Re: The Practical Benefits of the Relational Model

Paul Vernon wrote:

> The idea being (I think) that any two projections that have the same
> predicate should also have the same value.
>
> So if we have P (Part PART_#, Part_name STRING) S (Supp SUPPLIER_#,
> Part PART_#)
>
> Then SELECT Part FROM S should equal SELECT Part FROM P
>
> as they have the same predicate.

        But, they do? The predicate to a relvar would be all its constraints.   Without knowing all the constraints to P and S, we can't know if their projections on Part have the same predicate.

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