THE XML REVOLUTION - TECHNOLOGIES FOR THE FUTURE WEB

Type derivation and substitution groups

XML Schema contains a complicated type system.

Types can be derived from other types:

Derivation by extension:

<complexType name="car"> <complexContent> <extension base="n:vehicle"> <sequence> <element name="wheel" minOccurs="3" maxOccurs="4"/> </sequence> </extension> </complexContent> </complexType>
creates a car type from a vehicle type by extending it with 3 or 4 wheel sub-elements.
(Components from the base type are inherited as in an implicit sequence.)

Derivation by restriction:

<complexType name="small_car"> <complexContent> <restriction base="n:car"> <sequence> ... <element name="wheel" maxOccurs="3"/> </sequence> </restriction> </complexContent> </complexType>
creates a small_car type from the car type by restricting it to 3 wheel sub-elements.
(All components from the base type must be repeated, as indicated by "...".)

Subsumption:

Assume that we declare an element:

meaning that myVehicle elements are valid if they match the vehicle type.

Since car is derived from vehicle, myVehicle elements are also valid if they match car
- provided that we add xsi:type="n:car" to the elements (where xsi refers to http://www.w3.org/2001/XMLSchema-instance) in the instance document.

Substitution groups - an alternative to type derivation:

If we declare another element as:

then we may always use myCar elements whenever myVehicle elements are required (without using xsi:type).

This is independent of the extension/restriction inheritance hierarchy! - car is not required to be declared as a sub-type of vehicle.

Controlling derivation and subsumption:

In addition to all this,

derivation of types can be forbidden (e.g. using final="restriction")
subsumption can be forbidden (e.g. using block="#all")
use of elements and types can be forbidden (using abstract="true")