optional foreign keys one of which must be mandatory - how?

I have a table that has two optional foreign keys, each to a different table and one of which HAS to be filled in, but it doesn't matter which one. I was thinking about using a trigger to enforce this "constraint", but it feels wrong doing it that way. I am not able to redesign th开发者_Python百科e tables, so I am stuck with it.

We are using Oracle 10g

Is there a better way to do this?

EDIT: I accidentally left out some of the info. At least one column HAS to be filled in, and ONLY one column CAN be filled in.

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Use a check constraint. If they can both be populated then:

 alter table t add constraint c check (col1 is not null or col2 is not null)

Or if they are mutually exclusive:

 alter table t add constraint c check ((col1 is not null and col2 is null
                                       or (col2 is not null and col1 is null))

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Create another table to be parent of the two tables you are referencing at the moment. Reference that new table with just one foreign key (non-nullable) instead of two. In other words use a supertype / subtype pattern.