Slide 16.12: Implications of a class hierarchy

Desired Properties (Cont.)

Implications of a Class Hierarchy
Objects belonging to a subclass inherit those properties defined in its superclass, unless the properties are overwritten by the subclass.

Determines the qualifications of employees whose name is Peter Smith.
SELECT qualifications FROM employee WHERE name = 'Peter Smith';
The attribute name is not defined explicitly for class employee but is inherited from its superclass person.

Asks for employees aged over 50.
SELECT * FROM employee WHERE age > 50;
Assume that a method age is defined for person and thus inherited by employee.

IsA relationships can be used explicitly in query expressions. Determines employees and everyone else over the age 50.
SELECT * FROM person WHERE age > 50;
Selects only persons over 50 who are not employees.
SELECT * FROM p IN person WHERE age > 50 AND p NOT IN employee;

Queries of the above type may result in heterogeneous sets; that is, sets containing elements of different types. Two ways to remedy are
1. By stating that all objects in the resulting set are to carry all the attributes and that their values are to be null values if they would otherwise be undefined.
  
  Disadvantage: Causes the problematic issue of null values.