有一个Choice Indicator 在 XML Schema 中，它允许您使用一个包含的元素，但不能使用两个或更多.如果您想要 3 个中的任何 2 个，我建议您执行以下操作:

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>

(也许 maxOccurs 会阻止您选择同一个元素两次.)如果那行不通，我什么都不会想.

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

编辑:我第一次没有正确理解问题.如果您希望 dbtablename 始终与 fieldname 或 freetext 中的任何一个一起出现，那么这就是答案:

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>