Using W3C XML Schema
by Eric van der Vlist | Pages: 1, 2, 3, 4, 5, 6, 7, 8, 9

Table of Contents

•Introducing Our First Schema •Slicing the Schema •Defining Named Types •Groups, Compositors and Derivation •Content Types •Constraints •Building Usable and Reusable Schemas •Namespaces •W3C XML Schema and Instance Documents •W3C XML Schema Datatypes Reference •W3C XML Schema Structures Reference

Constraints

Unique

W3C XML Schema provides several flexible XPath-based features for describing uniqueness constraints and corresponding references constraints. The first of these, a simple uniqueness declaration, is declared with the xs:unique element. The following declaration done under the declaration of our book element indicates that the character name must be unique:

    <xs:unique name="charName">
      <xs:selector xpath="character"/>
      <xs:field xpath="name"/>
    </xs:unique>

This location of the xs:unique element in the schema gives the context node in which the constraint holds. By inserting xs:unique under our book element, we specify that the character has to be unique within the context of this book only.

The two XPaths defined in the uniqueness constraint are evaluated relative to the context node. The first of these paths is defined by the selector element. The purpose is to define the element which has the uniqueness constraint -- the node to which the selector points must be an element node.

The second path, specified in the xs:field element is evaluated relative to the element identified by the xs:selector, and can be an element or an attribute node. This is the node whose value will be checked for uniqueness. Combinations of values can be specified by adding other xs:field elements within xs:unique.

Keys

The second construct, xs:key, is similar to xs:unique except that the value has to be non null (note that xs:unique and xs:key can both be referenced). To use the character name as a key, we can just replace the xs:unique by xs:key:

<xs:key name="charName">
  <xs:selector xpath="character"/>
  <xs:field xpath="name"/>
</xs:key>

Keyref

The third construct, xs:keyref, allows us to define a reference to a xs:key or a xs:unique. To show its usage, we will introduce the friend-of element, to be used against characters:

<character>
  <name>Snoopy</name>
  <friend-of>Peppermint Patty</friend-of>
  <since>1950-10-04</since>
  <qualification>
    extroverted beagle
  </qualification>
</character>

To indicate that friend-of needs to refer to a character from this same book, we will write, at the same level as we defined our key constraint, the following:

<xs:keyref name="charNameRef" refer="charName">
  <xs:selector xpath="character"/>
  <xs:field xpath="friend-of"/>
</xs:keyref>

These capabilities are almost independent of the other features in a schema. They are disconnected from the definition of the datatypes. The only point anchoring them to the schema is the place where they are defined, which establishes the scope of the uniqueness constraints.

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