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Introduction to Relations in XML Schema


This is the first article in a series concentrating on implementing relations for designing robust XML schema definitions.

Author Info:
By: Jagadish Chaterjee
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April 17, 2006
TABLE OF CONTENTS:
  1. · Introduction to Relations in XML Schema
  2. · A composite “Primary Key” in XML Schema
  3. · Combining two different roots or hierarchies in a single XML Schema: the problem
  4. · Combining two different roots or hierarchies in a single XML Schema: the solution
  5. · Defining a simple relation in XML Schema: XML Schema example

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Introduction to Relations in XML Schema - A composite “Primary Key” in XML Schema
(Page 2 of 5 )

In our previous section, we discussed a simple “Primary Key” (or a primary key based on a single element).  Now, we shall work on a composite primary key.

When we define a primary key on more than one element, we can call it a composite primary key.  We need to make sure that the two elements are siblings to the same parent before enforcing the composite primary key.

For example, all the “OrderIDs” and “ItemIDs” would never repeat within the same order.  How do I enforce this?  Let us look into the following complete schema which does the same.

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" 
elementFormDefault="qualified" attributeFormDefault=
"unqualified">
      <xs:element name="OrderDetailsInfo">
            <xs:annotation>
                  <xs:documentation>Contains All Order details information</xs:documentation>
            </xs:annotation>
            <xs:complexType>
                  <xs:sequence>
                        <xs:element name="OrderDetails" maxOccurs="unbounded">
                              <xs:complexType>
                                    <xs:sequence>
                                          <xs:element name="OrderID" type="xs:int" />
                                          <xs:element name="ItemID" type="xs:int" />
                                          <xs:element name="Quantity" type="xs:int" />
                                          <xs:element name="UnitPrice" type="xs:float" />
                                          <xs:element name="TotalPrice" type="xs:float" />
                                    </xs:sequence>
                              </xs:complexType>
                        </xs:element>
                  </xs:sequence>
            </xs:complexType>
            <xs:key name="PK_OrderDetails">
                  <xs:selector xpath=".//OrderDetails" />
                  <xs:field xpath="OrderID" />
                  <xs:field xpath="ItemID" />
            </xs:key>
      </xs:element>
</xs:schema>

The most important code fragment to concentrate on from the above schema is as follows:

            <xs:key name="PK_OrderDetails">
                  <xs:selector xpath=".//OrderDetails" />
                  <xs:field xpath="OrderID" />
                  <xs:field xpath="ItemID" />
            </xs:key>

Within the above code fragment, you can see the definition of “Key.”  I provided some name (“PK_OrderDetails”) to identify the key.  I also specified that the elements “OrderID” and “ItemID” (in the “OrderDetails” element) are the ones which should be applicable for the “Key.”  Another issue to remember is that they can repeat individually, and cannot repeat together.


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