Q:

UML : Optional Foreign Key

I'm sitting with a little headache about a problem right now.
I have two entities PAYMENT ORDER and ACCOUNT
PAYMENT ORDER
PK - ID
FK - DEBITED ACC
FK - CREDITED ACC

ACCOUNT
PK - ACCOUNTNUMBER

I struggle to find the logic when the credited account is not an account from my financial institution. It can not be a foreign key anymore since the account will not be contained within the ACCOUNT table.
Have you guys an idea about a workaround?
EDIT
Question is : Do I have to change the constraint on CREDITED ACC to nothing ? Or can I keep it the way it is?

A:

I recommend you make your ACCOUNT table contain more than just an ACCOUNTNUMBER column.
If this is just a toy example, add a boolean to indicate whether this account number is internal versus external. If this is going to be an operational system, consider adding a FINANCIAL_INSTITUTION table and use its primary key as a foreign key in the ACCOUNTNUMBER table.

