"] Select the CrsTaken elements that pertain to the fall 1994 semester and the Class elements that describe fall 1994 course offerings. Notice that the set of nodes selected by this query is the union of elements of different types.

32 5.1 XPath (26) Functions XPath has a rich repertoire of functions that greatly increase its expressive power. Example //Student[Status = "U3" and starts-with(.//last, "P") and not(.//last =.//First)] The built-in predicate starts-with() selects only Last names that start with P. The query selects all the students who have the status U3, whose last name starts with P, and whose last and first names are different.

33 5.1 XPath (27) Functions Other string manipulation functions allow us to check for containment, perform concatenation, determine length, etc. Example //Student[contains(concat(Name//text()), "van")] Name//text() returns the set of all text nodes that are descendants of element Name which is a child element of the current node (Student in this case). The concatenation function makes a string out of these text nodes. The query selects all the students who have "van" as part of their name.

34 5.1 XPath (28) Functions Aggregate functions available in XPath include sum() and count(). Example //Student[count(CrsTaken) >= 5] CrsTaken returns the set of CrsTaken elements for the current node (Student in this case). Count(CrsTaken) returns the number of the courses which is then compared with 5. > stands for > (and < for <). The symbols < and > are encoded as < and > respectively as they are reserved in XML as tag delimiters.