A persistent class can be mapped to multiple tables or duplicated in several tables so that feature values of an object are split across rows in several tables. You can map multiple classes in a subclass hierarchy to a single table. Each JADE subclass instance produces a row in the table.
When subclass mapping is selected, the _type special column in the table is used to identify the subclass for each row. The RPS mapping contains a user-defined constant value to associate with each concrete class in the hierarchy. An example of a type hierarchy where subclass mapping might be useful is a bank account hierarchy comprising an abstract base Account class and several concrete subclasses representing different account types.
See also "Special Columns", later in this chapter, and "