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?
Three common strategies:
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.
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.
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)