XSD unique contraint based on multiple elements including an optional element

Question

I have defined a unique constraint on multiple elements : define unique constraint based on multiple elements

Now unique constraint looks like this:

<xs:unique name="specieSizeGroupLengthAssortment">
    <xs:selector xpath="DataRow"/>
    <xs:field xpath="Specie"/>
    <xs:field xpath="Group"/>
    <xs:field xpath="Length"/>
    <xs:field xpath="Type"/>
</xs:unique>   

Now imagine the element "Type" is optional. So far my search and my testing confirmed, that this unique constraint only works on elements which have all the subelements defined in the unique constraint. For example:

This should be invalid due to unique constraint:

<DataRow>
   <Specie>A</Specie>
   <Length>100</Length>
   <Group>A</Group>
</DataRow>

<DataRow>
   <Specie>A</Spacie>
   <Length>100</Length>
   <Group>A</Group>
</DataRow>

This should be valid :

<DataRow>
   <Specie>A</Specie>
   <Length>100</Length>
   <Group>A</Group>
</DataRow>

<DataRow>
   <Specie>A</Spacie>
   <Length>100</Length>
   <Group>A</Group>
   <Type>D</Type>
</DataRow>

This should be invalid :

<DataRow>
   <Specie>A</Specie>
   <Length>100</Length>
   <Group>A</Group>
   <Type>D</Type>
</DataRow>

<DataRow>
   <Specie>A</Spacie>
   <Length>100</Length>
   <Group>A</Group>
   <Type>D</Type>
</DataRow>

Is it possible to create an XSD schema that will do this kind of validation?

Answer 1

The specification states that each field in a unique constraint

must identify a single node (element or attribute) whose content or value, which must be of a simple type, is used in the constraint.

XML Schema part 1: Structures, §3.11.1, my bold.

So it appears that you can't use optional elements in a uniqueness constraint. This is backed up by the step-by-step rules for validating these constraints (§3.11.4):

3 For each node in the ·target node set· all of the {fields}, with that node as the context node, evaluate to either an empty node-set or a node-set with exactly one member, which must have a simple type. [Definition:] Call the sequence of the type-determined values (as defined in [XML Schemas: Datatypes]) of the [schema normalized value] of the element and/or attribute information items in those node-sets in order the key-sequence of the node.
4 [Definition:] Call the subset of the ·target node set· for which all the {fields} evaluate to a node-set with exactly one member which is an element or attribute node with a simple type the qualified node set. The appropriate case among the following must be true:
4.1 If the {identity-constraint category} is unique, then no two members of the ·qualified node set· have ·key-sequences· whose members are pairwise equal, as defined by Equal in [XML Schemas: Datatypes].
[...]

This explicitly defines the uniqueness check as applying only to the "qualified node set", i.e. those nodes matching the selector which have values for all their fields