dtd 属性#IMPLIED 有无皆可

<?xml version="1.0" encoding="utf-8"?>

<!DOCTYPE shopping [
	<!ELEMENT book (name,price,count,id)>
	<!ELEMENT name (#PCDATA)>
	<!ELEMENT price (#PCDATA)>
	<!ELEMENT count (#PCDATA)>
	<!ELEMENT id EMPTY>
	
	<!--	
		id标签的
		bookId属性 ID类型(只能是字母和下划线开头) 可以有也可以没有
	-->
	<!ATTLIST id 
		bookId ID #IMPLIED 
	>
	
]>

<shopping>

	<book>
		<name>庄子</name>
		<price>100</price>
		<count>3</count>
		<id bookId="zhuangzi"></id>					
	</book> 

	<book>
		<name>易经·系传</name>
		<price>200</price>
		<count>5</count>
		<id bookId="_yijingxizhuan"></id>	
	</book>

	<book>
		<name>坛经</name>
		<price>300</price>
		<count>7</count>
		<id></id>		
	</book>
	
</shopping>