predicate, constraints, header, relvar, and relation

I've been reading a lot of CJ Date's work, and I have some fairly general (but fundamental) questions.

Does a predicate apply to a relvar or a relation? It makes sense to me that you could have one relation that exists in two separate variables with two separate predicates, indicating to me that a predicate applies to the relvar, not the relation. However, if you join the values in two relvars, it makes sense that there is an inference being made, and therefore the result relation has some kind of derived predicate. Am I making any sense? Can someone clear this up for me?

Also, I have a similar question for constraints. Does a constraint apply to a relation or a relvar? It makes sense that this would be the same answer as above, because a constraint is an approximation of the predicate. Would it make any sense to apply a constraint to, for example, a relation literal?

It seems to me that the header must be a part of the relation itself, because a relation literal must have a header. However, it makes sense to me that, when you declare a relvar, you specify the header so that the relvar can only store relations with a matching header. Received on Fri Apr 20 2007 - 18:15:33 CDT