How to write a recursive XSD schema for self-similar XML tree?

Question

I am having difficulty understanding how to write a recursive XSD schema for a simple self-similar XML data tree.
I have an idea for the node schema, but not much clue how to make the schema recursive.

Here is an example of the XML, which is a financial risk spec document:

<CG SYM="ROOT" B="-1" S="-1">
  <CG SYM="IOU-AllContracts" B="100" S="100" P="100">
    <CG SYM="IOU100MAY14" B="-1" S="-1" P="50"/>
  </CG>
</CG>

All the nodes are CG nodes, and may contain only CG nodes, to any recursion depth.

I think I have the CG (Contract/Group) Node definition figured out, including some restrictions that I want to put on the contained attribute values. B (BuyLimit) and S (SellLimit) range -1 to whatever, P (Position) is any int, SYM (Symbol) must have no whitespace:

  <xs:schema 
xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://www.w3schools.com"
xmlns="http://www.w3schools.com"
elementFormDefault="qualified">

  <xs:element name="CG">
    <xs:complexType>
      <xs:attribute name="SYM" type="xs:string" use="required">
        <xs:restriction base="xs:string">
      <xs:whiteSpace value="collapse"/>
        </xs:restriction>
      </xs:attribute>
      <xs:attribute name="B" type="xs:integer" use="required">
        <xs:restriction base="xs:integer">
      <xs:minInclusive value="-1">
        </xs:restriction>
      </xs:attribute>
      <xs:attribute name="S" type="xs:integer" use="required">
        <xs:restriction base="xs:integer">
      <xs:minInclusive value="-1">
        </xs:restriction>
      </xs:attribute>
      <xs:attribute name="P" type="xs:integer" >
      </xs:attribute>
    </xs:complexType>
  </xs:element>

  </xs:schema> 

Thanks very much in advance for any help with this.

Answer 1

You should declare that your CG accepts an optional nested CG. You can use the ref attribute to refer to your element declaration:

<xs:element name="CG">
    <xs:complexType>
        <xs:sequence>
            <xs:element ref="CG" minOccurs="0"/>
        </xs:sequence>
        <xs:attribute name="SYM" use="required">
          ....
</xs:element>

A XSD containing the element declaration below will validate the sample instance you provided:

<xs:element name="CG">
    <xs:complexType>
        <xs:sequence>
            <xs:element ref="CG" minOccurs="0"/>
        </xs:sequence>
        <xs:attribute name="SYM" use="required">
            <xs:simpleType>
                <xs:restriction base="xs:string">
                    <xs:whiteSpace value="collapse"/>
                </xs:restriction>
            </xs:simpleType>
        </xs:attribute>
        <xs:attribute name="B" use="required">
            <xs:simpleType>
                <xs:restriction base="xs:integer">
                    <xs:minInclusive value="-1"/>
                </xs:restriction>
            </xs:simpleType>
        </xs:attribute>
        <xs:attribute name="S" use="required">
            <xs:simpleType>
                <xs:restriction base="xs:integer">
                    <xs:minInclusive value="-1"/>
                </xs:restriction>
            </xs:simpleType>
        </xs:attribute>
        <xs:attribute name="P" type="xs:integer"/>
    </xs:complexType>
</xs:element>