Help understanding types

I am reading through the 8th edition of "Introduction to Database Systems" and I am having some difficulty with the section on types. (5.3 Representations vs. Types)

"A nonscalar type is a type whose values are explicitly defined to have a
set of user visible, directly accessible components. " ....

"A scalar type is a type that is not non scalar." ...

Date then goes on to explain that types can have several representations, and the representations can have components. Here is one example he gave:

TYPE POINT
    POSSREP Cartesian { X Rational, Y Rational};

I don't understand the distinction between components that make up a representation, and components that make up a type. How can a value of type 'Point' be scalar if I (as a user) can access the X and Y members? Doesn't this exactly satisfy the definition of a scalar type as "A type whose values are explicitly defined to have a set of user visible, directly accessible components".? (Aren't the X and Y co-ordinates above components, user visible, and directly accessible?)

The explanation given in the book isn't entirely clear to me, so if somebody else could put it in different words I would be grateful.

Thanks,

Jim Received on Sun Nov 23 2003 - 21:02:20 CET