Re: The Practical Benefits of the Relational Model

In article <bdf69bdf.0210050531.4e420d83_at_posting.google.com>, Mikito Harakiri <mikharakiri_at_yahoo.com> wrote:
>
>Chapter 4 of Alice&Riccardo&Sergio&Vittorio book describes SPC Algebra
>for conjunctive queries. (I apologize for guessing folklore name of
>the book; corrections are welcome).

I don't think there is. I hereby propose "The Alice Book" with a slight hint to "The Dragon Book".

>Selection, Projection, and Cartesian product are primitive operations,
>while few others - intersection, for example - could be expressed as a
>composition of primitive ones. On the other hand, union is a notable
>exception, so that it is introduced as an additional primitive operation to
>form SPCU Algebra. IMHO, the fact that union is so different from
>intersection is very disturbing. They are dual operation in the traditional
>set theory: unions and intersections in any tautology formula can be
>interchanged and you'll get another tautology. If they are so symmetric,
>why they are so different in the relational theory?

Symmetric? I can simulate the intersection with the difference. What is the corresponding operator for the union?

• Jan Hidders