Representing the XML Schema as a DTD

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The following is an example of representating the XML schema with a DTD. The representation is not exact, however; and there are a number of problems:

• Schemas and DTDs use different content models. When the model is defined as open, the element can include ElementType elements, AttributeType elements, and mixed content not specified in the content model. When the model is defined as closed, the element cannot include elements and cannot include mixed content not specified in the content model. The DTD uses a closed model, and schemas use an open content model.
• Possible datatypes for elements and attributes are different. The datatypes parameter entity below lists the datatypes for both attributes and elements so for the dt:type attribute of the datatype element. But in the Microsoft® Internet Explorer 5 version, not all of these datatypes are supported on attributes and elements. The attributes only support the first 10 types (which map to DTD attribute types) and elements only support from "string" through to uuid and therefore an element cannot behave like an id or idref, for example.
• Placement of xmlns namespace declarations cannot be modelled very well using a DTD. In a real schema the xmlns:dt attributes can go anywhere. And for that matter any other xmlns declaration can also appear because schemas themselves follow an open content model. This way you can add extended information in your schemas for your own use.
• The maxOccurs attribute can only have the values "1" or "*" and this cannot be modelled using a DTD enumeration because "*" is not a valid name token.
• ElementTypes and AttributeTypes can only have one datatype child element. DTDs cannot model this sort of ordinality especially when other child elements are also allowed.

<!--
  The possible element datatypes according to
    http://msdn.microsoft.com/xml/schema/reference/datatypes.asp
-->
<!ENTITY % datatypes "(entity | entities | enumeration | id | idref | idrefs
| nmtoken | nmtokens | notation | string | bin.base64 | bin.hex | boolean |
char | date | dateTime | dateTime.tz | fixed.14.4 | float | int | number |
time | time.tz | i1 | i2 | i4 | r4 | r8 | ui1 | ui2 | ui4 | uri |
uuid)" >

<!--  *** datatype **** -->
<!ELEMENT datatype  (description)*>
<!ATTLIST datatype
    dt:type %datatypes;  #IMPLIED
    xmlns:dt CDATA #FIXED "urn:schemas-microsoft-com:datatypes"
>

<!--  ***** description  ***** -->
<!ELEMENT description (#PCDATA) >

<!-- ****  element  ***** -->
<!-- additional constraint on maxOccurs is that it must be 1 or "*" -->
<!ELEMENT element  (description)* >
<!ATTLIST element
   type IDREF        #REQUIRED
   minOccurs  CDATA  #IMPLIED
   maxOccurs  CDATA  #IMPLIED
>

<!-- ****  attribute   ***** -->
<!ELEMENT attribute  (description)* >
<!ATTLIST attribute
   type       IDREF    #REQUIRED
   default    CDATA        #IMPLIED
   required   (yes | no)   "no"
>

<!-- ****  AttributeType   ***** -->
<!ELEMENT AttributeType (datatype | description)* >
<!ATTLIST AttributeType
   name         ID              #REQUIRED
   default      CDATA           #IMPLIED
   dt:type      %datatypes;     #IMPLIED
   dt:values    CDATA           #IMPLIED
   required     (yes | no)      #IMPLIED
   xmlns:dt CDATA #FIXED "urn:schemas-microsoft-com:datatypes"
>

<!-- ****  ElementType   ***** -->
<!ELEMENT ElementType (datatype | description | AttributeType | attribute | element | group)* >
<!ATTLIST ElementType
   name  ID                       #REQUIRED
   model (open | closed)          #IMPLIED
   content (empty | textOnly | eltOnly | mixed) #IMPLIED
   order   (one | seq | many)     #IMPLIED
   dt:type %datatypes;            #IMPLIED
   dt:values  CDATA               #IMPLIED
   required  (yes | no)           #IMPLIED
   xmlns:dt CDATA #FIXED "urn:schemas-microsoft-com:datatypes"
>

<!-- ****  group   ***** -->
<!ELEMENT group  (group | element | description)*>
<!ATTLIST group
   minOccurs   CDATA              #IMPLIED
   maxOccurs   CDATA              #IMPLIED
   order      (one | seq | many)  #IMPLIED
>

<!-- ****  Schema   ***** -->
<!ELEMENT Schema  (AttributeType | ElementType | description )* >
<!ATTLIST Schema
   name  CDATA #IMPLIED
   xmlns:dt CDATA #FIXED "urn:schemas-microsoft-com:datatypes"
>