Thanks Mark. I think my basic design was flawed too. I moved the FK in the Accounts Table. The Account belongs to the customer. Not the other way around. So..it seems this design works better (and no fuss from SQL!):

Old Design:

CUSTOMERS

------

CustomerID (PK)

AccountNumID (FK)

FullName

Address

ACCOUNTS

------

AccountID

AccountNum

NEW Design:

CUSTOMERS

-------------

CustomerID

Name

Address....etc...

ACCOUNTS

-----------

AccountID

CustomerID (FK)

AccountNum

...etc...