Re: thinking about UPDATE

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"Marshall Spight" <mspight_at_dnai.com> wrote in message news:N0mLc.114236$WX.22912_at_attbi_s51...
> Hi all,
>
> I was thinking about the relationship between operations of the
> relational algebra and SQL statements.

> INSERT is a lot like union, but with an odd restriction that the
> intersection of the new set and the existing set must be empty. (Why?)

This is only if you define a primary or candidate key.

> DELETE is the same thing as set subtraction.

> But what the heck is UPDATE?

> If T : {a1, a2, ... an}

> UPDATE T set a1 = 0 where <cond>
> is
> T' = T { 0, a2, ... an | <cond> }
> T = T - T { a1, a2, ... an | <cond> }
> T = T union T'

> That's certainly more complicated than I would suspect
> for what seems like such a simple operation.

> Am I making it needlessly complex, or is this about right?
> Is there some other way to think about UPDATE?

You could think about it as a set transformation T --- f ---> T'
{t | t in T}---> {f(t) | t in T}

Some questions to think about:
Why there is a need for primary/candidate keys in RM ? If keys are essential, why the definitions of the relational operators don't include them ?

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