changing from relational schema to XML schema,HELP!

[iwsyang]

In a relational schema, we have such definition:
create demain( posInt AS int default 1 CHECK(value >1)

create table book( isbn posInt PRIMARY KEy,......)

I convert the domain to this XML schema,
<xsd:simpleType name="posInt">
<xsd:restriction base="xsd:integer">
<xsd:minExclusive value="0" />
</xsd:restriction>
</xsd:simpleType>

and for the table, I try to define "isbn" as an element like:
<xsd:element name="ISDN" type="posInt"/>

But how to express ISDN is a primary key for the table?

 

[tsuji]

http://www.w3.org/TR/xmlschema-1/#cIdentity-constraint_Definitions