The main aspect of XQuery is that everything is an expression. XQuery is not a programming language, so XQuery scripts (or programs) are expressions. This is where the SQL analogy is appropriate, because SQL statements are basically expressions that interact with backend data, although the expressions may become very complex. Here is a simple example of an XQuery expression:

let $value1 := 0
let $value2 := 1
let $rValue := ""
if ($value1 > $value2) then let $rValue := "true" else let $rValue := "not true"
return $rValue

The preceding few lines represent a simple XQuery expression. It creates and assigns values to variables, utilizes flow control via the if statement, and outputs a value via the return keyword. In this example, let is used to assign values and the dollar sign is prepended to variable names. In addition, the assignment operator is a colon plus the equal sign. The if structure follows the basic syntax of most languages. The return statement marks the point in the expression where a value is returned. The return value may be a simple variable (like the previous example), static text, or a mixture of extracted values and text.

While the central ingredient of XQuery is the expression, these expressions utilize the following common reserved keywords:

An overused acronym used for these common keywords is FLWR, often called FLWR-expression. Here is a basic XML document, which contains a sampling of books:

<?xml version="1.0" encoding="ISO-8859-1"?>
<books>
<book type="paperback">
<title>American Psycho</title>
<author>Bret Easton Ellis</author>+
<isbn>0679735771</isbn>
<price>14.00</price></book>
<book type="hardback">
<title>A Burnt-Out Case</title>
<author>Graham Greene</author>
<isbn>0370014995</isbn>
<price>13.00</price></book>
<book type="paperback">
<title>The Information</title>
<author>Martin Amis</author>
<isbn>0679735739</isbn>
<price>14.00</price>
</book></books>

The preceding XML is used in the following XQuery example:

let $doc = document("books.xml")
for $d in $doc/books/book
return
{$d/title/text}
?by
<td>
{$d/author/text}

In this simple example, all books in the format of title followed by the text and the author's name are returned. Notice that XPath (http://builder.com.com/5100-6387-1057652.html) notation is used to specify individual nodes in the for statement and in portions of the return statement. Two other noteworthy aspects of this example:

There are three books in the sample XML, but only two are paperbacks (the attribute of the book element). This attribute may be utilized in a where statement that extends the previous example to output only paperback books:

let $doc = document("books.xml")
for $d in $doc/books/book
where ($d/@type = "paperback")
return
{$d/title/text}
?by
<td>
{$d/author/text}

The where clause guarantees only paperback items are returned. Again, this is similar to the structure of a SQL statement where a condition is used. These examples provide a peek at the XQuery approach and syntax. It is a powerful language for retrieving necessary data, and industry support is swelling.

As with most developing technologies, you must search carefully to locate products that support it. Thankfully, XQuery is seen as an industry standard, so the rush to support it has been quick. You can find it in popular tools such as XML Spy and Corel XMetal. In addition, Microsoft has been quick to provide XQuery support for .NET, and the Java community has been on the bandwagon for some time. The support is overwhelming, so the prospect of XQuery becoming deprecated is dismal (I never say never).

The comparison between XQuery and SQL is good, but notable differences are apparent. A major variation is XQuery's lack of support for updating the data source (this is a basic aspect of SQL), nor can data sources be created on the fly. With this limitation in mind, some vendors have chosen to develop a proprietary approach to updating. This is just one item, but it does show the difference and highlights the fact that this is a relatively new technology (version 1.0) that will continue to evolve.