XQuery is a language that designed to query XML data. It is built on XPath expressions so one that wants to use XQuery must know how to use XPath.
XPath
XPath is the XML Path Language which uses a "path like" syntax to browse through elements and attributes in an XML document.
It contains over 200 built-in functions for such as string values, numeric values, booleans, date and time comparison, node manipulation, sequence manipulation, and much more. XPath expressions can also be used in JavaScript, Java, XML Schema, PHP, Python, C and C++, and lots of other languages.
In XPath, there are seven kinds of nodes: element, attribute, text, namespace, processing-instruction, comment, and document nodes.
XML documents are treated as trees of nodes. The topmost element of the tree is called the root element.
Selecting Nodes
XPath uses path expressions to select nodes in an XML document. The most useful path expressions are listed below:
| Expression | Description |
|---|---|
| nodename | Selects all nodes with the name "nodename" |
| / | 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 node |
| @ | Selects attributes |
For the following example of XML document we will be presenting a couple of XPath examples:
<bookstore>
<book>
<title lang="en">A Clockwork Orange</title>
<author>Anthony Burgess</author>
<year>1962</year>
<price>19.99</price>
</book>
<book>
<title lang="en">Nineteen Eighty-Four</title>
<author>George Orwell</author>
<year>1949</year>
<price>14.99</price>
</book>
</bookstore>
Here are some path expressions and the result of the expressions:
| Path Expression | Result |
|---|---|
| bookstore | Selects all nodes with the name "bookstore" |
| /bookstore | Selects the root element bookstore Note: If the path starts with a slash ( / ) it always represents an absolute path to an element! |
| bookstore/book | Selects all book elements that are children of bookstore |
| //book | Selects all book elements no matter where they are in the document |
| bookstore//book | Selects all book elements that are descendant of the bookstore element, no matter where they are under the bookstore element |
| //@lang | Selects 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.
In the table below we have listed some path expressions with predicates and the result of the expressions:
| Path Expression | Result |
|---|---|
| /bookstore/book[1] | Selects the first book element that is the child of the bookstore element. Note: In IE 5,6,7,8,9 first node is[0], but according to W3C, it is [1]. To solve this problem in IE, set the SelectionLanguage to XPath: In JavaScript: xml.setProperty("SelectionLanguage","XPath"); |
| /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 |
| //title[@lang] | Selects all the title elements that have an attribute named lang |
| //title[@lang='en'] | Selects all the title elements that have a "lang" attribute with a value of "en" |
| /bookstore/book[price>35.00] | Selects all the book elements of the bookstore element that have a price element with a value greater than 35.00 |
| /bookstore/book[price>35.00]/title | 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
XPath wildcards can be used to select unknown XML nodes.
| Wildcard | Description |
|---|---|
| * | Matches any element node |
| @* | Matches any attribute node |
| node() | Matches any node of any kind |
In the table below we have listed some path expressions and the result of the expressions:
| Path Expression | Result |
|---|---|
| /bookstore/* | Selects all the child element nodes of the bookstore element |
| //* | Selects all elements in the document |
| //title[@*] | Selects all title elements which have at least one attribute of any kind |
Selecting Several Paths
By using the | operator in an XPath expression you can select several paths.
In the table below we have listed some path expressions and the result of the expressions:
| Path Expression | Result |
|---|---|
| //book/title | //book/price | Selects all the title AND price elements of all book elements |
| //title | //price | Selects all the title AND price elements in the document |
| /bookstore/book/title | //price | Selects all the title elements of the book element of the bookstore element AND all the price elements in the document |
XPath Axes
An axis defines a node-set relative to the current node.
| AxisName | Result |
|---|---|
| ancestor | Selects all ancestors (parent, grandparent, etc.) of the current node |
| ancestor-or-self | Selects all ancestors (parent, grandparent, etc.) of the current node and the current node itself |
| attribute | Selects all attributes of the current node |
| child | Selects all children of the current node |
| descendant | Selects 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 |
| following | Selects everything in the document after the closing tag of the current node |
| following-sibling | Selects all siblings after the current node |
| namespace | Selects all namespace nodes of the current node |
| parent | Selects the parent of the current node |
| preceding | Selects all nodes that appear before the current node in the document, except ancestors, attribute nodes and namespace nodes |
| preceding-sibling | Selects all siblings before the current node |
| self | Selects the current node |
XPath Operators
Below is a list of the operators that can be used in XPath expressions:
| Operator | Description | Example |
|---|---|---|
| | | Computes two node-sets | //book | //cd |
| + | Addition | 6 + 4 |
| - | Subtraction | 6 - 4 |
| * | Multiplication | 6 * 4 |
| div | Division | 8 div 4 |
| = | Equal | price=9.80 |
| != | Not equal | price!=9.80 |
| < | Less than | price<9.80 |
| <= | Less than or equal to | price<=9.80 |
| > | Greater than | price>9.80 |
| >= | Greater than or equal to | price>=9.80 |
| or | or | price=9.80 or price=9.70 |
| and | and | price>9.00 and price<9.90 |
| mod | Modulus (division remainder) | 5 mod 2 |
The XQuery Update Facility is a relatively small extension of the XQuery language which provides convenient means of modifying XML documents or data. As of March 14, 2008, the XQuery Update Facility specification has become a "Candidate Recommendation", which means it is now pretty stable.
Why an update facility in XML Query ? The answer seems obvious, yet after all the XQuery language itself - or its cousin XSLT2 - is powerful enough to write any transformation of an XML tree. Therefore a simple "store" or "put" function, applied to the result of such transformation, could seem sufficient to achieve any kind of database update operation. Well, perhaps. In practice this would be neither very natural, convenient, nor very efficient (such an approach requires storing back the whole document and makes optimizing very difficult). So as we will see the little complexity added by XQuery Update seems quite worth the effort.
The Instructions below give a quick yet comprehensive practical introduction to the XQuery Update extension, while highlighting some of its peculiarities.
Prerequisites: the reader is presumed to have some acquaintance with XML Query and its Data Model (the abstract representation of XML data, involving nodes of six types: document, element, attribute, text, comment, processing-instruction).