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

> 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.

Sorry for not mentioning section, it is Customizing Queries/Using NULL
values.  Content on the web:

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

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