Designing Your own XML Schema: Restrictions and User Defined Types

Instead of explaining the whole XML schema at once (demonstrated in the previous section), let me elaborate in a few steps.  Let us consider the following fragment first:

The above fragment declares a new data type (of the simple type in XML schema).  The name of the new simple data type is “AgeType.”  Once declared, I can use this “AgeType” 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 “AgeType” is getting inherited from the “integer” type (xs:integer).  This means all characteristics of “xs:integer” are available to our new data type, “AgeType.” 

Once inherited from an existing type, we can refine it with our own restrictions, along with facets (if necessary).  According to the above definition, I restricted the “AgeType” in such a way that it would accept only the values between one and one hundred (both inclusive).  And that completes our user-defined type at the moment.

Now I need to use the new data type in my element definitions (in our sense, it would be the “Age” element).  Let us look at the following fragment:

The above line declares a new element, “Age,” which is based on our own user-defined simple type, “AgeType.”  And thus all the restrictions available for “AgeType,” get carried to the “Age” element accordingly for validations.  You can use the same “AgeType” any number of times within the same schema.  Availability of the “AgeType” to external schemas is also possible through namespaces and other means. I will cover this in future articles.

Apart from all of the above uses, there exists another advantage to the user-defined data types.  If we modify the “AgeType” definition (for instance, changed its restrictions), it would automatically apply to every element declared with that respective data type!