How to specify in an XML schema that either one of two fields must be present?

I want to specify that either fieldname or freetext must always be present in XML files that apply to this XSD. Is there a way to do that?

<xs:complexType name="tSome">
<xs:sequence>
  <!-- either one of the two below has to be present. -->
  <xs:element name="fieldname" type="xs:string" />
  <xs:element name="freetext" type="xs:string" />
  <!-- this one below must always be present -->
  <xs:element name="dbtablename" type="xs:string" />
</xs:sequence>
<开发者_如何学JAVA;/xs:complexType>

There is a Choice Indicator in XML Schema, which allows you to take one of the contained elements, but not two or more. If you want any 2 of 3, I suggest doing something like this:

<xs:choice>
  <xs:element name="fieldname" type="xs:string" minOccurs="0" maxOccurs="1" />
  <xs:element name="freetext" type="xs:string" minOccurs="0" maxOccurs="1" />
  <xs:element name="dbtablename" type="xs:string" minOccurs="0" maxOccurs="1" />
</xs:choice>
<xs:choice>
  <xs:element name="fieldname" type="xs:string" minOccurs="0" maxOccurs="1" />
  <xs:element name="freetext" type="xs:string" minOccurs="0" maxOccurs="1" />
  <xs:element name="dbtablename" type="xs:string" minOccurs="0" maxOccurs="1" />
</xs:choice>

(Maybe maxOccurs will prevent you from choosing one and the same element twice.) If that does not work, nothing will I think.

Edited: I didn't correctly understand the question the first time. If you want dbtablename to always be present with any one of fieldname or freetext, then this is the answer:

<xs:complexType name="tSome">
<xs:sequence>
  <xs:choice>
    <xs:element name="fieldname" type="xs:string" />
    <xs:element name="freetext" type="xs:string" />
  </xs:choice>
  <xs:element name="dbtablename" type="xs:string" />
</xs:sequence>
</xs:complexType>

So, you want either fieldname or freetext and not both? or maybe both? and then dbtablename optionally?

Here is 1 or 2 of the elements:

<xs:choice minOccurs="1" maxOccurs="2">
    <xs:element name="fieldname" type="xs:string"/>
    <xs:element name="freetext" type="xs:string"/>
    <xs:element name="dbtablename" type="xs:string"/>
</xs:choice>

Is this what you want? or did you want dbtablename to be separate?