atomic variability

[Mark]

Is it possible to have a record either be a fixed value or a foreign key in the same attribute of a particular table? For example, if I am using a value that I believe to be atomic, but later it might not be - a constant that I may later want to depend on other values - can I somehow store this quantity as an attribute in my database? Or is there some hack around doing this, like using a dummy foreign key value in the attribute that in some cases points to an atomic quantity and others to a quantity that is dependent on multiple atomic quantities?