2_5 IDREF和IDREFS类型的属性的使用及作用

IDREF/IDREFS类型

IDREF类型的属性用来引用同一文档中的另一元素的ID属性。IDREF属性的值必须是文档中某个元素的ID属性值勤。通过使用IDREF，可使一个元素和另一个元素发生联系。

如

可以为book元素定义一个IDREF类型的属性publisher，用于指出该书的出版社，其值勤为publisher元素的ID属性publisherID的值.

IDREFS是IDREF的复数形式。该类型的属性值是若干个ID属性的值，之间用空格分开。

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE bookInfo [
<!ELEMENT bookInfo (publishers,authors,books)>
<!ELEMENT publisher (publisher+)>
<!ELEMENT publisher (pname,address)>
<!ATTLIST publisher publisherID ID #REQUIRED>
<!ELEMENT pname (#PCDATA)>
<!ELEMENT address (#PCDATA)>
<!ELEMENT authors (author+)>
<!ELEMENT author (aname,age,sex)>
<!ATTLIST author authorID ID #REQUIRED>
<!ELEMENT aname (#PCDATA)>
<!ELEMENT age (#PCDATA)>
<!ELEMENT sex (#PCDATA)>
<!ELEMENT books (book*)>
<!ELEMENT book (title,price,year)>
<!ATTLIST book
publisher IDREF #REQUIRED
author IDREFS #REQUIRED
>
<!ELEMENT title (#PCDATA)>
<!ELEMENT price (#PCDATA)>
<!ELEMENT year (#PCDATA)>
]>
<bookInfo>
<publishers>
<publisher publisherID="p001">
<pname>tsinghua university press</pname>
<address>beijing city</address>
</publisher>
<publisher publisherID="p002">
<pname>electronic industry press</pname>
<address>beijing city</address>
</publisher>
<authors>
<author authorID="a001">
<aname>Wangchao</aname>
<age>45</age>
<sex>male</sex>
</author>
<author authorID="a002">
<aname>Zhangpeng</aname>
<age>38</age>
<sex>Female</sex>
</author>
</authors>
<book>
<book publisher="p001" author="a001 a002">
<title>Inside XML</title>
<price>35</price>
<year>2002</year>
</book>
</book>
</publishers>
</bookInfo>