At least one XML element with no duplicates in XSD

Question

I'm trying to create a validation scenario where I want to choose at least one non-repeating element from a list.

Somewhere along the lines of:

<xs:choice minOccurs="1" maxOccurs="7">
    <xs:element name="Sunday"/>
    <xs:element name="Monday"/>
    <xs:element name="Tuesday"/>
    <xs:element name="Wednesday"/>
    <xs:element name="Thursday"/>
    <xs:element name="Friday"/>
    <xs:element name="Saturday"/>
</xs:choice>

However the above solution allows duplicate elements to appear in a list, which I don't want.

Example of a valid XML:

<Monday/>
<Tuesday/>
<Friday/>

Example of an invalid XML:

<Monday/>
<Monday/>

Can this be achieved with an XSD without having to hardcode all the possible sequences? If so how?

Answer 1

XSD 1.0 Solution

I do not believe your constraint can be expressed in XSD 1.0 "without having to hardcode all the possible sequences" (but see C. M. Sperberg-McQueen's clever solution that cuts down the combinatorics considerably) . The values of elements could be forced to be unique using xsd:unique, but subset of XPath used by xsd:unique does not include name(), which could otherwise help make a uniqueness statement about the names of the elements.

XSD 1.1 Solution

Your constrain can be expressed in XSD 1.1 using xs:assert:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:vc="http://www.w3.org/2007/XMLSchema-versioning" elementFormDefault="qualified"
  vc:minVersion="1.1">
  <xs:element name="Days">
    <xs:complexType>
      <xs:choice minOccurs="1" maxOccurs="7">
        <xs:element name="Sunday"/>
        <xs:element name="Monday"/>
        <xs:element name="Tuesday"/>
        <xs:element name="Wednesday"/>
        <xs:element name="Thursday"/>
        <xs:element name="Friday"/>
        <xs:element name="Saturday"/>
      </xs:choice>
      <xs:assert test="count(*[name() = following-sibling::*/name()]) = 0"/>
    </xs:complexType>
  </xs:element>  
</xs:schema>