Re: "IS NOT NULL" != "NOT NULL"

> I've been told that Oracle fails to distinguish empty strings from
> NULL, which if true is a clear violation of the SQL specification.
> If you're used to Oracle then that might help explain your confusion :-(
>
> Another problem is that SQL's boolean operations act as though NULL
> is the logical value UNKNOWN, rather than explicitly setting up a
> boolean datatype with the three allowed values TRUE, FALSE, UNKNOWN.
> While the rules for propagation of NULL happen to be similar to the
> results that logic dictates you get for UNKNOWN, this is still a kind
> of type pun, and it doesn't help to reduce the confusion any.

My book does deal with this NULL distinction:

    http://www.postgresql.org/docs/awbook.html

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