Designing Your own XML Schema: Constraining with Restrictions - Defining a Primary Key in XML Schema
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This topic is a bit more complicated than any of the previous sections, as it involves a bit of XPATH as well. What is a Primary Key? Anyone familiar with database design would understand it on the spot. A Primary Key is a special type of constraint (or restriction) which makes sure that all the values are unique and without nulls (you can forget about indexing in XML here).
For example, all the Employee IDs should never repeat and are compulsory. How do I enforce it? 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="Employees">
<xs:annotation>
<xs:documentation>Contains All Employee information</xs:documentation>
</xs:annotation>
<xs:complexType>
<xs:sequence>
<xs:element name="Employee" maxOccurs="unbounded">
<xs:complexType>
<xs:sequence>
<xs:element name="ID" type="xs:string" />
<xs:element name="Name" type="xs:string" />
<xs:element name="Age" type="xs:int" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
<xs:key name="PK_Employee_ID">
<xs:selector xpath=".//Employee" />
<xs:field xpath="ID" />
</xs:key>
</xs:element>
</xs:schema>
Within the above code, you can see the definition of “Key.” I provided a name (“PK_Employee_ID”) to identify the key. I also specified that the element “ID” (in the “Employee” element) is the one which should be applicable to the “Key.” You can also provide more than one element as part of the “Key” (making it a composite primary key).
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