Foreign Key Constraints - basic question

[Colin B asterfield]

Hi,

I have just introduced foreign key constraints onto the tables in my Database Schema, and it is causing me a problem.

An example is my Booking table which has three columns which have foreign key constraints from three different tables, but inside my program only one of these columns will have a valid value in it, the other two are legitimately NULL.

Obviously one could argue the rights and wrongs of designing ones database in this manner, but is there some way of configuring the foreign key so that it can be either null or a value in the parent table?

many thanks in advance...
Colin