Designing Your own XML Schema: Restrictions and User Defined Types - Understanding the simple type with enumeration in XML Schema
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Instead of explaining the whole XML schema at one time (demonstrated in the previous section), let me elaborate in a few steps. Let us consider the following fragment first:
<xs:simpleType name="Category">
<xs:restriction base="xs:string">
<xs:enumeration value="TopManagement" />
<xs:enumeration value="Management" />
<xs:enumeration value="SalesMan" />
<xs:enumeration value="Clerk" />
</xs:restriction>
</xs:simpleType>
The above fragment declares a new data type (of the simple type in XML schema). The name of the new simple data type is “Category.” Once declared, I can use this “Category” as a data type for any element in my XML Schema.
In general, every new user-defined type will get inherited from an existing XML Schema type. According to the above scenario, our “Category” is getting inherited from the “string” type (xs:string). This means all the characteristics of “xs:string” become available to our new data type “Category.”
Once inherited from an existing type, we can refine with our own restrictions along with facets (if necessary). According to the above definition, I restricted the “Category” in such a way that it would accept any of the values within the following list (or enumeration):
<xs:enumeration value="TopManagement" />
<xs:enumeration value="Management" />
<xs:enumeration value="SalesMan" />
<xs:enumeration value="Clerk" />
Now I need to use the new data type in my element definitions (in our sense, it would be “EmpCategory” element). Let us look at the following fragment:
<xs:element name="EmpCategory" type="Category" />
The above line declares a new element, “EmpCategory,” which is based on our own user-defined simple type “Category.” And thus all the restrictions available for “Category,” get carried to the “EmpCategory” element accordingly for validations (based on enumeration values). You can use the same “EmpCategory” any number of times within the same schema. Availability of “Category” to external schemas is also possible through namespaces and other means, which I will cover in future articles.
You can also create several other new data types, which can even nest some other data types. Similarly, you can even create a data type based on some elements. This would help us to reuse every data type in several other schemas when necessary.
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