Techniques for database inheritance?

Question

What are the tips/techniques when you need to persist classes with inheritance to relational database that doesn't support inheritance?

Say I have this classic example:

Person -> Employee -> Manager
                   -> Team lead
                   -> Developer
       -> Customer -> PrivilegedCustomer
                   -> EnterpriseCustomer

What are the available techniques to design the database? Pros and cons of each?

p.s. I have searched and found several question regarding database inheritance but most were about changing to a database engine that supports it natively. But let's say I'm stuck with SQL Server 2005... what are my options?

Answer 1

Three common strategies:

1. Create a table for each class in the hierarchy that contain the properties defined for each class and a foreign key back to the top-level superclass table. So you might have a vehicle table with other tables like car and airplane that have a vehicle_id column. The disadvantage here is that you may need to perform a lot of joins just to get one class type out.

2. Create a table for each class in the hierarchy that contains all properties. This one can get tricky since it's not easy to maintain a common ID across all the tables unless you're using something like a sequence. A query for a superclass type would require unions against all the tables in question.

3. Create one table for the entire class hierarchy. This eliminates joins and unions but requires that all of the columns for all class properties be in one table. You'll probably need to leave most columns nullable since some columns won't apply to records of a different type. For example, the vehicle table might contain a column called wingspan that corresponds to the Airplane type. If you make this column NOT NULL then any instance of a Car inserted into the table will require a value for wingspan even though a value of NULL might make more sense. If you leave the column nullable you might be able to work around this with check constraints but it could get ugly. (Single Table Inheritance)