Question

If I have an element with one or more subelements, should the min/maxoccurs attributes be on the xsd:sequence element, the xsd:element, both, or neither?

<xsd:element name='books'>   <xsd:complexType>     <xsd:sequence minOccurs='1' maxOccurs='unbounded'> <!-- here? -->       <xsd:element ref='book' minOccurs='1' maxOccurs='unbounded'/> <!-- or here? -->     </xsd:sequence>   </xsd:complexType> </xsd:element> 

Answer 1

In almost all circumstances, you want to put the min/max Occurs on the element within a sequence and not on the sequence. Using your example:

<xsd:element name='books'>   <xsd:complexType>     <xsd:sequence>       <xsd:element ref='book' minOccurs='1' maxOccurs='unbounded'/>     </xsd:sequence>   </xsd:complexType> </xsd:element> 

This is unequivocal. If you have a series of book elements in a row, you can point to exactly which schema item is producing them. However:

<xsd:element name='books'>   <xsd:complexType>     <xsd:sequence minOccurs='1' maxOccurs='unbounded'/>       <xsd:element ref='book' minOccurs='1' maxOccurs='unbounded'/>     </xsd:sequence>   </xsd:complexType> </xsd:element> 

Here, if you have two ‘book‘ elements in a row, do you have two sequences in a row, or one sequence with two book elements? This fails the Unique Particle Attribution requirement.

Finally, if you put the min/max Occurs on the sequence and you later add an additional element:

<xsd:element name='books'>   <xsd:complexType>     <xsd:sequence minOccurs='1' maxOccurs='unbounded'/>       <xsd:element ref='book'/>       <xsd:element ref='ebook'/>     </xsd:sequence>   </xsd:complexType> </xsd:element> 

then this may allow the following XML, which is probably not what you intend:

<books>   <book/><ebook/><book/><ebook/><book/><ebook/><book/><ebook/> </books> 

whereas if you have:

<xsd:element name='books'>   <xsd:complexType>     <xsd:sequence/>       <xsd:element ref='book' minOccurs='1' maxOccurs='unbounded'/>       <xsd:element ref='ebook' minOccurs='1' maxOccurs='unbounded'/>     </xsd:sequence>   </xsd:complexType> </xsd:element> 

then it is clear and unambiguous what you intend: A sequence of one or more book elements followed by a sequence of one or more ebook elements.