When expressions conflict

And views contradict

The results that emerge

These rules will predict—

Throughout the book prior to this point, I’ve been concerned primarily with the question of updating through some individual relational operation (updating a restriction, updating a projection, and so on). What’s more, I’ve been assuming, more or less tacitly, that the rules for updating a relvar defined by means of some more complicated expression can be determined by combining the rules for the operations involved in that expression—for example, updating a union of two joins can be done by first applying the rules for updating a union and then applying the rules for updating joins. Now it’s time to take a closer look at that assumption.

I observe first of all that the assumption is surely correct in principle. The reason is that the alternative is just too horrible to contemplate! To spell it out, the alternative in question would mean treating every expression as a special case—i.e., defining one set of rules for updating the union of two joins, and another for updating the difference of two joins, and another for updating the join of a union and a difference, and so on and so forth ad infinitum.

To say it again, the assumption is surely correct. However, it does raise certain questions, questions to which the answers don’t always seem entirely clear (at least, not to me, and not at this time). Thus, I have two aims in this chapter: First, ...

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