Dr. Alexandra I. Cristea CS 253: Topics in Database Systems: C2

Previously we looked at: –XML –XSL –XSLT Next: –XPath –XQuery

XPath

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 )

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.

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.

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

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

Nodes Examples Harry Potter J K. Rowling Document nodeElement node Attribute node

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

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

Examples of selecting nodes Path ExpressionResult bookstoreSelects all the child nodes of the bookstore element /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

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

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

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

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

Example: selecting several paths Path ExpressionResult //book/title | //book/priceSelects all the title AND price elements of all book elements //title | //price /bookstore/book/title | //price Selects all the title AND price elements in the document Selects all the title elements of the book element of the bookstore element AND all the price elements in the document

Location Path Expression A location path can be absolute or relative. An absolute location path: /step/step/... A relative location path: step/step/... Location step: axisname::nodetest[predicate]

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

AxisNameResult ancestorSelects all ancestors (parent, grandparent, etc.) of the current node ancestor-or-selfSelects all ancestors (parent, grandparent, etc.) of the current node and the current node itself attributeSelects all attributes of the current node childSelects all children of the current node descendantSelects all descendants (children, grandchildren, etc.) of the current node descendant-or- self Selects all descendants (children, grandchildren, etc.) of the current node and the current node itself followingSelects everything in the document after the closing tag of the current node following-siblingSelects all siblings after the current node namespaceSelects all namespace nodes of the current node parentSelects the parent of the current node precedingSelects everything in the document that is before the start tag of the current node preceding- sibling Selects all siblings before the current node selfSelects the current node

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

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

More examples l/examples.htmlhttp:// l/examples.html –Check basics, //, *, predicates, attributes, functions (new ones: count, name, normalize-space, starts- with, contains, string-length, floor, ceiling), axes, 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

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

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

Naming ambiguity

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

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

Applying namespaces

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 …

XQuery

What is XQuery? XQuery is the language for querying XML data XQuery for XML is like SQL for databases XQuery is built on XPath expressions XQuery is defined by the W3C XQuery is supported by all the major database engines (IBM, Oracle, Microsoft, etc.) XQuery is a W3C recommendation (Jan 2007) thus a standard

XQuery and XPath XQuery 1.0 and XPath 2.0 share the same data model and support the same functions and operators.

XQuery - Examples of Use Extract information to use in a Web Service Generate summary reports Transform XML data to XHTML Search Web documents for relevant information

Usage Scenario: Document-Oriented Queries could be used –To retrieve parts of documents –To provide dynamic indexes –To perform context-sensitive searching –To generate new documents as combinations of existing ones

Usage Scenario: Programming Queries could be used to automatically generate documentation

Usage Scenario: Hybrid Queries could be used to data mine hybrid data, such as patient records

XQuery compared to XPath XQuery 1.0 is a strict superset of XPath 2.0 XPath 2.0 expression is directly an XQuery 1.0 expression (a query) The extra expressive power is the ability to: –Join information from different sources and –Generate new XML fragments

Relationship to XSLT XQuery, XSLT: both domain-specific languages for combining and transforming data from multiple sources different in design - historical reasons –XQuery: designed from scratch –XSLT: intellectual descendant of CSS technically, they may emulate each other

XQuery query Prolog –Like XPath, XQuery expressions are evaluated relatively to a context –explicitly provided by a prolog (header) ~ header with definitions Body –The actual query

XQuery Prolog (i.e., header(s)) Settings define various parameters for the XQuery processor language, such as: xquery version 1.0; module " default element namespace= " declare namespace xs= " import module " functions" at "logo.xq" define function addLogo($root as node()) as node()* { } (: etc :)

XQuery capabilities Generate Join Select

Generate: constructors XQuery expressions may compute new XML nodes Expressions may denote: –element, character data, comment and processing instruction nodes node is created with a unique node identity Constructors may be either –direct or –computed

Direct constructors in XQuery my fragment Evaluates to the given XML fragment

Explicit, computed constructors

Variable bindings {$name} {$job} {$deptno} {$SGMLspecialist }

How to Select Nodes with XQuery? Functions –XQuery uses functions to extract data from XML documents. (X)Path Expressions –XQuery uses path expressions to navigate through elements in an XML document. Predicates –XQuery uses predicates to limit the extracted data from XML documents.

Functions doc() –function to open a file Example: –doc("books.xml") Note: A call to a function can appear where an expression may appear.

Path Expressions Example: select all the title elements in the "books.xml" file: doc("books.xml")/bookstore/book/title

Predicates Example: select all the book elements under the bookstore element that have a price element with a value that is less than 30 : doc("books.xml")/bookstore/book[price<30]

At a glance: function, path, predicate

FLWOR For, Let, Where, Order by, Return = main engine ~ SQL syntax (SFHW) ~ programs and function calls

FLWOR by comparison with Path expressions select all the title elements under the book elements that are under the bookstore element that have a price element with a value that is higher than 30. Path expression : doc("books.xml")/bookstore/book[price>30]/title FLWOR expression : for $x in doc("books.xml")/bookstore/book where $x/price>30 return $x/title

Sorting in FLWOR for $x in doc("books.xml")/bookstore/book where $x/price>30 order by $x/title return $x/title

Present the Result In an HTML List { for $x in doc("books.xml")/bookstore/book/title order by $x return {$x} }

Result HTML List Everyday Italian Harry Potter Learning XML XQuery Kick Start

Eliminate element (here: title) { for $x in doc("books.xml")/bookstore/book/title order by $x return data($x) (: also text() :) }

New result HTML List Everyday Italian Harry Potter Learning XML XQuery Kick Start

Another FLWOR Expression

The Difference between for and let

FLWOR Basic Building Blocks

General rules for and let may be used many times in any order only one where is allowed many different sorting criteria can be specified (descending, ascending, etc.)

Joining documents for $p IN doc(" for $n IN doc("neighbors.xml")//neighbor[ssn = $p/ssn] return { $p/ssn } { $n/name } { $p/income }

Conditionals FOR $b IN doc(bib.xml)/book RETURN {$b/title} {IF count($b/author) and others }

Functions DEFINE FUNCTION depth($e) RETURNS xsd:integer { IF (empty($e/*) THEN 1 ELSE max(FOR $c in $e/* RETURN depth($c)) ) +1 } FOR $b in doc(bib.xml)/book RETURN depth($b)

Existential and Universal Quantifiers FOR $b in doc(bib.xml)/book WHERE SOME $author IN $b/author SATISFIES $author/text() = Ullman RETURN $b FOR $b in doc(bib.xml)/book WHERE EVERY $author IN $b/author SATISFIES $author/text() = Ullman RETURN $b Return books where all authors are Ullman Return books where at least one author is Ullman

XQuery on Distributed Sources

XQuery Syntax Declarative, functional language ~ SQL Nested expressions Case sensitive White spaces: –Tabs, space, CR, LF –Ignored between language constructs –Significant in quoted strings No special EOL character

Keywords and names Keywords and operators –Case-sensitive, generally lower case –May have several meanings depending on the context E.g. * or in –No reserved words All names must be valid XML names –For variables, functions, elements, attributes –Can be associated with a namespace

Comments

Comparisons Value comparisons Eq, ne, lt, le, gt, ge Used to compare individual values Each operand must be a single atomic value (or a node containing a single atomic value) General comparisons =, !=,, >= Can be used with sequences of multiple items

Example

Query Prolog

XQuery gives you a choice: Path Expressions: –If you just want to copy certain elements and attributes as is FLWOR Expressions: –Allow sorting –Allow adding elements/attributes –Verbose, but can be clearer

XQuery tools XStylus Studio oad.html (free trial version) oad.html –See also short its XQuery intro at:

XML and programming XSLT, XPath and XQuery provide tools for specialized tasks. But many applications are not covered: –domain-specific tools for concrete XML languages –general tools that nobody has thought of yet

XML in general-purpose programming languages parse XML documents into XML trees navigate through XML trees construct XML trees output XML trees as XML documents DOM and SAX are corresponding APIs that are language independent and supported by numerous languages. JDOM is an API that is tailored to Java.DOMSAXJDOM

XQuery Conclusion We have learned: –XQuery definition –Usage scenarios –Comparison w. XSLT and XPath –Capabilities –Functions, path expressions and predicates –FLWOR –Extensions for generic programming with XML