清单 4. 包含简单业务逻辑的 XQuery

(: ————————– 🙂
(: declares ……:)
(: ————————– 🙂
declare namespace local = "http://localhost/ns";
declare variable $title as xs:string external;
declare variable $author as xs:string external;
declare variable $year as xs:string external;
declare variable $emptymsg as xs:string external;
declare variable $oversizedmsg as xs:string external;
declare variable $outputformat as xs:string external;
(: ————————– 🙂
(: functions                  🙂
(: ————————– 🙂
declare function local:error_view($helpmsg as xs:string) {
    if($helpmsg) then $helpmsg
    else (‘<report status="0">Please call 1-800-123-4567</report>’)
};
declare function local:html_view($doc) {
    <table>
    <tr> <td>Library Name</td>
         <td>Book Title</td>
         <td>Number of Copies</td>
         <td>Status</td>
    </tr>
    {   for $book in $doc/book
            where $book/title/@lang="en"
            order by $book/library
        return <tr>
                    <td>{$book/library}</td>
                    <td>{$book/title/text()}</td>
                    <td>{$book/numberCopies}</td>
                    <td>{$book/status}</td>
                </tr>
    }
    </table>
};
(: ————————– 🙂
(: the business logic         🙂
(: ————————– 🙂
let $docs := (‘bookinfor/book_collection.xml’)
let $searchdoc := (
<report status=’1′>
{
    if($title and $year and $author) then (
        for $doc in collection($docs)//booklist/book[contains(title,$title)
                and (year = $year)]
        where contains($doc//authors, $author)
        return $doc
    ) else if($title and $year ) then (
        for $doc in collection($docs)//booklist/book[contains(title,$title)
                and (year = $year)]
        order by $doc//booklist/book/title
        return ($doc)
    ) else if($title and $author ) then (
        for $doc in collection($docs)//booklist/book[contains(title,$title)]
        where contains($doc//authors, $author)
        order by $doc//booklist/book/title
        return ($doc)
    ) else if($author and $year ) then (
        for $doc in collection($docs)//booklist/book[(year=$year)]
        where contains($doc//authors, $author)
        order by $doc//booklist/book/title
    return ($doc)
    ) else if($title) then (
        for $doc in collection($docs)//booklist/book[contains(title,$title)]
        order by $doc//booklist/book/title
        return ($doc)
    ) else if($author) then (
        for $doc in collection($docs)//booklist/book
        where contains($doc//authors, $author)
        order by $doc//booklist/book/title
        return ($doc)
    ) else if($year) then (
        for $doc in collection($docs)//booklist/book[(year=$year)]
        order by $doc//booklist/book/title
        return ($doc)
    ) else (
        for $doc in collection($docs)
        return ($doc//booklist/book)
    )
}
</report>
)
(: ————————– 🙂
(: return results             🙂
(: ————————– 🙂
return  if(count($searchdoc//book) eq 0) then local:error_view($emptymsg )
        else if(count($searchdoc//book) gt 100)
            then local:error_view($oversizedmsg)
            else if($outputformat eq $html)
                then local:html_view($searchdoc)
                else $searchdoc

　　在应用程序中调用 XQuery 查询

　　为了在内嵌引擎中执行查询，需要设置相关的参数并预编译查询。查询执行后，最终结果发送回 Web 客户机。客户端 JavaScript 代码组装数据并呈现给用户。请注意，这个示例应用程序将目录 c:/tmp/xmldatasources/ 设置为静态的查询上下文。一定要保证 bookinfor 目录在其中，并在测试该应用程序之前将 XML 数据文件复制过去。清单 5 显示了内嵌的 XQuery 调用。

　　清单 5. 在应用程序中调用 XQuery 查询

dynamicQueryContext.setParameter("emptymsg", "No results exist!");
dynamicQueryContext.setParameter("oversizedmsg",
                                   "Too many results, please refine queries");
   
staticQueryContext.setBaseURI("c:/tmp/xmldatasources/");
fileReader = new FileReader(request.getRealPath("/")+  "/queries/booklist_app.xql");
queryExpression = staticQueryContext.compileQuery(fileReader); 
    
writer = new StringWriter();
StreamResult result = new StreamResult(writer);
Properties props = new Properties();
queryExpression.run(dynamicQueryContext, result, props);
retContent = writer.getBuffer();

　　结束语

　　需要注意的是，XQuery 技术仍然存在一些局限性，学起来也不是很简单。要记住，没有一种技术能够解决所有的业务问题。很可能仍然需要把 XQuery 与其他技术(如 XPath 和 XSLT)结合起来才能满足要求。

　　虽然 XQuery 还没有成为最终标准，但它的主要特性和功能已经引起了很大关注。它有强大的 XML 处理功能，而且在建立 marshup 应用程序方面显示了很大的潜力。一旦用 XQuery 实现了您的动态 Web 应用程序，可以很快地扩展您的网站，并以相对简单和敏捷的方式达到您的业务目标。