Querying

Storing XML data
Querying XML data
XQuery

Characteristics

Query language for searching and extraction of XML nodes (elements, attributes) from a document and for an output XML document construction.

Characteristics (2nd)

The XQuery is a basic XML query language at present time (and it seems in the future as well).
The W3C specification since March 2011, see http://www.w3.org/XML/ Query.
Base on XPath 2.0 data model, operators and functions.
Supported by main database engines producers (IBM, MS, Oracle, etc)

Where to use XQuery (and where not)

The XQuery domain is useful for:

queries, where extraction (selection) part is more complicated than the construction part.

In other cases:

Use XSLT if more complex output is required (e.g. added some new markup and content).
Use a more general API (using DOM manipulation) if more complex operations are required.

Source code example

Example of source document, XML Queries on it and their results.


  
    
      Petr
      Novak
      1969-05-14
      novak@myfriends.com
      Very good friend
    
    
      Jaroslav
      Nováček
      1968-06-14
      novacek@myfriends.com
      Another good friend
    
    
      Jan
      Horak
      1970-02-0
      horak@mycompany.com
      Just colleague
    
    
      Erich
      Polak
      1980-02-28
      erich@myfriends.com
      Good friend

Example - Simple Query (XPath)

Task: "extract all surnames in the addressbook".
Query is more-or-less just an XPath expression, like "selects all lastname elements":
```
doc('myaddresses.xml')/addressbook/person/lastname
```

Running XQuery using Saxon 9.0j

XSLT processor Saxon contains the XQuery processor since version 8.x as well. To process XQuery you need:

to install Saxon 9.0.0.4J for example ("j" means implementation in Java, there is a .NET implementation as well) by unpacking into folder c: /devel/saxon9-0-0-4j for example.
Change working directory to the folder: cd c:/devel/saxon9-0-0-4j
put the above mentioned query into a file (lastnames.xq).
store the above mentioned XML document containing "addressbook" into the file myaddresses.xml in the same directory.

Run:

java -classpath saxon9.jar net.sf.saxon.Query -o result.xml lastnames.xq

Result

The query to above mentioned document will create the file result.xml:

Novák Nováček
Horák Polák

XQuery structure

FLWOR is an acronym of an XQuery structure. It roughly corresponds to the SQL query structure:

(F)or

Initial query part that specifies query cycle including control variable. Results of XPath expression behind the keyword " in" are assigned to the variable.

(L)et

You can assign values of next variable that can be used later in this section.

(W)here

specifies selection condition ie. which nodes (values) selected by for section will be used.The condition can utilize the variables defined in the "let" section.

(O)rder

Defines how the nodes should be oredered.

®eturn

Defines what is returned, constructed from extracted nodes (values).

FLWOR - simple example

Condition used to select requested nodes can be specified either in an XPath expression in "for" clause or in the "where" clause. "Return Mr. Polak’s birth-date."

for $person in
doc('myaddresses.xml')/addressbook/person where $person/lastname='Polák'
return $person/date-of-birth

XQuery returns:


l980-02-28

XQuery Implementation

SAXON since versions 7.x:

install (extract) Saxon with version 7.0 at least (8.x, 9.x) into some directory
change working directory to the Saxon directory and
run: java -classpath saxon9.jar net.sf.saxon.Query -o result.xml query-file .xq from command line.
There is a .NET Saxon implementation (means .DLL and .EXE files)

Native XML databases

Native XML database systems mostly support XQuery as a query language:

Berkeley DB XML:: http: //www.sleepycat.com/products/index.shtml
eXist:: http://exist.sourceforge.net/

Storing and Querying XML data; XQuery