Database schema which can support specialized properties

Question

I need to store a set of entities, of which there are several specialized versions. They have some common properties, but the specialized ones contain properties specific fo

Answer 1

This technique can have some issues with proper design and performance, but you seem to need a compromise with flexibility.

Instead of having to create specific tables or even a new field for each property you could have a table (And it will be quite large) for unique properties:

Unique_Property_ID
, FK_To_Some_Entity (Not sure what entity these link to: customers, bills, etc.) 
, Property_Type Not the data type but a link to a table describing this entity)
, Property_Value (Difficult to determine if all of your values will be of the same type: string, int, date, etc.)

Example: A used car dealership needing to track add ons for different makes and models (They never know what they're going to get). A few records may look like:

VehicleID   Property Type  Property Value
3           Sound System   Bose
3           Hybrid System  Battery
7           Starter Type   Hand Crank
7           Passenger Seat Rumble
9           Starter Type   Kick Start
9           Passenger Seat Side Car

Each value for each Set | property type would have its own record.

Another problem is when you do want each property to be represented as a column, you'll need to transpose this table.