Querying XML

Intro

• Not as mature as querying relational databases
• No underlying algebra
• XPath : Path expressions and conditions
• XSLT : XPath + Transformations, output processing
• XQuery : XPath + full featured query language
• XLink, XPointer : Use XPath as a component

XPath

• Think of XML as a tree
• Expressions in XPath as navigations down/across the tree with conditions
• / - root element + separator, Element name, * is wildcard, @ for attribute
• // - any descendant of the current element including self
• condition in square bracket. [price < 50]. Also [] used as array access.
• Many built-in functions : e.g. contains(s1, s2) : true/false. name() : returns element tag name
• Navigation axes : e.g. parent, following-sibling, descendants
• XPath queries operate on and return sequence of elements for XML document & XML stream
• Sometimes result of XPath query can be expressed in XML, but not always

Sample queries

• doc("Bookstore.xml")/Bookstore/Book/Title - returns titles of all books
• doc("Bookstore.xml")/Bookstore/(Book | Magazine)/Title - titles of all books or magazines
• doc("Bookstore.xml")/Bookstore/*/Title - wildcard
• doc("Bookstore.xml")//Title - any Title element anywhere in the tree - Double slash
• doc("Bookstore.xml")//* - Will print the whole tree for the root, then subtree for the child etc
• doc("Bookstore.xml")/Bookstore/Book/data(@ISBN) - Data operator needs to be specified
• doc("Bookstore.xml")/Bookstore/Book[@Price < 90] - Condition, price < 90 : Will print the whole book
• doc("Bookstore.xml")/Bookstore/Book[@Price < 90]/Title - Above, but return only title.
• doc("Bookstore.xml")/Bookstore/Book[Remark]/Title - Existence condition, Book must have a remark element
• doc("Bookstore.xml")/Bookstore/Book[@Price < 90 and Authors/Author/Last_Name = "Ullman" and Authors/Author/First_Name = "Jennifer" ]/Title : Bigger condition. The second part is actually a "there exists". So actually not doing an AND.
• doc("Bookstore.xml")/Bookstore/Book[@Price < 90 and Authors/Author[Last_Name = "Ullman" and First_Name = "Jennifer" ]/Title : This is the correct one.