Column A or Column B required MS Access DB I'm in the situation where I would like to enforce the requirement where a record must contain either one column or another at a data level, I'm not sure whether it's achievable.

I've got an Enquiry table which is used for new enquiries being recorded; this can either be based on an existing customer, which is contained within the Customer table or prospective customer which will be created and stored within a ProspectiveCustomer table.

 Enquiry
-----------------------------------------------
 EnquiryID  | CustomerID | ProspectID | Notes | 
       3    |     2      |     null   |   x   | 
       4    |     null   |     3      |   y   |


There must be a record number in either CustomerID or ProspectID, but not in both. Is this achievable at a data level alone? or would I have to simply validate my inputs in this way and hope for the best?
I think of a scenario, where you can have a MasterUser table that will asign ID to both Customer and Prospect, auto incremented PKId can be named as UserID of the table.

let Say you create a table MasterUser and stored common details like names etc. that both Customer and Prospect have, now the unique info can be better moved to table Customer and ProspectiveCustomer.

Now as you have uniqueId for each user, no matter the user is Prospect or Customer.

So you Enquiry table will be something like


Enquiry
-----------------------------------------------
 EnquiryID  | UserID| Notes | 
       3    |    2  |   x   | 
       4    |    3  |   y   |