foreign key question

Hi

this is a general question about relational databases, not specific to Oracle (I think), so if there is a better place to ask then please let me know.

I have a question about constraints/foreign-keys.

For example:

I have table A and table B.
Table A has a foreign-key column which points to a record in table B.

Is it possible to define this foreign-key so it can either be "null" (that is, there is no related record in table B), or (if it has a value) then it points to an existing record in table B.

It seems to me I can only define a foreign-key in table A so it must point to an existing record in table B - it cannot be "null". I can just not define a foreign-key at all - but then I cannot be sure that if there is a value it actually points to a record in table B.

Hope you can understand my question.

Thanks,
Peter Received on Fri Mar 11 2005 - 03:01:52 CST