1. A CUSTOMER can be either a COMMERCIAL CUSTOMER or a PERSONAL CUSTOMER, which both inherit characteristics from CUSTOMER.
2. A PERSONAL CUSTOMER is also a PERSON, and inherits characteristics from PERSON.
3. A COMMERCIAL CUSTOMER has a CONTACT, who is also a PERSON, and therefore inherits characteristics from both CUSTOMER and PERSON.

NOTE that the Physical Data Model can have be generated in different ways to reflect the ways in which these logical relationships can be implemented in a Database.

There are four alternatives :-
1. One Consolidated Table.
2. One SubType Product Table
3. One SuperType Table, Many SubTypes each in its own Table, each containing all the shared Attributes. 
4. One SuperType Table with an 'extensible' table defined to identify the Attributes 
   specific to each Product Type.
4.  A "products" table, an "attributes" table, and a "specs" table. 
  The products table is like in 3.  
  The attributes table simply lists all available attributes and
  the Product_Atttributes table maps an attribute to a product, and
  provides an attribute value.

Barry Williams
Principal Consultant
Database Answers