Re: relational algrabra division?

The text states that the operation is useful for a kind of query. It then gives an example query in natural language and the corresponding formal version using the operation and works through its evaluation. The very next thing is a formal definition of the operation. Then another definition of it. The some properies of the operation. Then a third definition. (The one you quote.) Then it relates the operation to natural language.

This is reasonable and clear.

It is not the job of the textbook to anticipate and answer the rhetorical questions of its readers. It is the job of the reader to accept, consider and absorb what a presentation gives.

The sentence that you think makes "no sense" is perfectly grammatical and sensible. Like any sentence it requires a certain amount of ability with the language and a certain amount of effort since it has a certain amount of structure. It might have been a good idea for the authors to use simpler sentences. You could have spent more time trying to parse it. Clearly your English comprehension is not what it could be, and your opinion of your English comprehension is not what it could be.

My suggestion is that you think more about where the authors are trying to take you. "Trust them" and your instructors. This involves holding multiple things in mind.

I agree that not every mentor can be trusted. There's certainly a lot of writing, and textbook writing, and CS textbook writing, and DB textbook writing that could be clearer or much clearer both in wording and in concepts. But your recent messages are simply about writing that is about topics of a certain complexity.

By the way, the whole idea of relational division is a bit dubious and it is much better to approach queries of the kind that it is used for plus queries of a more general kind via relation inequalities and "image relations". So there is an example of a relative conceptual weakness of this book's syllabus. Received on Sun Dec 21 2014 - 09:25:14 CET