@run with Coding Info: XQuery

XPath and XQuery are similar in some ways. XPath is even an integral part of XQuery. Both languages allow you to select bits of data from an XML document or an XML document store.
XPath
XPath is a domain-specific language (DSL) that is quickly becoming an important part of other more general-purpose languages. Programming languages are incorporating XPath through modules and classes, and in some cases directly into the languages' syntax. This is similar to what happened with regular expressions some time ago.
XPath is popular because of the considerable amount of time and effort that the language can save a developer when extracting specific bits of data from an XML document
XQuery
Because XQuery supports XPath natively, as part of XQuery's syntax, XQuery clearly can do everything that XPath can do. XQuery uses a simple syntax that is a mix of XML, XPath, comments, functions, and a special expression syntax to tie it all together. XQuery code consists entirely of expressions with no statements. All values are sequences and simplicity is important to the language. So both of the expressions Hi and 2 * 2 are valid XQuery code that will execute without any prelude or modification. XQuery is a high-level, strongly-typed, functional language (free of side-effects) that is ideal to express a query to obtain data both from an XML document and a large XML document repository. In this last respect, it is much like SQL. But XQuery additionally provides for expressing an arbitrary transformation of the result set. Much like the use of XPath can be rewarding when you want to retrieve some data from an XML document, the use of XQuery can be quite rewarding when you want to retrieve and transform data from a large repository of XML documents.

Summary
XPath is a mature DSL that should be your first choice to get to a piece of data that is buried deep in an XML document or repository. But, XPath was not designed to handle many kinds of problems. As you saw in this article, XQuery extends XPath vastly, emerging as the tool of choice when you have complex data selection requirements or you need to return results that are sorted, specially formatted, or otherwise transformed.

XQuery is a query and functional programming language that is designed to query and transform collections of structured and unstructured data, usually in the form of XML, text and with vendor specific extensions other data formats (JSON, binaries, ...).

"The mission of the XML Query project is to provide flexible query facilities to extract data from real and virtual documents on the World Wide Web, therefore finally providing the needed interaction between the Web world and the database world. Ultimately, collections of XML files will be accessed like databases".

XQuery provides the means to extract and manipulate data from XML documents or any data source that can be viewed as XML, such as relational databases or office documents.

XQuery contains a superset of uses XPath expression syntax to address specific parts of an XML document.

It supplements this with a SQL-like "FLWOR expression" for performing joins. A FLWOR expression is constructed from the five clauses after which it is named: FOR, LET, WHERE, ORDER BY, RETURN.

The language also provides syntax allowing new XML documents to be constructed. Where the element and attribute names are known in advance, an XML-like syntax can be used; in other cases, expressions referred to as dynamic node constructors are available. All these constructs are defined as expressions within the language, and can be arbitrarily nested.

The language is based on the XQuery and XPath Data Model (XDM) which uses a tree-structured model of the information content of an XML document, containing seven kinds of nodes: document nodes, elements, attributes, text nodes, comments, processing instructions, and namespaces.

XDM also models all values as sequences (a singleton value is considered to be a sequence of length one). The items in a sequence can either be XML nodes or atomic values. Atomic values may be integers, strings, booleans, and so on: the full list of types is based on the primitive types defined in XML Schema.

XQuery 1.0 does not include features for updating XML documents or databases; it also lacks full text search capability. These features are both under active development for a subsequent version of the language. However, the new standards such as XQuery Update Facility 1.0 supports update feature and XQuery and XPath Full Text 1.0 support full text search in XML documents.

XQuery is a programming language that can express arbitrary XML to XML data transformations with the following features:

Logical/physical data independence
Declarative
High level
Side-effect free
Strongly typed

What You Should Already Know

Before you continue you should have a basic understanding of the following:

HTML / XHTML
XML / XML Namespaces
XPath

If you want to study these subjects first, find the tutorials on our Home page.

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 supported by all major databases
XQuery is a W3C Recommendation

XQuery is About Querying XML

XQuery is a language for finding and extracting elements and attributes from XML documents.

Here is an example of a question that XQuery could solve:

"Select all CD records with a price less than $10 from the CD collection stored in the XML document called cd_catalog.xml"

XQuery and XPath

XQuery 1.0 and XPath 2.0 share the same data model and support the same functions and operators. If you have already studied XPath you will have no problems with understanding XQuery.

XQuery - Examples of Use

XQuery can be used to:

Extract information to use in a Web Service
Generate summary reports
Transform XML data to XHTML
Search Web documents for relevant information

The XML Example Document

We will use the following XML document in the examples below.

"books.xml":

<?xml version="1.0" encoding="ISO-8859-1"?>

<bookstore>

<book category="COOKING">
<title lang="en">Everyday Italian</title>
<author>Giada De Laurentiis</author>
<year>2005</year>
<price>30.00</price>
</book>

<book category="CHILDREN">
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>

<book category="WEB">
<title lang="en">XQuery Kick Start</title>
<author>James McGovern</author>
<author>Per Bothner</author>
<author>Kurt Cagle</author>
<author>James Linn</author>
<author>Vaidyanathan Nagarajan</author>
<year>2003</year>
<price>49.99</price>
</book>

<book category="WEB">
<title lang="en">Learning XML</title>
<author>Erik T. Ray</author>
<year>2003</year>
<price>39.95</price>
</book>

</bookstore>

View the "books.xml" file in your browser.

How to Select Nodes From "books.xml"?

Functions

XQuery uses functions to extract data from XML documents.

The doc() function is used to open the "books.xml" file:

doc("books.xml")

Path Expressions

XQuery uses path expressions to navigate through elements in an XML document.

The following path expression is used to select all the title elements in the "books.xml" file:

doc("books.xml")/bookstore/book/title

(/bookstore selects the bookstore element, /book selects all the book elements under the bookstore element, and /title selects all the title elements under each book element)

The XQuery above will extract the following:

<title lang="en">Everyday Italian</title>
<title lang="en">Harry Potter</title>
<title lang="en">XQuery Kick Start</title>
<title lang="en">Learning XML</title>

Predicates

XQuery uses predicates to limit the extracted data from XML documents.

The following predicate is used to 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]

The XQuery above will extract the following:

<book category="CHILDREN">
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>

XQuery Example

for $x in doc("books.xml")/bookstore/book

where $x/price>30

order by $x/title

return $x/title

Thanks http://www.w3schools.com

S.No.	XQuery	SQL
1.	XQuery is an expression oriented language	SQL is a statement oriented language.
2.	Concept of nodes are used in XQuery	Concept of rows are used in SQL
3.	A user cannot create and update tables.	A user can create and update tables.
4.	It is a new language.	It is an old language.

@run with Coding Info

Thursday, June 13, 2013

XPath and XQuery

XQuery