| <!-- 引入dtd文档到xml文档中 dtd无法约束标签内的内容 | |
| * 内部dtd：将约束规则定义在xml文档中 | |
| * 外部dtd：将约束的规则定义在外部的dtd文件中 dtd文件名字随意 | |
| * 本地：<!DOCTYPE 根标签名 SYSTEM "dtd文件的位置"> | |
| * 网络：<!DOCTYPE 根标签名 PUBLIC "dtd文件名字" "dtd文件的位置URL"> --> | |
| <!-- 内部dtd: | |
| <!DOCTYPE students [ | |
| <!ELEMENT students (student+) > | |
| <!ELEMENT student (name,age,sex)> | |
| <!ELEMENT name (#PCDATA)> | |
| <!ELEMENT age (#PCDATA)> | |
| <!ELEMENT sex (#PCDATA)> | |
| <!ATTLIST student number ID #REQUIRED> | |
| ]> | |
| --> | |
| <students> | |
| <student number="s001"> | |
| <name>zhangsan</name> | |
| <age>abc</age> | |
| <sex>hehe</sex> | |
| </student> | |
| <student number="s002"> | |
| <name>lisi</name> | |
| <age>24</age> | |
| <sex>female</sex> | |
| </student> | |
| </students> |