sql - Multiple foreign keys to a single column

Question

I'm defining a database for a customer/ order system where there are two highly distinct types of customers. Because they are so different having a single customer table would be very ugly (it'd be full of null columns as they are pointless for one type).

Their orders though are in the same format. Is it possible to have a CustomerId column in my Order table which has a foreign key to both the Customer Types? I have set it up in SQL server and it's given me no problems creating the relationships, but I'm yet to try inserting any data.

Also, I'm planning on using nHibernate as the ORM, could there be any problems introduced by doing the relationships like this?

Answer 1

No, you can't have a single field as a foreign key to two different tables. How would you tell where to look for the key?

You would at least need a field that tells what kind of user it is, or two separate foreign keys.

You could also put the information that is common for all users in one table and have separate tables for the information that is specific for the user types, so that you have a single table with user id as primary key.