Table of contents

Appendices

5.1 Specifying Uniqueness

ref29XML Schema enables us to indicate that any attribute or element value must be unique within a certain scope. To indicate that one particular attribute or element value is unique, we use the unique element first to "select" a set of elements, and then to identify the attribute or element "field" relative to each selected element that has to be unique within the scope of the set of selected elements. In the case of our report schema, report.xsd, the selector element's xpath attribute contains an XPath expression, r:regions/r:zip, that selects a list of all the zip elements in a report instance. Likewise, the field element's xpath attribute contains a second XPath expression, @code, that specifies that the code attribute values of those elements must be unique. Note that the XPath expressions limit the scope of what must be unique. The report might contain another code attribute, but its value does not have to be unique because it lies outside the scope defined by the XPath expressions. Also note that the XPath expressions you can use in the xpath attribute are limited to a [subset] of the full XPath language defined in [XML Path Language 1.0].

We can also indicate combinations of fields that must be unique. To illustrate, suppose we can relax the constraint that zip codes may only be listed once, although we still want to enforce the constraint that any product is listed only once within a given zip code. We could achieve such a constraint by specifying that the combination of zip code and product number must be unique. From the report document, 4Q99.xml, the combined values of zip code and number would be: {95819 872-AA}, {95819 926-AA}, {95819 833-AA}, {95819 455-BX}, and {63143 455-BX}. Clearly, these combinations do not distinguish between zip code and number combinations derived from single or multiple listings of any particular zip, but the combinations would unambiguously represent a product listed more than once within a single zip. In other words, a schema processor could detect violations of the uniqueness constraint.Going back to our purchase order example, suppose we want each item to have a unique combination of part number and product name. We could achieve such a constraint by specifying that for each item element, the combined values of its partNum attribute and its productName child must be unique.

To define combinations of values, we simply add use multiple field elements to identify all the values involved. So, to add the part number value to our existing definition, we add a new field element whose xpath attribute value, r:part/@number, identifies the number attribute of part elements that are children of the zip elements identified by r:regions/r:zip:

NOTE: 

A Unique Composed Value

 <unique name="dummy1">
  <selector xpath="r:regions/r:zip"/>
  <field    xpath="@code"/>
  <field    xpath="r:part/@number"/>
 </unique>
<xsd:element name="items" type="Items">
  <xsd:unique name="partNumAndName">
    <xsd:selector xpath="item"/>
    <xsd:field    xpath="@partNum"/>
    <xsd:field    xpath="productName"/>
  </xsd:unique>
</xsd:element>