The DTD FOR XML constraints

XML constraint technology:    DTD constraint: syntax is relatively simple and function is relatively simple.    schema constraints: The syntax is relatively complex and functions are relatively powerful. (namespace)

DTD constrained import DTD mode:

Internal Import <! DOCTYPE Note [  <! ELEMENT Note (to,from,heading,body) >  <! ELEMENT to      (#PCDATA) >  <! ELEMENT from    (#PCDATA) >  <! ELEMENT heading (#PCDATA) >  <! ELEMENT body    (#PCDATA) >]>

external import of    local file system    :<! DOCTYPE Note SYSTEM "Note.dtd" >     External import of public:<! DOCTYPE root element Public "HTTP://GZ.ITCAST.CN/ITCAST.DTD" >

DTD syntax    constraint tag         <! Element name category >  or <! Element elements name (element content) >     Category:        empty tag: null. Indicates that the element must be an empty element.        Normal string: (#PCDATA). Indicates that the content of an element must be a normal string (cannot contain child tags).        

(element content): Order problem:<! element name (child element name 1, child element name 2,.....) >: Number of sub-labels in order problem: Tags: must and only appear 1 times. Label+: Labels appear at least 1 times*: 0 or N times.  Label? :0or 1 times. Constraint Properties<! Attlist element Name Property Name property type default Value >Default Value: #REQUIRED property value is required #IMPLIED property is not required #FIXED The Value property is not required, but the property value is a fixed property type: A CDATA that controls the value of a property: normal String (En1|en2|..) : Indicates that it must be one of the value IDs: Indicates that the attribute value must be unique in an XML document. Value cannot start with a number

Code:

Create a new Note.dtd file  : <! ELEMENT Note (from?,to+,heading*,body+) >  <! ELEMENT to      empty>  <! ELEMENT from    (#PCDATA) >  <! ELEMENT heading (#PCDATA) >  <! ELEMENT body    (#PCDATA) >  <! Attlist to ID ID #REQUIRED >

Create a new note.xml<?xml version= "1.0"? ><! DOCTYPE Note SYSTEM "note.dtd" ><note>  <to id= "A1" ></to>  <to id= "A2" ></to>  <to id= "A3" ></to>  
The DTD FOR XML constraints