Thank you for your tips. I think I will change the tables and use some
minimal date instead of a null value to represent a constraint that is
valid all the time. An additional advantage of this approach is that I
can then make sure that the time intervals (I not only have a start
date, but also an end date) don't overlap.
Nevertheless, I think that there are some examples where a null value
does not exactly mean "unknown". But this is beyond the scope of this
discussion.
Regards,
Christian
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