self XPath Dongwon Lee Ph D

/*/*/self: : * XPath Dongwon Lee, Ph. D. IST 516 Fall 2011

XPath l Path-based XML query language l l V 1. 0 – 1999: http: //www. w 3. org/TR/xpath V 2. 0 – 2003: http: //www. w 3. org/TR/xpath 20/ Functional, strongly-typed query language http: //www. w 3 schools. com/xpath_intro. asp 2

Apps of XPath l XQuery: a full-blown query language for XML for $x in doc("books. xml")/bookstore/book where $x/price>30 order by $x/title return $x/title l XPointer/XLink: a standard way to create hyperlinks in XML As his fifth year at Hogwarts School of Witchcraft and Wizardry approaches, 15 -year-old Harry Potter is. . . . 3

Apps of XPath l XSLT: a style sheet language of XML that can transform XML from one to another format 4

XPath vs. SQL XPath l l XML Model Trees Hierarchy Order SQL l l Relational Model Tables Flat Orderless (except ORDER-BY) 5

XPath vs. XQuery XPath l l XML Model Trees Hierarchy Order XQery l l Can do all XPath does but not vice versa Turing-Complete general purpose PL Can retrieve, update, and transform XML data FLWOR expression 6

XPath Expression l Expression (basic building block) returns one of the 4 objects: 1. node-set (an unordered collection of nodes without duplicates). . . 2. boolean (true or false) number (a floating-point number) string (a sequence of characters) 3. 4. 7

XPath Nodes processing-instruction Tove comment Reminder Don't forget me this weekend! document text l Nodes: 7 types l l element, attribute, text, namespace processing-instruction, comment, document 8

Location Step l l l Location Steps are evaluated in order from left to right Preferred – Absolute: /step/… Faster to evaluate Relative: step/… axis : : node-test [predicate] l l l Axis: Specifies the node relationship Node Test: specifies node type and name Predicate: Instructions to filter nodes 9

1. Axis l / selects the root of the node hierarchy l l Forward Axis l l child: : , descendent: : , attribute: : , self: : , descendentor-self: : , following-sibling: : , following: : Backward Axis l l as the default root of XML document ancestor: : , preceding-sibling: : , preceding: : , ancestor-or-self: : Relative to the current context (Axis: : context) l l child: : emp: “emp” is the child element of current node attribute: : date: “date” is the attribute of current node 10

Node Relationships Courses Self Undergrad Ancestors Graduate Parent Sibling Descendants Room Name Instructor Office Child Phone Grandchild 11

1. Axis Abbreviation l l l Descendent-or-self: : node() // child: : / attribute: : @ self: : node() . parent: : . . Eg l l /child: : doc/descendent: : chapter /doc//chapter //doc/attribute: : type //doc/@type 12

2. Node Test l l l node(): matches all nodes text(): matches all text nodes Element. Name: matches all elements of type ‘Element. Name’ *: matches all elements @*: matches all attributes 13

2. Node Test l * (wildcard) is often used to match unknown XML elements l /catalog/cd/*: all the child elements of all the cd elements of the catalog element /* : all children of the root /*/*: all grandchildren of the root //*: all elements of the XML document l l l 14

3. Predicate l l Path-expresson[ filtering condition ] Pathexpression that satisfies the filtering condition Eg l l l //doc [@type=‘PDF’] finds all elements whose attribute “type” values are ‘PDF’ This returns elements, not its attributes “type” Filtering condition does not affect the returned answers (ie, projection) of XPath l It just adds more constraints to satisfy 15

Location Step Examples 16

Examples of usage 17

IST Example What IST Classes are in Room IST 110? /Courses/*[child: : Room=‘ 110 IST’] Original XML Result 110 IST Robert Luo 210 IST 110 IST Robert Luo 18

IST Example What IST courses have TA’s? /Courses/*/TA/parent: : * Original XML Result 110 IST Robert Luo 210 IST 110 IST Robert Luo 19

IST Example What rooms are used by IST courses? /Courses/*/Room/text() Original XML Result 110 IST Robert Luo 210 IST 110 IST 20

Comparison l Comparison can be performed using l l =, !=, <, >=, and > Examples l l [child: : Room != ‘ 205 IST’] [child: : Time > 1220] NOTE: When used within Predicate, Child: : Room == Child: : Room/text() 21

Math Operators l + : performs addition - : performs subtraction * : performs multiplication div : performs division mod : returns the remainder of division l Examples: l l l [child: : Time mod 100 = 30] 22

Node Functions l l last() : returns the numeric position of the last node in a list position() : returns the numeric position of the current node count() : returns the number of nodes in a list name(): returns the name of a node id() : selects elements by their unique ID 23

Node Function Example Which courses have more than 2 child elements? /Courses/*[count(child: : *)>2] Original XML Result 110 IST Robert Luo 210 IST 110 IST Robert Luo 24

String Functions l l concat(string, string) : concatenates the string arguments starts-with(string, string) : returns true if the first string starts with the second string contains(string, string) : returns true if the first string contains the second string Eg, l concat(‘sh’, ‘oe’) = ‘shoe’ l starts-with(‘cat’, ‘ca’) = true l contains(‘puppy’, ‘upp’) = true 25

String Functions l l l substring(string, number, [number]) : returns a substring of the provided string-length(string) : returns the number of characters in the string Eg, l l l substring(‘chicken’, 3, 4) = ‘icke’ substring(‘chicken’, 3) = ‘icken’ string-length(‘cat’) = 3 26

String Functions Examples l //Book [startswith(child: : Title, “X”)] / price l //Book [stringlength(Author/FN)=3] / Title