进入W3C官网的DTD学习中：
点击进入DTD实例：
上面的案例代码是内部DTD文档，现在需要创建一个外部dtd文档并拷贝代码：
<!ELEMENT TVSCHEDULE (CHANNEL+)>
<!ELEMENT CHANNEL (BANNER,DAY+)>
<!ELEMENT BANNER (#PCDATA)>
<!ELEMENT DAY (DATE,(HOLIDAY|PROGRAMSLOT+)+)>
<!ELEMENT HOLIDAY (#PCDATA)>
<!ELEMENT DATE (#PCDATA)>
<!ELEMENT PROGRAMSLOT (TIME,TITLE,DESCRIPTION?)>
<!ELEMENT TIME (#PCDATA)>
<!ELEMENT TITLE (#PCDATA)>
<!ELEMENT DESCRIPTION (#PCDATA)>
<!ATTLIST TVSCHEDULE NAME CDATA #REQUIRED>
<!ATTLIST CHANNEL CHAN CDATA #REQUIRED>
<!ATTLIST PROGRAMSLOT VTR CDATA #IMPLIED>
<!ATTLIST TITLE RATING CDATA #IMPLIED>
<!ATTLIST TITLE LANGUAGE CDATA #IMPLIED>
然后创建xml文档引入上面的dtd文档并编写代码：
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE TVSCHEDULE SYSTEM "TV.dtd">
<TVSCHEDULE NAME="TV">
<CHANNEL CHAN="CHANNEL">
<BANNER>aaa</BANNER>
<DAY>
<DATE>2022-2-2</DATE>
<HOLIDAY>xxxx</HOLIDAY>
</DAY>
<DAY>
<DATE>2022-2-3</DATE>
<PROGRAMSLOT VTR="PROGRAMSLOT">
<TIME>2022-2-3</TIME>
<TITLE LANGUAGE="CHINESE" RATING="TITLE">title</TITLE>
<DESCRIPTION>description</DESCRIPTION>
</PROGRAMSLOT>
</DAY>
</CHANNEL>
</TVSCHEDULE>
上面的实例结论：
先写根元素，再看复杂元素，然后按照复杂元素中简单元素的顺序和次数进行编写（每当增加一个复杂元素标签的时候，都必须按顺序对其中的简单元素进行编写）；
例如上面代码中的<DAY></DAY>每当增加一个DAY标签，都必须按照其中的元素顺序和次数要求进行编写；
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE TVSCHEDULE SYSTEM "TV.dtd"><TVSCHEDULE NAME="TV"><CHANNEL CHAN="CHANNEL"><BANNER>aaa</BANNER><DAY><DATE>2022-2-2</DATE><HOLIDAY>xxxx</HOLIDAY></DAY><DAY><DATE>2022-2-3</DATE><PROGRAMSLOT VTR="PROGRAMSLOT"><TIME>2022-2-3</TIME><TITLE LANGUAGE="CHINESE" RATING="TITLE">title</TITLE><DESCRIPTION>description</DESCRIPTION></PROGRAMSLOT></DAY></CHANNEL></TVSCHEDULE>
