A complex type aggregates a collection of simple types into a logical unit. For example, a customer type might include the customer number, name, street address, town, city, and zip code. A complex type can also reference other complex types or element and attribute groups.

XML supports complex type inheritance. When you define a complex type, you can create other complex types that inherit the components of the base type. In a type relationship, the base type is the complex type from which you derive another type. A derived complex type inherits elements from the base type.

An extended complex type is a derived type that inherits elements from a base type and includes additional elements. For example, a customer_purchases type might inherit its definition from the customer complex type, but the customer_purchases type adds item, cost, and date_sold elements.

A restricted complex type is a derived type that restricts some elements from the base type. For example, mail_list might inherit elements from customer, but restrict the phone_number element by setting the minoccurs and maxoccurs boundaries to zero.

The following figure shows derived complex types that restrict and extend the base complex type:

In the above figure, the base type is PublicationType. BookType extends PublicationType and includes the ISBN and Publisher elements. Publication_Minimum restricts PublicationType. Publication_Minimum requires between 1 and 25 Authors and restricts the date to the year.