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 foreign key for column B matches Table 2?

Answer 1

A check constraint can handle this. If this is SQL Server, something like this will work:

create table A (Id int not null primary key)
go
create table B (Id int not null primary key)
go
create table C (Id int not null primary key, A_Id int null, B_Id int null)
go
alter table C add constraint FK_C_A
foreign key (A_Id) references A (Id)
go
alter table C add constraint FK_C_B
foreign key (B_Id) references B (Id)
go
alter table C add constraint CK_C_OneIsNotNull
check (A_Id is not null or B_Id is not null)
go
alter table C add constraint CK_C_OneIsNull
check (A_Id is null or B_Id is null)
go