One-One, One-Many and Many-Many Relations in XML Schema - Implementing a “One to Many” relation in XML Schema: the schema
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Until now, we have looked at only the one to one relation. Now, it is time to look at implementing the one to many (or conversely many to one) relation.
Let us consider the following schema.
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified" attributeFormDefault=
"unqualified">
<xs:complexType name="DeptType">
<xs:sequence>
<xs:element name="Deptno" type="xs:int"/>
<xs:element name="Dname" type="xs:string"/>
<xs:element name="Loc" type="xs:string"/>
</xs:sequence>
</xs:complexType>
<xs:element name="Organization">
<xs:complexType>
<xs:sequence>
<xs:element name="dept" type="DeptType"
maxOccurs="unbounded"/>
<xs:element name="Employee"
type="EmployeeType" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
<xs:key name="PK_Dept_Deptno">
<xs:selector xpath=".//dept"/>
<xs:field xpath="Deptno"/>
</xs:key>
<xs:key name="PK_Employee_Empno">
<xs:selector xpath=".//Employee"/>
<xs:field xpath="Empno"/>
</xs:key>
<xs:keyref name="FK_deptEmployee"
refer="PK_Dept_Deptno">
<xs:selector xpath=".//Employee"/>
<xs:field xpath="Deptno"/>
</xs:keyref>
</xs:element>
<xs:complexType name="EmployeeType">
<xs:sequence>
<xs:element name="Empno" type="xs:int"/>
<xs:element name="Ename" type="xs:string"/>
<xs:element name="Sal" type="xs:float"/>
<xs:element name="Deptno" type="xs:int"/>
</xs:sequence>
</xs:complexType>
</xs:schema>
I already explained similar types of hierarchies in my previous sections; I would like to show you the most important bits now. The most important code fragment to concentrate on from the above schema is the following:
<xs:keyref name="FK_deptEmployee"
refer="PK_Dept_Deptno">
<xs:selector xpath=".//Employee"/>
<xs:field xpath="Deptno"/>
</xs:keyref>
The above code fragment says that the “deptno” in “Employee” should “refer” to the values in “PK_Dept_Deptno” (which is nothing but “deptno” in “dept”) for integrity. We also named the relation “FK_deptEmployee.” It became a one to many relation because the “deptno” in “Employee” is not a primary key (or unique key).
How about an XML document for the above XML Schema? The next section shows you that.
Next: Implementing a “One-Many” relation in XML Schema: a sample XML document >>
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