Γιωργος Βαλκανας wrote:
>
> Are there any particular semantics for the "NOT IN" statement that
> cause the correlated query to execute for every row of the outter
> query, as opposed to the "NOT EXISTS" ? Or are there any other
> practical reasons, related to "IN / NOT IN", for this to be happening?
> Or is it simply due to implementation details of each RDBMS? I guess
> the former (or the 2nd one), since, as you say, this is common in most
> databases, but I would most appreciate an answer to clarify this.
>
> Thanks again!
>
> Best regards,
> George
>
>
>
Well, I really hoped that Bruce, Robert or Greg would take on this one,
but since there are no more qualified takers, I'll take a shot at this
one. For the "NOT IN (result of a correlated sub-query)", the sub-query
needs to be executed for every row matching the conditions on the
driving table, while the !EXISTS is just a complement of join. It's
all in the basic set theory which serves as a model for the relational
databases.
--
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