例如：定义所有Item里的Key为唯一，
<
xs:complexType
>
<
xs:sequence
minOccurs
="
0
"
maxOccurs
="
unbounded
">
<
xs:element
name
="
Item
"
type
="
IdentifiersWithSourceItemType
">
</
xs:element
>
</
xs:sequence
>
</
xs:complexType
>
<
xs:unique
name
="
KeyUnique
">
<
xs:selector
xpath
="
.//Key
"/>
<
xs:field
xpath
="
.//NamespaceID
"/>
</
xs:unique
>
<
xs:unique
name
="
KeyUnique2
">
<
xs:selector
xpath
="
.//Key
"/>
<
xs:field
xpath
="
.//UniversalIDType
"/>
<
xs:field
xpath
="
.//Universal/UniversalID
"/>
</
xs:unique
>
