Require Both Elements or None : Sequence « XML Schema « XML Tutorial

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2. XML Schema
3. Sequence

File: Schema.xsd

<?xml version = "1.0" ?>
<xs:schema xmlns:xs = "http://www.w3.org/2001/XMLSchema">
   
    <xs:element name="Book">
       <xs:complexType>
          <xs:sequence minOccurs = "0" maxOccurs = "1">
                <xs:element name="Title" type="xs:string" />
                <xs:element name="Author" type="xs:string"  />
          </xs:sequence>
       </xs:complexType>
    </xs:element>
   
</xs:schema>

So the following two examples would be allowed:

<Book></Book>
<Book>
   <Title>XML</Title>
   <Author>author</Author>
</Book>

But the third is invalid because it only contains one of the child elements:
<Book>
   <Title>Professional XML Schema</Title>
</Book>