Foreign Key for either-or column?

Question

Is it possible to have a foreign key that requires either column A or column B to have a value, but not both. And the foreign key for column A matches Table 1 and the forei

Answer 1

It depends on which database you're working with. If you want a table Foo that has FK relationships to Table1 and to Table2 but only one at a time, then you'll need to set up either some sort of trigger (my links assume SQL Server, but the ideas's the same) or Constraint to enforce your rule that only one column have a value.