XQuery 条件判断语句

XQuery 条件判断语句 语法

XQuery 提供了一个非常有用的 if-then-else 构造来检查传递的输入值的有效性。下面给出了 if-then-else 结构的语法。

语法为：

if (condition) then
 ... 
else
 ... 

XQuery 条件判断语句 示例

我们将使用以下 books.xml 文件，并对其应用包含 if-then-else 构造的 XQuery 表达式，以检索价格值大于 30 的书籍的书名。

books.xml：

<?xml version="1.0" encoding="UTF-8"?>
<books>
    <book category="JAVA">
        <title lang="en">Learn Java in 24 Hours</title>
        <author>Robert</author>
        <year>2005</year>
        <price>30.00</price>
    </book>

    <book category="DOTNET">
        <title lang="en">Learn .Net in 24 hours</title>
        <author>Peter</author>
        <year>2011</year>
        <price>40.50</price>
    </book>

    <book category="XML">
        <title lang="en">Learn XQuery in 24 hours</title>
        <author>Robert</author>
        <author>Peter</author>
        <year>2013</year>
        <price>50.00</price>
    </book>

    <book category="XML">
        <title lang="en">Learn XPath in 24 hours</title>
        <author>Jay Ban</author>
        <year>2010</year>
        <price>16.50</price>
    </book>
</books>

books.xqy：

<result>
{
   if(not(doc("books.xml"))) then (
      <error>
         <message>books.xml does not exist</message>
      </error>
   )
   else (
      for $x in doc("books.xml")/books/book
      where $x/price>30
      return $x/title
   )
}
</result>

解析程序

package com.yiidian;

import com.saxonica.xqj.SaxonXQDataSource;

import javax.xml.xquery.*;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStream;

public class XQueryTester {
   public static void main(String[] args){
      try {
         execute();
      }
      catch (FileNotFoundException e) {
         e.printStackTrace();
      }
      catch (XQException e) {
         e.printStackTrace();
      }
   }
   private static void execute() throws FileNotFoundException, XQException{
      InputStream inputStream = new FileInputStream(new File("books.xqy"));
      XQDataSource ds = new SaxonXQDataSource();
      XQConnection conn = ds.getConnection();
      XQPreparedExpression exp = conn.prepareExpression(inputStream);
      XQResultSequence result = exp.executeQuery();
       while (result.next()) {
         System.out.println(result.getItemAsString(null));
      }
   }
}

输出结果为：