Using W3C XML Schema - Part 2
by Eric van der Vlist | Pages: 1, 2, 3, 4, 5

Constraints

Table of Contents

•Content Types •Constraints •Building Usable and Reusable Schemas •Namespaces •W3C XML Schema and Instance Documents

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 xsd:unique element. The following declaration, within the context of our book document, indicates that the character name must be unique.

<xsd:unique name="charNameMustBeUnique"> 
  <xsd:selector xpath="character"/>
  <xsd:field xpath="name"/>
</xsd:unique>

This location of the xsd:unique element in the schema gives the context node in which the constraint holds. By inserting xsd:unique under our book element, we specify that the character has to be unique in the context of a 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 xsd:field element. is evaluated relative to the element identified by the xsd:selector and can be an element or an attribute node. This is the node whose value will be checked for uniqueness. Uniqueness over a combination of several values can be specified by adding other xsd:field elements within xsd:unique.

Keys

The second constraint construct, xsd:key, is similar to xsd:unique, except that the value specified as unique can be used as a key. This means that it has to be non-null, and that it can be referenced. To use the character name as a key, we can replace the xsd:unique by xsd:key.

<xsd:key name="charNameIsKey">
  <xsd:selector xpath="character"/>
  <xsd:field xpath="name"/>
</xsd:key>

The third construct, xsd:keyref, allows us to define a reference to a key. To show its usage, we 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 the same book, we write, at the same level as we defined our key constraint, the following:

<xsd:keyref name="friendOfIsCharRef" refer="charNameIsKey">
  <xsd:selector xpath="character"/>
  <xsd:field xpath="friend-of"/>
</xsd:keyref>

These capabilities are nearly 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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