Re: thinking about UPDATE

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"Marshall Spight" <mspight_at_dnai.com> wrote in message news:YhxLc.4607$8_6.1154_at_attbi_s04...

> > Some questions to think about:
> > Why there is a need for primary/candidate keys in RM ?

> It's not a set if it allows duplicates.

It is one thing to not allow duplicates, it is another thing to have a primary/candidate key that is a strict subset of all relation attributes.

> > If keys are essential, why the definitions of the relational operators
don't
> > include them ?

> For one thing, what you get back from a select (I guess I'm talking
> SQL now) is a value that you can't really do much with. You
> can't assign it to a relational variable and manipulate it further,
> so any constraints you might expect to see on the value are
> irrelevant. Which bugs me.

You are not alone :-)

> Presumably one could infer the keys on the results of a relational
> operator.

Try it. For project operator for example.

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