Re: Logical equivalence of simple and complex types under the relational model?

"Neo" <neo55592_at_hotmail.com> schrieb im Newsbeitrag news:4b45d3ad.0411301048.4a23356b_at_posting.google.com...
>> It seems to me at first sight that
>> 1) RM with simple types
>> 2) RM with complex types
>> are indistiguishable at the logical level.
>
> What is a type? What distinguishes simple from complex?

There are two things to consider for type. What is a type is fairly simple. Then there is the more complicated part of how a type is defined.

The type of a variable can be viewed as a constraint on that variable, or the allowed set of the values that variable takes. e.g. the variable x only takes integers.

The type(s) of a value can be viewed as to which categories to value belongs.
e.g. 2 is a natural number.

Type also normally includes that operations that are allowed on that variable or value.

Simple in the sense of RM means that the value is atomic with respect to RM. i.e. there are no operations within RM that can access parts of the value. e.g. the number 2 is atomic in RM if there are no operations that let you look into the structure of a '2'.

All other types are complex.

These things are quite well covered in Codds paper "Extending the database relational model to capture more meaning "

Rene. Received on Tue Nov 30 2004 - 21:12:37 CET