mnickel67@g... <mnickel67@g...> wrote in
<f0461272-9799-42a5-a909-1673f42564c8@d...>:
> I'm still learning, so apologies if this is trivial...

W3C's XML Schema Primer is a good, solid tutorial, if a bit
on the chewy side.

> <Record>
>    <foo id="a">... </foo>
>    <foo id="b">....</foo>
>    <foo id="c">.....</foo>
> </Record>
> 
> Is it possible to create an xsd file that enforces that
> within the <Record> block there exists exactly three foo
> tags each containing a different id value (from the enum
> a, b, and c )?

It is possible.

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="data">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="Record"
          minOccurs="0" maxOccurs="unbounded"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="Record">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="foo" minOccurs="3" maxOccurs="3"/>
      </xs:sequence>
    </xs:complexType>
    <xs:unique name="abc">
      <xs:selector xpath="foo"/>
      <xs:field xpath="@id"/>
    </xs:unique>
  </xs:element>
  <xs:element name="foo">
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base="xs:string">
          <xs:attribute ref="id" use="required"/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>
  <xs:attribute name="id">
    <xs:simpleType>
      <xs:restriction base="xs:string">
        <xs:pattern value="(a|b|c)"/>
      </xs:restriction>
    </xs:simpleType>
  </xs:attribute>
</xs:schema>

Note that the fact that it is, indeed, possible doesn't yet
mean it's a good idea. My knee-jerk reaction would be to
redefine foo as three separate elements (foo-a, foo-b,
foo-c or something). Keep your grammars as simple as
possible, or you'll be in for a nasty surpise when you run
into unavoidable limitations of XML Schemata.

-- 
<>There is no phenotype</>