Decomposing an IS-A Relationship
Create a table for the parent entity and for each of the child entities or subtypes
Move the associated attributes from the parent entity into the child table to which they correspond
From the parent entity take the entity key and add it as the primary key to the corresponding table for each child entity
In the event a table corresponding to a child entity already has a primary key then simply add the entity key from the parent entity as an attribute of the table corresponding to the child entity
CLIENT (CLIENT_ID, AMOUNT_DUE, …)
INDIVIDUAL_CLIENT (CLIENT_ID, SIN#, …)
CORPORATE_CLIENT(CLIENT_ID, GST#, …)