Question

I am learning SQL and have some tables much like the following:

Person (id*, name)
Customer(id*, is_active, ...)
Employee(id*, department_id, ...)

(the * indicates the primary key, which in the case of Customer and Employee is both a PK and a FK back to Person)

Both Customer and Employee are types of Person, and I want to ensure that when a record is inserted into Person a record must also be inserted into EITHER Customer or Employee but NOT both. A Person cannot be both an employee and customer within the limits of this example.

I have been told that a Trigger would be useful to enforce this constraint. Could somebody please explain the usage of a trigger with this simple example?

Answer 1

Depending on what your DBMS supports and other factors, you can either:

• Insert via a stored procedure that is implemented to ensure the correct behavior.
• Or use a trigger on a view (that JOINs a supertype with particular subtype) to make the view “updatable” and then insert into the view.
• Or ensure both exclusivity and presence of subtypes through purely declarative means, as explained here.
• Or use an implementation strategy for inheritance different from “all classes in separate tables”, as mentioned here.